Note: Descriptions are shown in the official language in which they were submitted.
~2~7
AUTOMATIC LINE TERMIMATION IN
DISTRIB~TED INDUSTRIAL PROCESS CONTROL SYSTEM
BACKGROUND OF THE INVENTION
This application is a division of Canadian Serial
No. 368,795, filed January 19, 1981.
The present invention relates to control systems of
the type having a plurality of remotely located process
control units connected together through a communications
link and, more particularly, to a control system in which
each of the remote units is capable of testing the communica-
ti.on integrity of the communications link between it and
other remotes in the system and automatically inserting
a line-termination impeda~nce when a degraded or interrupted
channel in the communications link is detected.
Many system-type industrial installations, for example,
those related to industrial process manufacturing and el-
ectrical power generation, employ a large number of physically
distributed controlled-devices and associated sensors for
ef~ecting coordinated opera-tion of the overall sys-tem. In
the past, coordinated control of the various devices has
been achieved by manual operation and various types of semi-
automati.c and automatic control systems including electro-
magnetic relay systems, hardwired solid-state logic systems,
and various types of computer control systems. The computer
systems have included central systems in which ~he various
sensors and controlled devices are connected to a central
computer; distributed control systems in which a remotely
located computer is connected to each of the controlled
devices and to one another; and hybrid combinations of -the
central and distributed systems. The successful functioning
of the control system is vital to any industrial process,
and, accordingly, distri~uted systems have generall~ been
preferred over central systems because the failure of one
-- 1 --
of the remotely located control computers generally does
not cause a system wide ~ailure as in the case of the ~ailure
of the central computer in the central system~ ~owever, in
the distributed systems, a communication link or buss inter-
connects each of the various remotes and deterioration or
interruption o~ the communication link can effectively divide
the system into inoperative portions. Such degradation
or interruptions of the communication link can occur when
the communication link physically passes through an area
of high electrical noise that can induce errors into the
data transmissions or where the communications link is act-
ually severed. In the lattex case, the open communications
link causes an lmbalanced line impedance which can adversely
affect the data signal-current levels in the communications
link. When the system transmits information in digital
form at high and very high data rates, an interrupted trans-
mission link can cause reflected signals which, in turn,
can cause false triggering and garb].ed data transmission.
In view of this, a control system must be able to detect
a deyradation or interruption of its communication link
and be able to minimize the adverse af~ect on the communicat-
ion capability of the remaining remotes in the system in
order to maintain high reliability data transmission between
remotes.
SUMMAR~ OF THE INVENTION
Accordingly, the present invention seeks to provide
an industrial control system for controlling an industrial
process or the like having a high overall system operating
reliability and more particularly seeks to provide an industrial
control system which can detect th~ degradation or inter-
ruption o~ the communication link and take steps to minimi.ze
the adverse a~fect such degradation or interruption has
on communication between remotely located controlled devices.
The present invention also seeks to provide an industrial
control system defined by a plurality of remotely located
process control units (remotes~ interconnected through a
communication link or buss with each of the remotely located
units adapted to test the communication integrity of the
communications link in accordance with a predetermined sequence.
The present invention further seeks to provide an
industrial control system having a plurality of remotely
located process control units interconnected through a link
or buss in which high reliability information transfer i5
achieved between remotes even when one or more signal carrying
channels of the communication link between remotes is inter-
rupted or degraded.
The invention to which this divisional application
is directed in one aspect pertains to an information transfer
system comprising a multiplicity of stations with a communicat-
ions link interconnecting -the stations. Each station is
provided with ~a) a stored-program controller and (b) trans-
mitting and rece:iving means connected between the stored-
program controller and the communications link for trans-
mitting digital information from the stored-program controller
over the communications link in messages of predetermined
format to another one of the stations and for receiving
digital information from another one of the stations over
the communications link in messages of predetermined format.
Each station also has (c) switch means selectively operable
under the control of the stored-program controller to connect
an impedance to the communications link. The stored-program
controllers include message checking means for determining
if the transmitted messages are successfully received by
the transmitting and receiving means of ~he receiving station.
The invention herein in another aspect pertains to
an information transfer system for transferring digital
information between stored-proyram controllers interconnected
-- 3 --
by a communications link, each controller adapted to test the
communications integrity of the communications link between it
and other of the controllers and connect an impedance to the
communications link in the event of an interruption or degra-
dation of the communications integrity of the communications
link between the controllers. Each system includes a plurality
of stored-program controllers interconnected through a com-
munications link, each controller is provided with (a~ tran-
smitting and receiving means operatively connected to the
communications link for transmitting and receiving digital
information in messages of predetermined format to and from
each other over the communications link and with (b) message-
checking means operably connected to the transmitting and
receiving means for determining if transmitted messages
are successfully received by the receiving controller. Each
system also includes (c) selectively operable switch means
to selectively connect an impedance to- the communications
link. Each controller includes means to cause the trans-
mltting and receiving means ko transmit messages to other
of the controllers and to provide an actuation signal to
the switch means to cause the switch means to connect the
impedance to the communications link in response to the
message checking means determining that the number of messages
successfully received by the other of the controllers is
below a predetermined threshhold.
The invention in this divisional application also
comprehends a method of providing a line-matching impedance
in a communications link of an industrial process control
system having a plurality of process controlling remotes
interconnected by the communications link in the event of
an interruption or degradation of the communications integrity
of the communicat~ons link. The method comprises the steps
of providing at least a first one of the remotes with an
impedance that is selectively connectable to the communica-tions
~8~7
link, transmitting a message from the first one of the remotes
to a second one of the remotes, evaluating the validity
of the message received at the second one of the remotes
and sending a valid-message-received message from the second
one of the remotes to the first one of the remotes if the
test message is valid, evaluating the cornmunications integrity
of the communications link between the first one of the
remotes and the second one of the remotes as a function
of a number of valid-message-received messages received
by the first one of the remotes, and connecting the impedance
means to the communications link if the number of valid-
message -received messages received by the first one of
the remotes is less than a predetermined value.
The invention also comprehends a method of providing
a line matching impedance in a commu~ications link of an
industrial process control system having a plurality of
O, Rl, Rx 1~ Rx~ R~+l -- Rn_L, Rn interconnected
by a common communications link in the event of an inter-
ruption or degradation of the communications integrity of
the communlcations link. rrhe method comprises the steps
of (a) providing at least one of said remotes Rx with an
impedance means that is selectively connectable to the com-
munications li.nk~ (b~ transmitting a test message from the
remote Rx to a selected one of the remotes R~l or Rx 1'
(c) evaluating the validity of the test message sent to
the selected one remote from the remote Rx, (d~ responding
to a validly received message by the selected one remo-te
by transmitting a valid-message-received message to the
remote Rx, (e) evaluating the communicati.ons integrity of
the communications link between the remote Rx and the selected
one remote as a function of the valid message--received messages
by the remote Rx, (f) connecting the impedance means to
the communications link when the number of valid-message-
received messages is below a predetermined value, and
-- 5
(g) repeating the steps (b) through (f) for -the other
one of the remotes RX+l or RX_l.
In accordance with these objects, and others,
the present lnvention provides a control system for control-
ling an industrial process including a plurality of remote
process control units R (remotes) connected to various
controlled devices and sensors and communicating with
one another through a communications link having at least
two independent communication channels. Each remote is
assigned a uni~ue succession number or position in a pre-
determined succession order with each remote unit assuming
supervisory communication control of the communications
link on a revolving or master for the moment basis in
accordance with the remote's relative position in the
succession order. Information transfer including process
data and command control information is accomplished bet-
ween a source remote RS and a des-tination remote Rd by
successively transmitti~g two identical informati~ blocks
over each communication channel with the des-tination remote
Rd testing the validity of the blocks on one of the chan-
nels and, if valid, responding with an acknowledgement
signal (ACK), and, if invalid, then testing the validity
of the two blocks received on the oth~r, alternate channel.
An acknowled~ement (ACK) or a non-acknowledgement signal
(NAK) is sent by the destination remote Rd if -the inform-
ation on the alternate channel is found, respectively,
valid or invalid. Each remote in the system is adapted
to test the communications integrity of both
communications channels between it and its immedia-tely
lower order and higher order remote in the succession
order and connect a line-balancing impedance tilereto in
the event that one or both of the communication channels
- 5a -
are interrupted or degraded to the point where reliable
information trancfer can not take place.
The system advantageously maintains a high over-
all operating efficiency in the event of a communications
link interruption since the remote on each side of the
interruption will maintain, to the extent possible,
a system-like integrity with a line-terminating impedance
inserted by the remotes at the appropriate point in the
communications link to prevent or minimize problems
incident to the interruption or degradation of the com-
munications link including loss of signal strength due
to the unbalanced line conditions and reflected signals
that can cause false triggering and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
The above description, as well as the aspects,
features, and advanta~es of the present invention will
be more fully appreciated by reference to the following
detailed description of a presently preferred but nonethe-
less illustrative embodiment in accordance with the present
invention when taken in connect:ion with the accompanying
drawings wherein:
FIG. 1 is a schematic diagram of an exemplary
process control system including a plurality of remote
process control units (remotes), including both primary
control remotes and redundant remotes, connected to a
common, dual-channel communications link;
FIG. 2 is a schematic block diagram of an exem-
plary remote process control unit of the type shown in
FIG. l;
FIG. 3 is a schematic block diagram of an ex-
emplary modulator/demodulator (MODEM) for the remote
process control unit shown in FIG. 2;
- 5b -
FIG. 4 i.s a schematic block diagrarn of an
e~emplary communication pro-tocol controller for the remote
process unit shown in FIG. 2;
FIG. 4A is a schematic block diagram of an
exemplary input/output management device for the remote
process control unit shown in FIG. 2;
FIG. 4B is a flow diagram illustrating the manner
in which the change~in-status even-ts of the controlled
devices of FIG. 1 are detected by the input/output manage-
ment device oE FIG. 4A;
FIG. 5 illustrates the format of an exemplaryor illustlative information block for transferring infor-
mation between remotes;
-- 6 ~
FIG. 5A illustrates the fornat of a header frame
of the inforTnation bloc~ shown in FIG. 5;
FIG. 5B illustxates the format for a data/
information rrame of the info~nation block shown in
FIG. 5i
FIG. 5C illustrates the forrnat for an
acknowledgement block (ACX) for acknowledging
successful receipt of an information bloc~;
~ IG. 5D illustxates the format for a non-
acknowledgement block (NAK) for indica~ing the
unsuccessful trans~ission of an information bloc~ between
remotes;
FIG. 6 illustrates, in pictorial form, two
identical data blocks having the format shown in FIG. 5
successively transmitted on each comrnunication chanrel of
the co~nunication link illustrated in FIG. l;
FIG. 7 is a flow diagram summa~y of the manner in
which a source and a destination remote effect comm~mi-
cations with one another;
FIG. 8A is 2 partial flo~- diagram illustrating
in detail the manner in which a source and a destination
remote communicate and validate info~nation transfexred
between one another;
FIG. 8B is a partial flow diagram which com-
pletes the rlow diagram of FIG. 8A and illustrates in
detail the rnanner in which a source and a destination
remote communicate and validate infoxmation transferred
between one another;
FIG. 9 is a legend lllustrating the manner in
the flow diagrams of FIG. 8A and FIG~ 8B are to be read;
FIGS. lOA through lOF are exemplary tables
illustrating the manner in which supervisory control of
the communication link is transferred from remote to remote;
FIG. 11 is a schematic block diagram of an
exemplary redundant remote that is adapted to assume control
from a failed or otherwise inoperative primary remote;
FIGS. llA and llB are flow diagrams of the manner
in which the central processing unit of the redundant
remote R4 monitors the operating condition of its assigned
primary remotes Rl, R2, and R3 and takes over operation when
one of the primary remotes fails;
FIG. 12 is a flow diagram summary of the manner
by which an interrogating remote Rx tests the integrity
of the communication link between it and the remotes Rx 1
and RX+l immediately adjacent thereto in the succession
order;
FIG. 12A is a partial flow diagram illustrating
in detail the manner by which an interrogating remote Rx
tests the communications integrity of the communications
link between it and the next lower number remote Rx 1 in
the succession order;
FIG. 12B is a partial flow diagram illustrating
in detail the manner in which an interrogating remote ~x
tests the communiations integrity of the communications
link between it and the next higher number remote ~x~l in
the succession order;
FIG. 12C is a partial flow diagram illustrating
in detail the manner by whioh a line termination impedance
is applied to the communications link in the event of a
communications link degradatlon or interruption
- 8 ~
FIG. 13 is a legend illustrating the manner in
which the flow diagrams of FIGS. 12A, 12B, and 12C are to
be read; and
FIG~ 14 is an exemplary table illustrating the
status of various counters when an interro~ating remote Rx
is evaluating the integrity of the communications link in
accordance wi~h th~ flow diagram shown in FIG. 12A.
DESCRIPTION OF THE PREFERRED EMBODI~E21T
_
An industrial control system in accordance with the
present invention is shown in schematic form in FIG. 1 and
includes a communications link CL (C-link) having a plurality
of remotely located process control units (remotes) Rl,
R2,...R7, R~ connected thereto wikh the eight remotes
(Rl-R8) shown being exemplary; it being understood that the
system is designed to be used with a much larger number of
remotes. Of -the eight remotes illustrated, the remotes Rl-R3
and R5-R7 are 'primary' remotes and the remotes R4 and R8 are
'redundant' remotes. The communications link CL is shown
as an open line, double channel configuration formed from
dual coax, dual twisted pair, or the like with the
individual communication links identified, respectively,
by the reference characters CL0 and CLl. While the
system configuration shown in FIG. 1 is a distrib~ted open
loop or shaxed global bus t~pe, the invention is equally
suitable for application to central systems or central/
distributed hybrid configurations. The system of FIÇ. 1
is adapted for use in controlling an industrial process,
e.g., the operation of a power generating plant, with each
primary remote unit Rl-R3 and R5-R~ connected ~o one or more
associated or corresponding input/output devices I/01-
I/03 and I/05-I/07, respectively. Each input/output
device is, in turn, connected to an associated controlled
device CDl-CD3 and CD5-CD1 (of which only CD6 and CD7 are
~2~
illustrated in FIG. 1) such as, but not llmited to, various
types of sensors (t~mperature, pressure, position, and motion
sensors, etc.) and various types of actuators (motors,
pumps, compressors, valves, solenoids, and relays, etc.).
Each primary remote may control a large number of output
devices and respond to a large number of input devices, and
the blocks labeled I/O in FIG. 1 can each represent many
input and output devices.
The redundant remote R~ monitors the operation of
primary remotes Rl, R2, and R3; and the redundant remote
R8 monitors the operation of primary remotes R5, R6,
and R7. Should any one of the remotes Rl R2, and R3
fail, -the failure will be detected by the remote R4 in
a manner to be described and the remote R4 will take over
control of the input and output devices of the failed remote
by receiving the data from the failed remote over the
communications link CL and sending commands to the failed
remote over the communications link CI. in formated information
blocks. Similarly, if one of the remotes R5, R6, or R7 fails,
the redundant remote R8 will take over control of the operation
of the input/output devices for the failed remote as described
above ~ith respect to redundant remote R4. Although only eight
remotes have been shown in Figure 1, any number of remotes
Rl, R2, R3, ...... Rn 1~ Rn could be utilized in a particular
system.
The architecture of an exemplary remote Rn is
shown in FIG. 2. While the architecture of the remote
Rn can vary depending upon the control process require-
ments, the remote shown in FIG. 2 includes a modem 10; a
communication protocol controller 12; an input~output
management device 14; a central processin~ unit lCPU) 16;
~ 10 -
5~7
a memory 18; a peripheral device 20 that can include,
e.g., a CRT display, a printer, or a keyboardi and a
common bus 22 which provides addressing, control, and
information transfer between the various devices which
constitute the remote. The devices shown in dotted line
illustration in FIG. 2 (that is, the central processing
unit 16, the memory 18, and the peripheral device 20)
are provided depending upon the process control require-
ments for the remote Rn. For example, in those primary
remotes Rn which function as an elemental wire replacer,
only the modem lO, the communication protocol controller
12, and the input/output management device 14 are pro-
vided. In more complex process control requirements, an
appropriately programmed central processing unit 16 and
associated memory 18 are provided to effect active con-
trol according to a resident firmware program. In still
other remotes requiring a human interface, the appropriate
peripheral device(s) 20 may be connected to the common buss
22.
As shown in more detail in FIG. 3, the modem lO
provides two independent communication channels CH~ and
CHl connected, respectively, to the communication links
CL0 and CL1. Each of the communication channels CH~
~nd CHl is provided with substantially identical communi-
cation devices, and a description of the communication
devices of the first communication channel CH0 is
sufficient to provide an understanding of the second
communication channel CHl. The communication channel
CH~ includes an encoder/decoder 24~ for providing appropriate
modulation and demodulation of the digital data trans-
mitted to and received from the communication link CL~.
In the preferred form, the encoder/decoder 24~ converts
digital information in non-return-to-æero binary (NRZ)
format to base-band modulation (BB.~) signal format for
transmission and effects the converse for reception.
~mplifiers 260 and 280 are provided, respectively, to drive
a passi~e coupling transformer T0 with digital information
provided from the encoder/decoder 24~ from the coupling
transformer T0. A set of selectively operable relay
contacts 30~ are provided between the coupling transformer
T~ and the corresponding communication link CL0 to effect
selective interruption thereof to isolate the remote Rn
from the communications link CL, and another set of relay
contacts 320 are provided to selectively connect the signal
output of the coupling transformex T~ with a termination
impedance Z~. The termination impedance Z~ is used when
the particular remote Rn is at the end of the communication
link CL to provide proper line termination impedance for
the link, or, as described in more detail below, to assist
in terminating an open or degraded portion of the communi-
cations link CL.
A selectively operable ].oop-back circuit 34 is
provided to permit looping back or recirculation of test
data during diagnostic checking of the remote Rn. While
not specifically shown in FIG. 3, the loop-back circuit
34 can take the form of a double pole, single throw relay
that effects connection between the channels CH~ and CHl
in response to a loop-back command signal 'LB'. During
the diagnostic checking of a remote, which checking takes
place when a ~articular remote is a master-for-the-moment
as explained below, the relay contact~ of the loop-back
12 -
25~
circuit 34 are closed and a predetermined test word is sent
from the channel CH~ to the channel CHl and from the channel
CHl to the channel CH0 with the received word in each case
being checked against the original test word to verify the
transmlt/receive integrity of the particular remote.
The isolation relays 300 and 31l, the
impedance termination relays 32~ and 321, and the loop-back
circuit 34 are connected to and selectively controlled by
a communications link control device 38 which receives its
communication and control signals from the communications
protrocol controller 12 described more fully below. A
watch-do~ timer 40 is provided to cause the C-link control
device 38 to operate the isolation relays 30~ and 30l to
disconnect the remote Rn from the communication link CL in
the event the timer 40 times-out. The timer 40 is
normally prevented from timing ol~t by periodic reset
signals provided from the communication protocol controller
12. In this way, a remote Rn is automatically disconnected
from the communication link CL in the event of a failllre
of its communication protocol controller 12.
As shown in more detail in FIG. 4, each communi-
cation protocol controller 12 includes input/output ports
42, 44, and 46 which interface with the above described
modem lO for the communication channels CH~ and CHl and the
modem C-link control device 38 (FIG. 3). A first-in first-
out (FIFO) serializer 48 and another first-in first-out
5~
serializer 50 are connected between the input/output
ports 42 and 44 and a CPU signal processor 52. The
first-in first-out serializers 48 and 50 function as
temporary stores for storing information blocks
provided to and from the modems 10 as described more
fully below. The CPU 52, in turn, interfaces with the
buss 22 through buss control latches 54. A read only
memory (ROM) 56 containing a resident firmware program
for the CP~ 52 and a random access memory (RAM) 58 are
provided to permit the CPU 52 to effect its communication
protocol function as described more fu].ly below. Timers
62 and a register Ç0 (for example, a manually operable
DIP switch register or a hardwired jumper-type register)
that includes registers 60a and 60b are also provided to
assist the CPU 52 in performing its communication proto-
col operation. An excess transmission detector 64,
connected to input/output ports 42 and 44 (corresponding
to communication channels CH~ and CHl) determines when
the transmission period is in excess of a predetermined
limit to cause the C-link control device 38 (FIG. 3) to
disconnect the transmitting remote from the communications
link CL and thereby prevent a remote that is trapped in
a transmission mode from monopolizing the communications
link CL.
The input/output management device 14, the
architecture o~ which is shown in FIG. 4A, is preferably
a firm~are controlled microprocessor-based device which
- 14 -
567
is adapted to scan the various input/output hardware points
of the controlled device, effect a point-by-point status
comparison with a prior scan, and record the change-in-
status events along with the direction of the change and
the time the event occurred (time-tagging), effect data
collection and distrlbution to and from the input/output
points, format the collected data in preferred patterns,
and assemble the patterned data in selected seauences.
As shown in FIG. 4A, the input/output
management device 14 includes a processor 14A connected
to the remote buss 22 through a processor buss 14B; read-
only-memories 14C and 14D connected to the processor 14A
through appropriate connections with these memories in-
cluding the firmware necessary to effect the above-
described functions of the input/output management device
14 including the change-in-status event monitoring
(described in more detail below); a read/write memory
14E (RAM) for temporarily storing information incident to
the operation of the processor 14A including the change-
in-status event in~ormation; a time base 14F for providing
time information for time tagging the change-in-status
events; and an input/output interface 14G for connection,
either directly or indirectly, to the controlled devices.
In the preferred embodiment, the input/output
interface 14G is defined by one or more printed circuit
control cards generally arranged in rack formation with each
card having hardware points arranged in predetermined sets
of eight points with each hardware point carrying a binary
~L~8~
indication for controlling or sensing the operation of the
controlled device. The control and operational status of the
controlled device can generally be represented by one or
more eight-bit words (e.g., 00010001) with each bit position
representing a control or operational characteristic of the
controlled device.
As described in fl~rther detail below in connection
with FIG. 4B, the input/output management device 14 effects
the aforedescrihed change-in-status monitoring and associated
time-tag~ing by periodically scanning the input/output hard-
ware points in eight-bit groups and effecting a comparison
between the so-obtained eight-bit group and the eight-bit group
obtained during the previous scan. If a change is detected
in one or more of the bit positions, the latest eight-bit
group, along with the time-of-day information obtained from
the time base 14F, and other information, if desired,
representing the dixection of change, is placed in a
first-in first-discard memory (FIFO) of predetermined
size. Thus, each change-of-s~atus event along with its
time tag and other information such as direction of
change, etc. is placed in a memory of selected size as
the changes occur. When all the memo~y locations are
filled, the first entered event (which now represents
the oldest chronological event) is discarded as the
latest event enters the memory. The memory loading is
inhibited by the occurxence of any one of a selected
number of inhibit signals. In the system, various con-
ditions including alarm conditions which represent partial
or full system failures can be assigned a priority with
- 16 -
~8~
those conditions or con~lnations thereof designated as
"high" priority signals being permitted to disable or
inhibit further accessing of the memory. In the event
one of these high priority conditions occurs, the memory is
inhibited from storing additional change-in-status
information and the change-in-status events occurring prior
to the high priority condition are preserved for
subsequent analysis. Alarm conditions which are not
designated as high priority, of course, do not inhibit the
memory. This technique advantageously differs from those
prior techniques in which the controlled device status was only
placed in memory at the moment of a high priority signal
(in which case a historical pre-failure record-of-events
was not available) or those techniques in which the change-
in-status events were logged in a memory which was
periodically cleared, refilled, and cleared in which case
the probability of obtaining a complete history of events
prior to a predetermined high priority condition diminished
in those instances in which the logging memory was cleared
~ just prior to the occurrence of the high priority condition.
The manner by which the input/output management
device 14 effects the change-of-status event logging is shown
in FIG. 4B. During initialization, the processor 14s (referred
to~ also as the RTZ in FIG. 4B) moves an image of the various
input/output points, that is, the current status of the
various input/output hardware points, to preassigned locations
in the memory 14E (local) of the input/output management
device 14 and the memory 18 (system) of the remote Rn (FIG. 2).
Thereafter, the address(s) of the first input/outpuk card is
obtained and the input/output hardware points for that card
are scanned to obtain an inpu-t/output i~age which takes the
form of an eight-bit word ~e.g., 00000000) with each bit
position representing the control or operational status of the
controlled device. The input/output points so obtained
are then compared with the previously obtained image of the
points (e.g., 00100000), for example, by effecting a bit-
by-bit exclusive OR (XOR) comparison. If the comparison
indicates no change in status, (that is, the words are
identical) the input/output points in the remaining cards
are likewise scanned with the process repeated on a
cyclic or looped basis. However, if a change is detected in
the exclusive OR comparison, that new input/output scan,
along with the time tag information and the direction of
change is placed in the memory 18 of the remote Rn, and,
in addition, the latest scan is moved to the memroy 14E
of the input/output management device. This process continues
with each new change-in status event loaded into the memory
18 of the remote on a first-in first-discarded basis. The
first-in first discard memory may be configured by assiyning
a preselected number of memory locations in the memory 18
of the remote Rn (e.g., flfty locations) for the logging
information and providing an address pointer that points
to each successive location in a serial manner with the
pointer returning to the first location arter pointing at
the last available pre-assigned location in the memory.
In the preferred embodiment, the processor
14A of the input/output management device 14 (FIG. 4A)
and the processor 52 (FIG. 4) of the communication
protocol controller 12 is 8X300 micro-controller
manufactured by the Signetics Company of Sunnyvale,
- 18 -
~alifornia, and the central proces~ing unit 16 (FIG. 2)
is an 86/12 single board 16-bit micro-computer manu-
factured by the ;ntel Company of S~nta Clara, California
and adapted to and configured for the Intel MULTIBUSIM
Each remote Rn is adapted to communicate with
~he other by transitting diyital data organized in pre-
determined b1Qck formats. A ~u~ta~1e and illustrative
block format 66 is shown in FIG. S and include~ a multi-
word header frame 66A, a multi-word data fr~me 66B, and a
block termination frame or word 66C. Sel~cted of the
informa~ion block configuration~ aro adapted to transfer
process control information to and from salected remote
units Rn and other of th~ b10ck configuration~ are adapted
to transfer super~isory control of the communications link
CL from one remote to the other remote as explalned in
greater detail helow.
An exemplary torm~t for the header and data
fram~s of an information block 66 i5 shown, rèspectively,
in FIGS. 5A and SB. The headcr frame 66A preferab1y
2~ includes a 'start of heade~' word(~) that indica~es to
all remotes that informaton is being tran~mitt d; a 'source'
identification word (8) that ind~c~t~ ~h~ id ntity of the
source remote Rs that i~ tran~erring the infor~tion;
'destination' wordts) that indicat~ th~ identiy of th~
receiving or destination r~mot~ Rd; a 'h~adcr-typ~' word(~)
th~l~ indicates whethar th~ data block is tran3fer~ing dat~,
a parame~ered command block, or a paramcter1eY~ comm~nd block;
'block-type' word indicating the type of block ~that is, a
co.~mand block or a data block)s a 'block numher' word that
~ 19 -
indicates the number of blocks being sent; a 'block size'
word indicating the length of the data framei a 'security
code' word(s) that permits alteration of the resident soft-
ware programming in a remote; and, finally, a two-byte
'cyclic redundancy code' (CRC) validity word. The data
frame for each data block, as shown in FI5. SB, can in-
clude a plurality of data carrying bytes or words B1,
B2~...Bn of variable length terminated with a two-byte
cyclic red~ndancy code word. As described moxe fully
below, each of the remotes is adapted to acknowledge (ACK)
successful receipt of da~a and command blocks and non-
acknowledge (N~C~ the receipt of data in which a trans-
mission error is detected. When transmit~ing an
acknowledgement block or a non-acknowledgement block, the
header format used is shown in FIGS. 5C and 5D in which an
acknowledgement (ACK~ or non-acknowledgement (NAK) word
occupies the 'block type' word position. The block
~ormats disclosed above are intended to be illustrative
only and not limiting.
The various remots units Rl, R2~ R3,.. Rn communi-
cate with one anokher by having each remote successlvely
take contxol of the communications link CL and the controlli~g
remote Rs then sending digital information between itself
and a destination remote Rd using a double transmission
alternate line technique that provides for high
reliability data transfer between remotes even when one of
the two communication li~ks C~ or C~l is inoperative, for
example, when one of the two communication cables is
severed or otherwise degraded as occassionally occurs in
harsh industrial environments.
- 20 -
When a remote unit assumes control of the communi-
cation link GL (as explained more fully below) and, as a
source remote Rs, desires to send data blocks to another,
destination remote Rd, the data block is assembled at the
source remote Rs.in accordance with the block formats
discussed abo~e in connection with FIGS. 5-5D and trans-
mitted through ~he information channels CL0 and CLl of the
source remote Rs to the communication links CL~ and CL1
wi~h the header frame containing both the source remote
Rs and the destination remote Rd identi~ication information.
In accordance with the data transmission
technique, the communication protocol controller 12
of the source remo e Rs transmits the information
blocks twice on each communication link CL~ and CL1
as schematically illustrated in FIG. 6 to provide a
first data block DBA and then a second, following data
block DBB on each communication link CL~ and CLl.
The transmitted infonmation block headers include the
identity of th~ destination remote, Rd, which causes the
destination remote Rd to receive and act upon the
information blocks. At the des~ination remote Rd, the
two data blocks DBA~ and DBB~ on the communicatlon link
C~0 are passed through the communication channel CH0
and the two data blocks DBAl and DBBl on the communication
link CLl are passed through the communication channel C~1
to, respectively, the first-in firs~-ou~ serialiæ~rs
48 and SO (FIG~ 4).
A5 shown in the summary flow diagram of FIG. 7,
the destingation remote Rd checks the validity of the
reseived data by selecting one of the two communication
links (e.g., CL0 in FIG. 7) and then checks the first
- 21 -
2~7
data block on the selected line (that is, DBA~) by
performing a cyclic redundancy check of the header frame
and, if valid, performing a cyclic redundancy check of the
data frame. If the data frame is valid, the communi-
cation protocol controller 12 of the destination remote
Rd then performs a bit-for-bit comparision between the
CRC-valid first data block DBA~ and the second following data
block DB~. If the bit-for-bit comparision is good, an
acknowledgement (ACX) signal is sent from the destination
remote Rd to the source remote Rs to indicate the receipt
of valid information and complete that data ~lock
information transaction. On ~he other hand, if the CRC
validity checks of the header or the data rame or the
bit-for-bit comparison check indicate invalid data, the
protocol controller 12 of the destination remote Rd then
selects the other, alternate line (in this case, CLl)
and performs the aforementioned cyclic redundancy checks
of the header and data frame and the bit-for-bit comparison
between the flrst and second data blocks DBAl and DB
on the alternate line CLl. I~ these checks indicate
valid data on the alternate line, the destinat1on remote
Rd responds with an acknowledgement signal (ACK) to
conclud~ the data block transmission transaction. On
the other hand, if ths~e checks indicate invalid data
on the alternate line (which mean~ that tha data ~locks
on bo~h the first-selected lin~ and the alternate line
are in~alid) the destination remote Rd responds with a
non acknowledgement signal (NAK) to cause retransmission
of the data blocks from the source remote Rs. The non-
acknowledgement block (NAK) includes a byte or bytes
- 22 -
indicating the identity of the data block or blocks
which should be retransmitted. A counter (not shown) is
~rovided that counts the number of retransmissions from the
source remote Rs and, after a fini~e number of re-
transmissions (e.g., four), halts further retransmission
to assure that a source remote Rs and a destination remote
Rd do not become lost in a xepetitive transmit/NAK/re-
transmit/NAK... sequence in the avent of a hardware or
software failure of the destination remote Rd error checking
mechanism.
The double message alternate line checking
sequence summarized in FIG. 7 may be more fully appreciated
by referring to the detailed flow diagram shown in FIGS. 8A
and 8B (as read i~ accordance with the flow diagram map of
FIG. 9). At the start of the information validity
checking procedure, the 'line ~-first' flag register is
checked; if a flag is present, the 'first-attempt fail'
flag register is checked, and, if there is no flag in thIs
register, the two data blocks DBAl and DBBl on channel C~l
are stored while the tWQ data blocks D~A~ and DB~0 on channel
CH~ are used for the first attempt information check.
Thereafter, the header frame of the first data block DBA~
on channel CH~ undergoes a CRC check, and, if acceptable,
the data frame of this data block DBA0 undergoes a CRC check.
If the header and data frames CRC checks indicate valid data
a 'good message' register is incremented. If the number of
good messages is less than two, the error checking procedure
returns o the initial part of the 10w diagram and, after
- 23 -
~25~97
dPtermining there is no channel CH0 first flag or first-
attempt flag present, checks the second following data
block DBB~ by repeating the header and data CRC cyclic
redundancy checks. If the header and data frames pass the
CRC checks, the 'good messagel register is incremented
again to indicate that a total of two messages in succession
(that is, DBA~ and DBB0) have passed the cyclic redundancy check
for the header and data frames. Thereafter, the two data blocks
DBA~ and DBB~ received on line CH~ are checked by performing
a bit-by-bit comparision between the two. If the data blocks
DBA~ and DBB~ pass the bit-by-bit comparision test, the communi-
cations protocol controller 12 of the destination remote Rd
sends an acknowledgement (ACR) message to the source remote
~5 to conclude the information block trans~er and resets the
various registexs. If, on the other hand, either the data
block DBA~ or DBB~ on line CL0 fail the header and data frame
C~C checks or these two data blocks fail the bit-by-bit
comparison check, the communication protrocol controller 12
sets ~he 'first-at~emp~ fail' flag and returns to the start
of the procedure ~o de~ermine tha~ the 'line ~ first' flag
a~d the 'first~attempt' fail flag are present. The communi-
cation protocol controller 12 then uses the stored da~a blocks
DBAl and DBBl from line CLl (which data blocks were previously
stvred in FIFO 50). The header block and data block of
the data blocks DBAl and DBBl from line C~l undergo the CRC
check and, if successful, cause the incrementing of the 'good
- 2~ -
message' register to cause the ccmmunication protocol
controller 12 to then check the validity of the second
data block DBBl. If the data blocks DBAl and DBBl pass the
CRC checks, they are compared with one another in a bit-
by-bit comparison test and if this comparison check is
successful, an acknowledgement ~ACK) is sent. If, on the
other hand, either data block DBAl or DBBl does not pass the
CRC checX or the data blocks do not pass the bit-by-bit
comparison test, a non-acknowledgement (NAK) is sent to the
souxce remote Rs including information requesting the
retxansmission of the data blocks which failed the validity
test at the destination remote Rd. The source remote RS then
retransmits the improperly received information blocks as
described above with retransmission limited to a finite number.
A register is provided for each of the communication links for
recording, in a cumulativa manner, the number of time~ an
invalid message is received for each communication link. In
this manner, it can be determined, on a statistical basis,
whether one of the two communication links has suffered a
de~erioration in signal transmission capability and, o course,
whether one of the communication links is severed.
As can be appreciated, the dual transmission of the
identical messages on plural communication links vastly
enhances ~he ability of the destination remote Rd to detect
errors and determine whether the information being.transmitted
is valid or not. In addition, the destination remote Rd is
able ~o operate and successfully receive messages even if one
of the communication links CL~ or CLl is severed since the
communication protocol controller 12 at the destination Rd
- 25 -
will examine the received signals on each line and will find
invalid data on the severed line, but will always examine
the data blocks on the other line and, if necessary, request
retransmission of the lnformation blocksO
In selecting one of the two channels CH~ or CHl for
the ~lrst validity check, it is preferred that one of the two
channels (e.g., CH~) be selected for the first check on every
other information transaction and that the other of the two
channels (e~g., C~l) be selected for the first check for the
other intermediate information transactions. While thP system
has been disclosed as having dual communication links CL~ and
CLl f ~he invention is not so limited and can encompass more
than two communication links with the remotes adapted to
sequentially examine signals received on the various channels.
As mentioned above, each remote Rn ~ the control
system is adapted to accept and then relinquish supervisory
control of the communication link CL on a master-for-the-
moment or revolving master arrangement. The communication
protocol controller 12 of each remote Rn includes a register
which contains the remote succession number, another register
which contains the total number of remotes in the system, and
another register which contains the relative position of the
remote from ~he present system master. The first two registers
are schematically illustrated by th2 reference character 60 in
~IG. 4. In addition, each remote Rn includes a variable transfer~
monitor timex having a time-out interval that is set in accordance
with a predetermined control-transfer time constant (50 micro-
seconds in the preferred embodiment) and the positioll or the
- 26 -
particular remote relative to the present system master
to permit, as explained in more detail below, the master-
for~the-moment transfer to continue even in the event of
a disabled remote (that is, a remote that is unable to
accept supervisory control because of a malfunction).
Another timer is provided to force transfer of supervisory
control of the communications link CL in the event a
remote, because of a malfunction, is unable to transfer
supervisory control to its next successive remote. The
operation of the master-for-the-moment transfer technique
can be appreciated bv consideration of the following
example of an illustrative system that includes five
remotes arransed in the open loop configuration of FIG. 1
and transferring supervisory control of the communications
link CL in accordance with the tables of FIGS. lOA-lOF. The
upper row of each table indicates the succession se~uence
or order of the five remotes Ro~ Rl, R2, R3 and R4 that
comprise the system; the intermediate row identifies the
remote that is the present master-for-the moment an~ also
identifies the relative successive position of the other
remotes from the present master, tha~ is, the first (or
next) successive remote from the present master, the second
successive remote from the pxesent master, the third remote
from the present master, etc.; and the third row of each
table lists the setting of the variable transfer-monitor
timer for the particular remote.
~ 27 -
The system is provided with initialiæation
software so that the first remote in the succession, Ro~
assumes supervisory control of the communication link
CL after system start-up and becomes the initial master
of the system (FIG. lOA). When the initial master Ro
is in control of the communications link CL, it can send
data to any of the other remotes, request status or
other data from another remote, and send control blocks and
the like over the communications link CL. When the master
Ro determines that it no longer desires possession o the
commu~ications link CL, it passes supervisory control of
the communications link CL to the next or first successive
remote in accordance with the succession order~ Thus, when
the present master Ro concludes its information transfer
transactions, it transfers supervisory contxol of the
communications link CL to its next or first successive
remote Rl by transmitting a control block to the remote R
with all the remaining remotes (that is, R2, R3, R4)
being cognizant of the transfer o~ supervisory control
from the present master Ro to its first or next succ~ssive
remote Rl. Since, in the present system, the transfer o
supervisory control of the communications linX CL is
expected to take plaoe within 50 micro seconds, the
second successive remote R2, as shown in the third row of
the table of FIG. lOB, sets its variable transfer monitor
timer to 50 micro-seconds, the third successive remot~ R3 sets
its variable transfer-monitor timer to 100 micro~seconds,
- 28 -
and the fourth successive remote R4 sets it transfer-
monitox timer to 150 micro-seconds. ~en the first
successive remote Rl receives the control block from the
present master Ror it accepts supervisory control of
the communications link CL by responding with an
acknowledgement message (ACX). If the control block
is misreceived, the first successive remote Rl can
respond with a non-acknowledgement (NAK) to request
retransmission of the control block transferring
supervisory control of the communications link C~. During
the time interval that the present master remote Ro is
attempting to transfer supervisory control of the communi-
cation link CL to its next successive remote Rl, the
transfer-monitor timers of the remaining remotes are
counting down. If, for any reason, the next or first
successive remote Rl ~ails to take control (e.g., a
malfunction of the xemote), the transfer-monitor timer
of the second successive remote R2 will time-out at 50 micro
secor.ds and cause the second successive remote R2 to then
accept supervisory control of the communication link CL
fxom the present master Ro and thus bypass the apparently
malfunctioning ~irst successive remote Rl.
Aassuming that the initial system master Ro
successively transfers supervisory control of the com~uni-
catins link CL to its first successive remote Rl~ that
successive remote Rl then becomes the present master with the
remaining remotes changing their position relative to the
present master and setting their transfer-moni~or timers
in accordance with the second and third rows o the table
of FIGo lOB. When the present master Rl concludes its
_ zg _
~il2~q67
information transfer transactions, if any, it attempts ~o
transfex supervisory control to its first or next successive
remote R2 by sending an appropriate control block to remote
R2 which responds with an acknowledgement signal (ACK) or,
in the event of a mistransmission of the control block, a
non-acknowledgement signal (NAK) which causes re
transmission of the control block. When the control block
requesting transfer of supervisory control of the communi-
cation link CL is sent from the present master Rl to its
next successive remote R2, all the remaining remotes reset
their transfer-monitor timers in accordance with their
position relative to the present remote as shown in the third
row of the table of FIG. lOC. Should the next successive
remote R~ be unable to accept supervisory control of
the communication link CL from the present master Rl,
the transfer-monitor timer of the second successive xemote
R3 will time-out in 50 micro-seconds and cause the second
successive remote R3 to assume supervisory control of the
communiations link CL to thereby bypass an apparently
malfunctioning first successive remote R~. A~ can be
appreciated from a review of the transfer-monitor time-out
settings of the various remotes, supervisory control of the
communicatiorls link CL will transfer even if one or more
successive remotes are malfunctioning, when the trans~er
monitor timer of the next operable remote times out. This
transfer sequence continues in succession as shown in the
remaining tables of ~IGS. lOD to lOF with supervisory
control of the communication link CL being passed from
remote to remote in succession with the last remcte R4
retuxning supervisory control to the first remote Ro~
- 30 -
By employing a master-for-the-moment transfer
technique in which the recelving remote acknowle~gPs
control from the transferring remote and in which re-
transmission of a mis-received control block is provided
for in re~ponse to a non-ack*owledgement signal from the
re~eiving remote, it is possi~le to positively tran~Pr
' supervisory control of the communication lin~, This
technique advantageously transfers co~trol u~ing the
data and i~formation carrylng communlcation link rather
th~n, as in other systems, by providing separatQ communl
cation lines or channels dedicated solely to ~upervisory
control transfer function~. Also, the provi~ion o~ a
variable ~ranser-monitor timer at each remote that is set
in accordance with the remote's relative position to the
present ma~ter and a trans~er time~constant automatically
transfers supervisory control o~ the co~unication~ l.ink
even if one or more of the succe~siva remotes are mal~
functioiling .
The architecture of a redundant remote (R ~ and
~8 in FXG. 1), as shown in ~IG. 1~, i3 es~entially the 5am(?,
as that o a primary remote except that it has no i.nput/
output devices as~igned to it. Each redundant xemo~e
functions to take over eontrol responsibility o a con~rolled
device from a primary remote in tha event the primary
remote malfunctions.
~ 3~ -
In each primary remote, preassigned memory
locations are designated to act as a 'mailbox' register
for that remote. Each time th~ eentral processing unit
16 or the primary remote cycles through its applications
program, in which it responds to and controls the input/
output devices of the remote via the input/output management
device 14, it stores a predetermlned number in its mailbox.
Each time the processor 14A of the input/output management
device 14 cycles through its program, it decrements the
number stored in the mailbox. The time for the CPU 16
to cycle through its program and for the input/output
management device 14 to cycle through its program ls
approximately 1:1 so that the number stored in the mailbox
will be maintained at or near the predetermined value set
by the applications program of the CPU 16 unless the
CPU 16 caases to cycle through its applications program.
Should this happen, the number stored in the mailbox memory
18 will be decremented by the input/output management
device 14 until it reaches a zero value.
Each time a redundant remote which is serving
as a back-up for its associated primary remotes takes its
turn in the master-for-the-mome~t sequence described above,
the redundant remote will request and obtain the value of
the number in the mailbox of its assigned primary remotes.
If the number in the mailbox is not zero, the redundant remote
will know that the centxal processing unit 16 in the 50-
queried primary remote is carrying out its applications
program and has not gone into an emergency mode of operatlon
or otherwise ceased to operateO If the redundant remote
- 32 -
detects that the number in the mailbox for one of its
assigned primary remotes is zero, then the redundant remote
will determine that ~he central processing unit 16 of the
zero-mailbox remote is not carrying out the applications
program and, in response to this determination, the redundant
remote will first attempt to restart the applications program
in the central processing unit 16 of the prima~y remote. If it
fails to successfully restart the applications program, the
redundant remote will carry out the applications program
for the failed remote. In carrying out the applications
program, the redundant remote will respond to the input
devices and control the output devices assigned to the
~ailed primary remote by sending commands and receiving
data from the failed remote over the com~unications link CL.
The redundant remote, in addition to checking the
status of its assigned primary remotes for which the
redundant remote serves as a back-up, also must maintain
an up-to-date record of the status of the applicakions
program in each of these assigned primary remotes. The
redundant remote checks the status of the mailbox and gets
the current applications program status from each of the
primary remotes by sending requests for information over the
communications link CL when the redt~dant remote takes its
turn in the master-for-the-moment sequence as described
above.
- 33 -
~B~
The operation of the redundant remote in carr~ing
out its function as a back-up for the primary remotes will
be more fully undexstood with reference to FIGS. llA and llB
which illustrate a flow chart of the program in the redundant
remote R4 (FIG. 1), which serves as a back up for its assigned
primary remotes Rl, R2~ and R3. The other redundant remote
R8 will have the same program except that it will be
applied to its assigned remotes R5, R~, and R7.
As shown in FIGS. llA, after the program in the
redundant remote R4 is started, it enters into a decision
instruction sequence lOl to check the status of remote
Rl. As explained above, it does this by sending a request
~or information over the communications link CL to remote
Rl asking for the current number in the mailbox of remote
Rl. It then determines whether this number is greater than
zero. If the number is greater than zero, the status of
remote Rl is determined to be opera~ing and the program of
the redundant remote R4 advances to instruction step 103
in which it resets a fail 1~g for Rl to 'off' and then enters
subroutine 105, in which the cuxrent applications program
status in remote Rl is obtained. This means that the
redundant remote R4 requests and obtains the current status
of the input and output devices in remote Rl and the current
status of the timers and the counters and the flags being
used in the applications program of remote Rl. In other
.
- 34 -
2~
words, in subroutine 105, all of the info~nation tha'c
would be needed f~r the redundant remote R4 to take over
the applications progr~m is obtained from remote R1
This informatlon is obtained by sending requests for
data and receiving data bac~ over the communications
link CL.
Following the obtaining of the current appli~
cAtions program status of remote Rl, the redundant remote
R4 program proceeds to decision instruction sequence 107,
in which the status of remote R2 is checked in the same
manner that was done with respect to Rl. If the status
of remote R~ is operating, the program ad~ances to
instruction step 109, in which the program sets a fail
flag for remote R2 and then proceeds into subroutine 111,
in which the status of the applications program for
remote R2 is obtained in the same manner as for Rl in sub-
routine 105. The program then proceeds into a decision
instruction sequence 113 to check the status of remote
R3. If the status of remote R3 is operating, then the
program resets the ~ail flag for remote R3 in instruction
step 115 and proceeds into subroukine 117 to obtain the
applications program status for remote R3 in the same manner
as Lor Rl in subroutine 105O Following subroutine 111, the
program returns agaln to decision instruction sequence 101
to check the status of remote Rl and the process cyclically
repeats.
If in decision instruction sequence 101, the
program determines that the status Rl is not operating as
indicated by the number in the mailbox of the remo~e Rl,
being zero, the program then advances to decision instruction
sequence 119, in which the program determines if the fail
flag for Rl is 'on' or 'off'. If the fail flag is iof', the
- 35 -
5~
program proceeds into instruction sequence 121, in
~hich the program attempts to restart the applications
program for remote Rl. It does this by sending a command
over the communications link CL to remote Rl to direct
the communications protocol controller 12 (FIG. 2) to
attempt a hardware restart of the applications pxogram.
This is carried ou~ by the communications protocol controller
1~ pulling a restart wire to ground in the common buss
22. When this restart wire is pulled to ground, it starts
the applications program back thro~gh its initialization
program and sets all of t~e flags, timers, and counters
just as if power had been turned on. Such a restart
is called a har~ware restart. Alternatively, the
redundant remote R4 could efect a sof~ware restart in
the failed remote. A software restart would merely start
the applications program through its initialization program
with the timers, counters and flags left in their present
status.
Ater completing instruction sequence 121,
the redundant remote R4 program then sets the fail flag
for remote Rl to 'on' in instruction step 123 and then
proceeds into decision instruction sequence 125 to again
check the status of remote Rl by checking the number in
the mailbox of remote Rl in the same manner as in decision
instruction sequence lOL. If the applications program
in remote Rl was successfully started in instruction
sequence 121, the num~er in the mailbox will not be zero
and the program will detenmine that the status of remote
Rl is operating, whereupon the pxogram will jump to
decision instruction sequence 107 ~o ch~ck the status of
remote R2 as already described.
- 3~ -
~%~
If the program determines khat the status.
of remote Rl is not operating in decision instruction
sequence 125, then ~hls means that the attempt to restart
the applications program in remote Rl in instruction
sequence 121 failed and the redundant remote R4 program
then proceeds into instruction sequence 127 to initialize
the input/output management device 14 (also identified
in FIG. llB as ' RTX ' ) in remote Rl to receive instructions
and data from the redundant remote R4 instead of from the
central processing unit 16 in the remote Rl and to send
data on the status of the input and output devices to the
redundant remote R4.
If the. program of the red~ndant remote R~
dete~nines that the fail flag was 'on' instead o~ 'off' in
decision instruction sequence 119, the redundant remote
program would proceed dixectly into the instruction
sequence 127 to initialize the input/output management
device 14 of remote Rl to respond to the redundant remote
R4.
The purpose of the fail flay which is set to 'on'
in instruction step 123 and is reset to 'off' in instructlon
step 103 i~ to prevent the redundant remote program from
getting hung-up in a condition in which it successully
restarts the remote Rl on~y to have the remote Rl fail again
by the time the program of the redundant remote recycles
around to checking the mailbox of the remote Rl again in
decision instruction sequence 101. If this should happen,
the fail flag for remote Rl will have been set ~o 'on' in
instruction step 123 after the successful restarting of -the
- 37 -
5~
applications program. Then, the next time that the
redundant remote program cycles back to decision
instruction sequence 101, and determines that the status
of remote Rl is not operating, the fail flag for remote
Rl will be 'on'. Accordingly, the program will jump from
decision instruction sequence 119 into the instruction
sequence 127 to initialize the remote Rl to respond to
redundant remote R4. If the next time the redundant remote
program recycles back to decision instruction sequence
101 to check the status of Rl, it determines that the
s~atus of Rl is operating, the program will then reset
the fail ~lag to 'off' in instruction step 103 so that in
subsequent cycles, should the program determine that the
remote R1 has again failed, the program will again go into
the restart instruction sequence 121 instead of immediately
jumping to the initialization instruction sequence 127.
After the redundant remote program has completed
the initialization instruction sequence 127, it then proceeds
to subroutine 129. In this subroutine, the status of the
applications program o~ remote R1 last received by khe
redundant remote R4, which status is stored in the memory
of the redundant remote R4, is loaded into predetermined
registers of the memory of the redundant remote R~ in order
to carry out the applications program of remote Rl in the
redundant remote R4. After this subroutine is completed,
the program proceeds into instruction sequence 130 and
then into the subroutine 131 in which it starts and
carries out the applications program. The redundant remote
R4 carries out the Rl applications program by receiving data
from remote Rl as to the status of the input and output devices
38 -
of ~he remote Rl and sending instructions to remote R1
to direct operation of the input/output management device
14 of the remote Rl. The program in the redundant remote
R4 will then continue to cycle through the applications
program for the remote Rl until it receives a command from
the operator to reset it back into its main cycle of checXing
the status of the remotes Rl, R2, and R3.
Should the redundant remote ~4 determine that
the status of remote R2 or remote R3 is not operating,
it then performs the same program with respect to these
remotes as described with respect to remote Rl as is
illustrated in FIGS. llA and llB.
The redundant remote R8 will take over the
applications program should any of the primary remotes
R5-R7 become nonoperative in the same manner as described
above with respect to R4 serving as a back-up for the
primary remotes Rl-R3.
It will be appreciated that the provision of
the redundant remotes decreases malfunctioning of the control
system due to one of the primary remotes becoming inoperative
as a result of Eailure 0f the central prc)cessing unit 16 of the
primary remote. Because each redundant remote serves as
a back-up for several primary remotes, the cost of providing
the redundancy is significantly reduced. Because the
redundant remotes are themselves each a remote control unit
which takes its turn in the master-for-a-moment sequence
communicating wi~h the other remotes over the dual channel
communications link, the redundant remotes can be provided
in the system very inexpensively.
- 39 -
Each remote Rn, as described above, is provided
with termination impedances Z~ and Zl for the first and
second communication channels C~ and CHl (FIG. 3) and a
line termination relay 32~ and 321 under the contxol of the
communications link control device 38. The termination
impedances are connected across each channel of the communi-
cations link when the particular remote is the first or the
last remote in the system (eOg., Rl and R8 in FIG. 1) to
establish proper line termination impedance to prevent
signal level degradation and the presence of reflected
signals, both conditions which can advexsely affect the
performance of the system. The termination impedances
Z~ and Zl are also applied across the appropriate communi-
cations channels when a remote determines, as described
below, that the communications link CL between it and its
immediately adjacent higher or lower number remote is
severed or suf~iciently degraded that reliable data
transmission cannot be maintained therebetween. The
determination as to communications link degradation can be
made b~ providing each remote with a register for each
communications channel that records, in a cumulative manner,
the number of invalid messages received from the immediately
adjacent remote(s) and terminate one or both of the
communications link CL~ and CLl in the direction of the
remote from which the number of in~alid messages received
exceeds a threshhold valueO ~ore preferably, however, each
remote is provied ~ith an active testing diagnostic routine
to enable it to test the communication integrity of the
communications link between it and its immediately adjacent
remote(s) in accordance wi~h the flow diagrams illustrated
in FIGS. 12, 12A, 13B and 12C as xead in accordance with
FX5. 13 and the table of FIG. 14.
-- ~0 --
The flow diagram illus~rated in FIG. 12 is a
summary of the manner by which each remote ls capable of
testing the communication integrity of the communications
link CL between it and its immediate adjacent remote or
remotes and terminating one or both of the communications
links, CL0 and CLl, when a degraded or interrupted line
condition is detected. As shown in FIGo 12 ~ the remote
Rx is initialized and then, in sequence, tests the communi-
cations integrity of the communications link CL~ in the
downstream direction between it and its immediately adjacen~
lower number remote (that is, Rx 1) and then tests the
communication integrity of the communications link CLl
in the downstream direction with the same remote. If
either the communications link C~ or CLl in the downstream
direction is faulty, an appropriate flag is set in a
register in the remote Rx reserved for this purpose. In
a similar manner, the remote Rx then tests the communications
integrity of the communications link CL~ and CLl in the up-
stream direction with its immediately adjacent higher number
remote (that is, remote RX~l3 and sets the appropriate ~lag,
as and if required. After this initial diagnostic checking
takes place, the remote Rx will terminate the failed communi-
cations line CL~ and/or CLl by actuating the appropriate
relay contacts 320 andfor 321 as required~ The line check-ng
test utilized in FIG. 12 pre~erably takes place when the
remote Rx is master-for-the-moment (that i5, Rm)o
A more detailed explanation of the communications
line integrity check and automatic line termination may be had
by referring to FIGS. ~2A, 12~ and 12C (as read in accordance
- 41 -
~2~ii~
with the flow chart legend of ~IG. 13) in which FIG. 12A
represents the downstream integrity check with the next
lower number remote, FIG. 12B represents the upstream
integrity check with the next hlgher number remote, and
FIG. 12C represents the line termination function in
response to the results of the integrity test performed
in FIGS. 12A and 12B.
In FIG. 12A, the line che~king diagnostic is
started by first loading ~hree registers or counters,
namely, a 'retry counter', a 'CL0 retry counter', and a
'CLl retry counter' with an arbitrarily selected number,
for example, five. The 'retry counter' is then decremented
by one and a message sent from the remote Rx to the
remote Rx 1 requesting an acknowledgement ACK signal. If the
communications link CL0 and CLl between the interrogaking
remote and the responding remote is fully functional, a
valid ACR signal will be received by the interrogating
remote Rx on both C~0 and CLl. The diagnostic checkiny will
then route to the part of the program ~FIG. 12B~ ~or
checking the communications integrity of the communications
link CL0 and CLl between the interrogating remote Rx and
the next higher number remote in the system, that is,
RX~l. On the other hand, if a valid ACK signal is not received
on one or both of the communications links CL0 or CLl by
the requesting remote Rx fxom the immediately adjacent lower
number responding remote R~ 1~ the appropriate retry counter
(that i5, ~ CL0 retry counter' or 'CLl retry counter') will
be decremented by one and the procedure repeated until the
'retry counter' is zero at which time the appropriate CL0
- 42 -
5~i~
and/or CLl termina e flag register will be set; thereafter,
the program will route to the upstream communications
integrity check shown in FIG. 12B.
The flow diagram o~ FIG. 12B is basically the same
as that of FIG. 12A except that the communication~ integrity
check occurs for that portion of the communications link
CL between the interrogating remote Rx and the next
higher number responding remote RXtl More specifically,
the three registers or counters, that is, the 'retry
counter', the 'CL0 retry counter', and the 'CLl retry
counter' are loaded with the arbitrarily selected value of
five. The 're~ry counter' is then decremented by one and
a message sent from the interrogating remote Rx to the
remote RX+l requesting an acknowledgement signal. If the
communications link CL~ and CLl between the interrogating
remote Rx and the responding remote RX~l is integral, a valid
acknowledgement signal will be received by the interrogating
remote Rx and the pxogram will route to the termination
impedance portion of the procedure shown in FIG. 12C.
On the other hand, if a valid acknowledgement signal is
not received on one or both of the communications lines CL~
or CLl by the interrogating remote Rx from the higher order
responding remote RX+1, the appropriate retry counter, that is,
the 'CL0 or CLl retry counter' will be decremented by one
and the procedure repeated until the 'retry counter' is
zero at which point the appropriate C~ and/ox CLl
termination ~lag register will be set; thereafter, the
program diagnostic will route to the line impedance
termination portion shown in FIG. 12C.
- 43 -
In the flow diagram of FIG. 12C, the various
termination registers are ex~mined for set flags and
appropriate commands issued to the C-link control device
38 (FIG. 3) to terminate the line by appropriate actuation
of the relay contacts 32~ and/or 32l. As is also shown in
FTG. 12C, a line termination relay can also be released
(that is, reset) to remove a previously applied line
termination impedance. Accordingly, the system provides
each remote with the ability to remove a line termination
as well as apply a line termination. This particular
feature is desirable when a communication link is
temperarily degraded by the pxesence of non-recurriny
electrical noise to permit the system to automatically re-
configure its line impedances.
The following specific example illustrates the
operation o~ the line termination procedure in which it is
assumed that the communications link CL~ in FIG. l is
severed at point A as shswn therein and that the remote
R4 is the pxesent master ~Rm) of the system and testing the
communications integrity of the communications link between
itself as the interxogating remote (Rx) and its next lower
order ~umber remote R3 (that is, Rx 1)~ In accordance with
the flow diagram of FIG. 12A, the 'retry counter', and the
'CL~ retry counter', and the 'C~l retry counter'~ as shown
in the tabulation table of FIG. 14, are set to ~he pre-
determined value of five. The 'retry cou~ter' is
decremerlted by one and the requesting interrogating remote
R4 (R~) requests an acknowledgement from the responding
remote R3 (that is, Rx_l). The requested acknowledgement
will be provided on line C~l but not line CL~ because of the
- 44
.
aforementioned interruption at point A (FIG. 1).
The interrogating remote R4, not receiving the requested
acknowledgement signal on communications link CL~, will
decrement the 'CL~ retry counter' by one. Thereafter,
the retest procedure will be sequentially continued with
the 'CL0 retry counter' being decremented with each
additional unsuccessful attempt to obtain an acknowledgement
from remote R3 through the communications link CL0. When
the 'retry counter' decrements to zero, the 'CL~ retry
coun~er' will also be decremented to zero at which time the
CL0 lower order termination flag will be set. The remote
R4 will thereafter continue the diagnostic checking procedure
to ~est the communications integrity of that portion of
the communications link between the remote R4 (Rx) and the
next adjacent higher remote R5 (that is, RX+l) in accordance
with the flow diagram of FIG. 12B. At the conclusion o~
the test o~ the communications link between the inter-
rogating remote R4 and the immediately adjacent lower number
and higher n~mber remotes R3 and R5, the termination relay
contacts 320 (FIG. 3) will be set to terminate the communi~
cations link Ch0 at the remote R~. In a ~imilar manner, the
remote R3, when it becomes master-for-the~moment, will also
apply a termination impedance across the communications link
CL~.
As can be appreciated from the foregoing, the
remotes Ro...~n have the ability, even when one or both of
the commur.ication links CL0 and CLl are severed to still
- 45 -
function on a master-for-the-moment basis and also to
effect appropriate line termination to minimize the adverse
effect on digital data signal strength and the generation
of xeflected signals from mismatched line impedance caused
by deteriorated or sevexed communication linesO In
addition, the system is self-healing, that is, when
reliable communications is restored over the severed or
degraded portion o~ the communications link the remotes
Rn will then again functi.on to remove the line impedances
to resume full system operation.
As will be apparent to those skilled in the art,
various changes and modifications may be made to the
industrial control system of the present invention without
departing from the spirit and scope of the invention as
recited in the appended calims and their legal equivalent.
- 46 -
