Note: Descriptions are shown in the official language in which they were submitted.
~2~748
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DIJAL FUNCTION INPUT/OUTPUT FOR A PROGRAMMABLE CONTROLLER
The present invention relates in general to
methods and app~ratus for use with "programmable
controllers": and in particular to an intelligent
inputtoutput system therefor.
Backqround of the Invention
Process control with a programmable controller
involves the acquisition of input signals from
various process sensors and the provision of output
signals to controlled elements of the process. The
procesfi is thus controlled as a function of a stored
program and of process conditions as reported by the
sensors. Numerous and diverse processes are, of
course, subject to such control, and sequential
operation of industrial processes, corlveyor systems,
and chemical, petroleum, and metallurgical processes
may all, for example, be advantageously controlled by
programmable controllers.
Programmable contcollers are of relatively
recent development. A state of the art programmable
controller compri~es a central proce~;fiing unit (CPU)
made up, broadly, of a data processor for executing
the stored program, a memory unit of sufficient size
to store the program and the data relating to the
status of the inputs and outputs, and one or more
power supplies. In addition, an input/output (I/O)
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system pro~ides the interface between the central
processing unit and the input devices and controlled
elements of the process being controlled.
Input/output systems have remained relatively
unchanged since the advent of programmable
controllers and are the feature most in nee~ of
improvement. While some ad~ances have been made in
I/O systems, the improvements have generally been
along the same lines as those followed in the past.
For example, U. S. Patent ~293,924 describes an I/O
~yct~ ~her~;n the density of the interface s
increased. Another approach, illustrated by U. S.
Patent ~,247,BB2, has been to concentrate on
improving the housing for the input/output system.
lS With the increased complexity of the processes
requiring control, and with a need for a greater
exchange of information between the process and the
central processor, however, other improvement
approaches to I/O problems have been needed.
The conventional I~O system is composed of a
number of individual I/O points, each one of which is
devoted to either receiving the signal from an input
device (e.g., a limit switch, pressure switch, etc.)
or to providing a control signal to an output device
(e.g., a solenoid, motor starter, etc.), depending on
h~w the circuitry for the particular I/O point is
configured. That is, an I/O point is Aedicated to
being either an input point or an output point and is
not readily converted from one use to the other.
One problem with state of the art I/O system8
(particularly when used with a complex process) is
the high cost of installation. Typically, I/O
modules, or circuit cards, are housed in card racks
or cages. For control of an extensive or complex
7~4~
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process, a large number of I/0 poir~ts must be
provided in each rack or cage. This necessarily
entails a great deal of wieing expense (both for
labor and foc materials) since wires from all of the
input and OlltpUt devices must be brought into the l/O
rack.
Additional problems then arise from use of a
large I/0 rack since it is frequently difficult to
bring all of the wires into the rack to make the
termin~tions. Although it is well-known to provide
~t least a ~4rtion of an I/O system in an enclosure
or rack remote fcom the CPU (in an attempt to get the
I/0 closer to the pcocess being controlled), these
problems are still not overcome since there is a
concentration of input/output wiring into a single
(albeit remote) location. Further complications
arise in dissipating heat in a concentrated I/O
system and, for that reason, it is frequently
necessary to operate an I/O system at less than its
~timum rating.
Another pcoblem with present I/O systems is that
they are difficult to diagnose and troubleshoot -
whether the malf~Jnrtions occur in the programmable
controller, per se, or in the controlled process.
Experience has shown that most on-line failures
associated with a controller occur in the I/O
system. The CPU portion is now highly eefioed,
having benefited greatly ~rom the advances in
microprocessor technology and data processing, for
example. ~hen an electrical failure does occur,
however, early detection and diagnosis of the precise
nature of the problem is often critical. It is
naturally desirable to detect a failed part through
an advanced warning rather than after some part of
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the process is ~ut of control.
~ ith state of the art I/O systems, early
detection or failures is dif~icult, and even when a
failuce is signaled its precise loca~ion and nature
may not be appacent. In many cases it is even
difficul~ to separate controller I/O failures from
failed elements (e.g., motors, pushbuttons. etc.) in
the process. Diagnostic features, particular for the
controller I/O system, have simply been lacking.
Improvements for diagnosing and preventing I/0 system
f~;lures h?'.'e ~herefore been eagerly sought.
The problem of diagnosing failures is at times
made dif~icult because each I/O point is oedinarily
protected by a fuse. Although the fuse protects the
particular I/O module from overcurrent, frequently it
adds to the problem. For example, mere transient
current may blow the fuse, leaving the I/O point
completely inoperative until the failed point can be
located and the fuse replaced.
Somewhat related is the problem of exchanging
diagnos~ic and control information between a
controlling portion and a controlled portion of an
I/O system. For example, it may occur that
distributed I/O modules ace used to configure an I/O
system. In such case it is desirable to provide
simple, reliable means and methods for exchanging
such information.
Yet another drawback of conventional I/0 sy6tems
is that (as was mentioned above) each I/0 point
functionS strictly as an input point or as an output
point. The same point may not readily be converted
from one use to the other. The user of a
programmable controller is therefore required to
select in~ut and output functions separately, based
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on an an initial estimate of needs. There is a
decided lack of flexibility for unforeseen future
needs. Moreover, since I/O points are typically
available in gcoups (e.g., six or eight points per
5 CilCUit card), there is freq~ently a large number Gf
unused I/O points in a control system.
Accordingly, the principal object of the present
invention is to provide an input/output system which
overcomes these shor~comings of conventional I/O
systems. More particularly, however, it is sought to
~rovid~ an I/O s~st~m wherein each I/O point may be
selected to operate either as an input point or as an
output point.
In addition, it i5 sought to pcovide an
input/output system wherein each I/O point is
self-protected against overcurrent and overvoltage
conditions without the use of fuses or circuit
bre~kecs and wherein each I/O point is continuously
and automatically diagnosed for failure, both within
the I/O system and within the controlled process, and
wherein detected failures are identified and
automatically reported. A further, specific object
of the invention is to provide an I/O system which is
simple and economical to wire and use and which
provides individual I/O points in distributed groups,
or modules, for location in close proximity to the
process, or particular part of the process to be
controlled. An additional object of the invention is
~o provide an I/O system which includes means for
monitoring, co~ rolling, and troubleshooting each I~O
point independent of the conventional central
processor unit. Still furt~ler objects, features, and
advantages of the invention will appear from the
~nsuing detailed description.
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SummarY of the Invention
The present invention provides circui~ry which
is selectively operable either as in input point or
as an output point in a programmable controller
having multiple input and output points for
exchanginq signals between a central processing unit
(CPU) of the controller and a process being
controlled. Preferably, operation as an input point
or as an output point is under control of the CPU.
In preferred form, the invention includes a
common inPut/output t.er~;n*l for terminating both
input and output devices associated with the process
being controlled; an input return terminal for
terminating a return conductor from the input device;
an output return terminal for t~rminating a return
conductor from the output device; a preload re~istor
connected between the input/output terminal and the
output return terminal so that a status signal is
developed across the preload resistor indicating the
status (generally, open or closed) of the input
device; and an insulated gate transistor having its
collector to emitter current path connected between
the input/output terminal and the input return
terminal. The insulated gate transistor, capable of
being ~riggered on and off from a gate terminal, is
held in an off state whenever the circuit is selected
to be operated a~ an input point and is triggered on
and off when the circuit i8 selected to operate as an
output point so as to provide power to the output
device as necessary. Most preferably, the IGT is of
the type having a main current section and an
emulation current section and arranged such that a
bl~rden resistor in series with the emulation section
provides a diayrlo~tic signal indicative of the total
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IGT cucrent.
Brief Description of the Drawinqs
While the specification concludes with claims
particulaely pointing out and distinctly claiming the
5~ subject matter regarded as the invention, the
invention will be better understood from ~he
following description taken in connection with the
accompanying drawinys in which:
Fig. 1 is ~ simplified block diagram of a
pro~r~mmable controller sVst~m including an
intelligent input/output (I/O) system in accordance
with the present invention:
Fig. 2 is a perspective illustration of one
possible physical form for an individual I/0 module
and a hand-held monitor, both configured for use in
~he I/0 system of Fig. 1:
Fig. 3 is a block diagram illustr~l;ing in
greater detail one of the I/O modules of Fig. l;
Fig. 4 is a simplified block diagram of a
communications section and a control and sensing
section for an I/0 point of the type illustrated in
Fig. 3;
Figs. 5 and 6 are illustrations of waveforms
showing the relationship between certain signals
relevant to the circuitry of Fig. 4;
Figs. 7A, 7B, and 7C are schematic diasrams
illustrating various input/output switchit~ circuits
usable with the I/O circuit of Fig. 4 -- Fig. 7A
showing a dc source circuit, Fig. 7B showing a dc
sink circuit, and Fig. 7C showing an ac circuit;
Fig. 8 is a schematic diagram illustrating in
detail a control and sensing section for the I/O
point of Fig. 4;
i Z ~ 7 ~ 4 8
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Figs. 9A, 9B, and 9C ace schematic diagrams,
illustrating in detail, a communications section for
the I/0 poin~ of Fig. 4; and
Fig. 10 is a truth table relating diagnostic and
status data to a 4-bit coded signal for providing
combinatorial logic in a state encoder for the
communications section of Fig. 4.
Detailed DescriPtion of the Invention
The programmable controller of Fig. 1 includes a
central processirg unit (_PU~ 20, an input/output
(I/0) controller 22, a plurality of input/output
modules 24-26, and a data communications link 28
which interconnects each I/0 module 24 - 26 with the
I/0 oorltroller 22. These items, exclusive of CPU 20,
generally ~omprise the input/output sys~em of the
controller. The CPU 20 is substantially of
conv~nl.ional design and may include one or more
microprocessors for data handling and control, plus
memocy for storage of operating programs,
input/output data, and other computed, interim, or
pe~manent data for use in executing the stored
program and for implementation of control. In
addition, other conventional elements, such as power
supplies, are included as necessary ~o make the CPU
20 fully functional. The I/0 controller 22 provides
for control of information exchanged betwoen the
various I/0 modules 24-26 and the CPU 20.
Each I/0 module 24 - 26, may be separately
located, remote from CPU 20 and I/0 controller 22,
and in close proximity to the process being
controlIed. Although only three I/0 modules are
illustrated in Fig. 1, it will be understood that the
actual numbec may be considerably greateL. For
21-IYP-2470
_ g_
example, sixteen separate I/0 modules may be readily
accommodated in the system to be described herein.
Each I~O module is independent of the other and each
may be devoted to control of a process separate from
that controlled by all other I/O modules.
In Fig. 1. for example, the Nth I/O module 26 is
illustrated to control a generalized process 30. The
input and output signals associated with process 30
are conveyed by conductors 32 which run between the
process 30 and the I/O module 26. The process 30
may, of course, .ake ~iL-uaily ~ny t`orm. In any
case, however, it includes various sensors, ~witches,
etc. (not specifically illustrated) for sensing the
status and condition of the process 30. The
information from the process is in the form of input
signals to I/O module 26. The process 30 also
includes controlled elements (e.g., pumps, motors,
etc. ~ also not illustrated) which receive the output
signals from the I/0 module 26 and which thereby
effect control of the process 30. In similar fashion
each of the other I/O modules 24, 25 is
interconnected to input and output devices and
apparatus associated with a process.
The data communications link 28 is preferably a
serial link although parallel transmission of signals
~eLween the CPU 20 and the I/0 modules 24 - 26 may be
readily prt)vided. In either case, I/O modules 24 -
26 are connected to the communications link Z8 for
communication with CPU 20. The communications link
28 may comprise a twisted pair of conductors, a
coaxial cable, or a fiber optics cable; all are
acceptable depending on such consideration~ as cost
and availability.
In Fig. 1, I/O module 24 illustrates in block
48
21-IYP-2470
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diagram form the general overall electronic structure
of each I~0 module.
Thus, there is included a microcontroller ~6
having an interface port for excharlging information
with CPU 20 and including an associated memory (not
illustrated) for implementation of a stored program
of operation according to which the various elements
of the I/0 modules are controlled and diagnosed for
incurred faults; a plurality of individual I/0 points
(or, "I/0 circuits") 37 - 39, each of ~hich may be
selectably operate~ aither as 2~ i~pU~ po~nt or as an
output eaint and each of which interfaces
i.ndividually through conductors directly to input or
output elements of the controlled process; and a
conductor bus 40 for interconnecting the I/0 points
37 - 39 to the microcontroller 36. The number of I/0
points 37 - 39 in any particular I/0 module 24 - 26
depends on practical considerations such as heat
dissipation and the limitations of the
microcontroller 36. As an example, however, it has
been found quite practical and convenient to provide
sixteen I/0 points per I/0 module~
For verifyin~ the integrity and functionality of
the input and output components and for maintenance
and troubleshooting, monitoring apparatus 42 is
provided. The monitor 42 i6 preferably sized to be
hand held so that it can be readily and conveniently
moved from one I/0 module to the other. It i6
adapted foe connection into each I/0 module by a
cable which includes a connector for mating with
another connector affixed to the I/0 module. The
cable and mating connectors are schematically
illustrated in Fig. 1 which shows the monitor 42
connected to l/0 module 24 through an interface port
~Z~7~48
~ 21-IYP-2470
of the microcontroller 36.
When connected to an I/0 module, the hand-held
monitor 42 allows the I/0 points of that module to be
monitored and controlled and pIovides a display of
diagnostic information pertaining to the module.
Advantageously, the hand-heid monitor performs these
func~ions independently of the central processing
unit 20 and everl without the CPU 20 being present.
The monitor 42 is operative, for example, to turn
output points on and off and to read the state of the
input points. In case a fa~llt ~ occurred, the
monitor 42 can also provide an indication of the
nature and location of the fault. The~hand held
monitor 42 may be noted to include a data display
panel 44 which displays alpha numeric characters and
a set of key switches 46 which provide for address
programming and for effecting operation of the I/0
modules 24 - 26.
Referring now to Fig. 2, preferred physical
forms for a hand-held morlitor and an individual I/O
module are ill~strated. Thus, the illustrated I/0
module 51 is substantially in the form of a terminal
block which includes a row of conductor terminals 53
for making connection to the conductors that connect
with the input and output devices of of the
controlled process. The terminals 53 may be in the
form of screw-type connections in which the screws
are tightened down on a connecting wire or terminal
lug. Each I/0 circuit is assigned to a corresponding
terminal connection. In addition, terminals are
assigned for connecting an external power source (ac
or dc) and for making connections to the data
communication link a~ fihown in Fig. 1. Visual
indicators are provi~ed, in the form of light
IYP-2470
-12-
emitting diodes (LEDs) 55 to indicate the status of
each I/O point. Additional LE~s 57 and 58 provide an
indication of the operational status of the module
51. For example, LED 57 indicates that a fault
condition is present (ei~her internal or external to
the module) and LED 5B indicates normal operating
conditions. A connector 59 is provided on the module
Sl for mating with a cable connector 60, and, through
cable ~1, to hand-held monitor 49.
The illustrated hand-held monitor 49, as
described above and in connection with Fig. 1, is
able to exercise the I/0 module to which it îs
connected. That is, the hand-held monitor allows an
I~O module to be opera~ed and thoroughly checked out
even if it is not connected to a central processing
unit as shown in Fig. 1.
The block diagram of Fig. 3 illustrates an I/O
module 80 (substantially the same as any one of
modules 24 - 26 of Fig. 1) in greater detail. The
I/O module 80 thus includes a group of 8 separate I/O
points 81 - 88, each one of which exchanges control
and diagnostic information signals with
microcontroller 90. Electrical power, either ac or
dc, is supplied at terminals H and N. The power
source connected to terminals H and N provides power
both to an internal dc power supply 94 and to any
extecnal output. loads (e.g., controlled elements)
which are controlled by the progeammable controller
of which module 80 is a part. Power supply 94 i8
simply the dc power supply foL all elements ~olltained
in the I/O module 80 which require dc power in their
operation.
Each I/O point 81 - 88 is connected to the
microcontroller 90 by a pair of conductors 95 - 102,
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Zl-IYP-2470
-13-
cespectively. One conductor of each pair, desi~nated
the D line, conveys control data to the associated
I/O point: the other line, designated the M line,
conveys status and diagnostic information from the
I~O point to the microcontroller 90. Each I~O point
81 - 88 is also connected to receive dc power (e.g.,
15 volts) from power supply 94 and each is connected
to the power source terminals H and N. If the
external power source connected to terminals H and N
is a 115 or 230 volt ac line, for example, the H and
N terminals m~rely r~er to the ho~ and neutral sides
of the line, respectively. However, if the external
power source is dc, the H terminal may be the
positive side of the source and the N terminal the
negative side. In addition, each I/O module 81 - 88
includes an IN/OUT terminal which is of dual
function. If the I/O point is to be operated as an
output point, the IN/OUT terminal for thd~ point is
connected to the controlled element (or load) in the
process which that point is as~igned to control. On
the other hand, if the I/O point is to be operated as
an input, the IN/OUT line for that point receives the
input signal from the input device. The same IN/OUT
line thus serves both func~ions, depending on the
command from the microcontroller 90 and the second
(or reference) connection of the input or output
device. As an example, I/O point 82 is shown
operating as an output point, turning power on or off
to a load device 89. Load ~9 is connected beLween
the IN/OUT line of I/O point 82 and the N line to the
power source. By contrast, I/O point 84 is shown
opecating as an input point with an input switching
device 91 connected between the IN/OUT line and the H
line of the power source. Any one of I/O points
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81 - 88 may be operated in the Oueput mode either as
a dc source, as a dc sink, or as an ac source,
depending somewhat on the internal circuitry of the
I/O point. That aspect of the circuitry is discussed
more fully herein below.
Information provided to the microcontroller go
from each I/O point 81 ~ 88, ~ia the M line
connection, includes data reporting the status of
load current (nigh or low), the level of power
supplied to that I/O point, the temperature condition
of ~he l/O point, the status of an~ input device, and
still othec infocmation, all of which will be set
forth irl greater detail subsequently herein.
Control of each I/O point 81 - BB i 5 ultimately
determined by a central processing unit as outlined
in connection with Fig. 1. In Fig. 3, communication
with such a CPU is through ~n interface port
tPreferably a serial port) of microcontroller 90 and
through a data communications link 106 (28 of Fig.
1). Other I/O modules substantially similar to
module 80 of Fig. 3 may also be connected to the data
communications link 106. While microcontroller 90 is
responsive to the commands of the central processing
unit, it also pcovides localized, distributed control
of each I~O point within the I/O module ~0.
Microcontroller 90 is an operations control unit and
operates in accordance with a stored program and as a
function of commands from the central processing unit
and the signals received on the M line from each I/O
point 81 - 88. Although not specifically illustrated
in Fig. 3, microcontroller 9o also includes memory
for program storage and for storage of other data
necessary to carry out program execution and to
achieve the intended control.
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The simplified block diagram of Fig. 4 shows a
preferred embodiment of an I/O circui~ exclusive of
the output switching device. The I/O point thus
includes a communications section 111 and a control
and sensing section 113. The communications section
111 (to be discussed first) includes timer 117,
output data filter 119, output selector 120, two-bit
counter 121, hold last state latch 123, default latch
~24, state encoder 125, state latch 127, and data
selector 129.
The communications section 111 receives, on li,le
D, a signal SIG from the operations control unit
(e.g., as from microcontroller 90 of Fig. 3) and a
set of state indica~ive ~diagnostic) signals on a six
conductor bus 115. The communications section 111
produces an ON/OFF command signal to the control and
sensing section 113 and transmits a diagnostic signal
(STATE) to the microcontroller on line M. The ON/OFF
command signal ultimately controls a ~witching device
~preferably an insulated gate transistor, or IGT, to
be discussed subsequently) whose operation depends on
whether the I/O point is to serve as an input or as
an output. Figs. 5 and 6 illustrate the relationship
bel.ween certain signals involved in the operation of
the communications section 111 and should be referred
to in conjunction with Fig. 4.
The control signal SIG is a coded pulse train
containing on/off information, hold last state (HLS)
;nformation, default state (~EF) information, and
tilning information. It consists of a series of
"frames", each of which contains either two or four
pulses followed by the omission of a pulse, i.e., a
"missing pulse". The "missing pulse" serves to
resynchronize opecation of the communications section
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21-IYP-2470
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111. Each of the two or four pulses has a duty cycle
of either 25 percent or 75 percent. The time between
pulses within a frame, T, is fixed and is also the
time duration of the "missing pulse". The con~rol
signal SIG is initially applied to a timer 117
wherein its rising edge causes the timer 117 to reset
and to initiate its timing cycle. Thus, the timer
117 puts out a rising edge of the clock signal CLK
approximately 0.5T after each rising edge of SIG.
The CLK signal is used to clock two bit counter 121,
output data filter 119, and l~ches 12~ and 124.
Unless first reset, the timer 117 also puts out a
rising edge of the synchronizing signal SYNC
approximately l.ST after a rising edge of SIG, and it
puts out a falling edge of the LOS signal at some
significantly longer time after a rising edge of SIG
(e.g., 2.5T). Normally, rising edges of SIG occur at
intervals of T so that the timer 117 is reset before
the SYNC or LOS transitions can occur. Howe~er, upon
the occurrence of a "missing pulse" (synchronizing
interval), a time 2T occurs between rising edges of
SIG, causing SYNC to go high for approximately 0.5T.
The SYNC pulse resets the communications section 111
and thus signals that a new frame is about to start.
If a period of more than 2.5T occurs between rising
edges of SIG, LOS will go low, signalling to the
communications section 111 that a los6 of r;ignal ha6
occurred.
The on/off information pa6sing to the I/O point
on line D is contained in the first two pulses of
each frame of the control signal. A 75 percent duty
cycle pulse corresponds to a logical "1" (switch on)
and a 25 percent duty cycle corresponds to a logical
"O" (switch off). As will become clear, the clock
21-IYP-2470
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pulse which occurs at 0.5T af~er the rising edge of a
SIG pulse, effectively causes a sampling of the SIG
pulse at that time. Thus, if a 25% duty cycle
(0.25T) pulse has been transmitted, a low level or
"zero" is obtained at 0.5T. On the other hand, if a
75% duty cycle (0.75T) pulse has been transmitted, a
high le~el or "one" is obtained at 0.5T. The first
two pulses are also transmitted redundantly: that is,
the ficst two pulses must agree (both 1 or both 0) in
order for the communications section 111 to respond
to the ON/OFF command. For these purpcses. ~ho
control signal SIG is provided to output data filter
119 which effectively samples and compares the first
two pulses of the control signal. If the two pulses
are different (due, for example, to noise
interference). the output data filter 119 maintains
the last valid ON/OFF command which was received.
If a frame of the control signal contains four
pulses rather than two, then the third and fourth
pulses are used to update the hold last state latch
123 and the default latch 125, respectively. The
contents of these latches 123 and 124 are only
changed when third and fourth pulses are received. A
logical one in the third pulse position sets the hold
last s~ate signal HLS high; a logical zero in the
third pulse position causes the HLS signal to go
low. The HLS signal appear 5 at the output of the ~ILS
latch 123 and is provided to the output selector 120
and to the state encoder 125. Similarly, a fourth
pulse sets the default signal DEF high or low (high =
On, low = Off). The default signal DEF and its
complement DEF appear as outputs from the default
latch 124. The default signal DEF is supplied to the
state encoder 125 and its complement DEF is supplied
7~8
21-IYP-2470
to the output selec~oL 120. In the event of a loss
of communications from the microcontroller (i.e., a
loss of ehe control signal causing LOS to go low),
the HLS signal commanas the output selector 120 to
either hold the previous on/off state or to assume
the default state. If HLS is a logical one, then the
previous state will be maintained: if HLS is equal to
zero, then the default state will be assumed as soon
as LOS goes low. The advantage of this operation is
apparent: in the event of a loss of communications
between the I/O point and the controllin~ d~ic~
(i.e., the microcontroller of Figs. 1 and 3) the
on/off condition is forced into a pre-selected,
preferred state.
The two-bit counter 121 counts CLK pulses to
provide an output count, S0 and Sl, which takes
binary values between zero and three. This count
value is indicative of whi~h pulse in a frame is
being received and is provided (as S0 and Sl) to the
output data filter 119, hold last state latch 123,
default latch 12~, and data selector 129 so that each
circuit responds only to the appropriate pulses of a
frame.
The waveforms of Fig. 5 illustrate the signal
relationships SIG, CLK, SYNC, LOS, and the On/Off
signal for various conditions. For the first frame
(~he frames are arbitrarily designated with frame
numbers for ease of reference), redundant 25 percent
duty cycle pulses are sent corresponding to "0" or an
Off switch state. Clock pulses are produced at 0.5T
after each rising edge of a SIG pulse. Following the
two redundant pulses, there is a synchronizing
interval or "missing pulse". The missing pulse
causes a SYNC pulse to be produced, signifying the
L~ 77 ~
21-IYP-2470
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.
end of a frame. Since the two SIG pulses are both of
25 percen~ duty cycle, the ON/OFF value remains low
and the LOS value cemains high.
For the second frame, the first SIG pulse is of
25 percent duty cycle and the second is of 75 percent
duty cycle. The lack of identity may result from
noise interferencè, for example. In such case the
CLK and SYNC pulses are again produced as in the
first frame and LOS remains high. Since the SIG
pulses a~e different, however, the ONJOFF signal
retains its previous value, which, in tiliS CdS~ i~
low. In the third frame, the SIG pulses are both of
75 percent duty cycle duration, signalling that the
ON/OFF switch signal should ~e raised to the ON
level. This occurs at the rising edge of the clock
pulse following the second SIG pulse. For the fourth
frame, pulse identity is lost between the control
pulses and so the on/off line remains high. The
fif th f~ame returns the on/off line to a low level
with the occurrence of redundant pulses both having
25 percent duty cycles. The sixth frame of SIG
pulses includes four 75 percent duty cycle pulses.
The sixth frame is somewhat extended in time duration
~o accommodate the four pulses and the "missing
pulse". The first and second SIG pulse6 return the
ON/OFF signal to high. Although not shown, the third
pulse of the frame causes ~LS to go high
simultaneously with the rising edge of the resulting
clock pulse, and the fourth pulse of the frame causes
DEF to go high.
In addition to on/off, default, and hold last
state information, the control signal SIG provides
timing for returning status or diagnostic data to the
microcontroller. State encoder 125 accepts, as
~4~7~$8
21-IYP-2470
-20-
inputs, six switch sta~es on conductor bus 115 from
the control and sensing section 113, along with the
~N/OFF, DEF, and HLS bits. The state encoder 125
combines these input signals to form a four-bit
encoded status message which is provided to state
latch 127. Data selector 129 is a one-of-four
selector which accepts the four bits of data from the
s~ate latch 127 and then sequentially sends this four
bit state information to the microcontroller via the
M line. The output of the two-bit counter 121
indicates the count of the sIr7 plllsec a~d ccntrols
the data selector 129 such that it sends out one bit
for each SIG pulse received. The four bits are coded
so that the first bit (XO) indicates whether or not a
~ault condition exists and the second bit (Xlj
indicates whether or not voltage appears on the
output load. If a fault occurs (XO = O), the third
and fourth bi~s (X2 and X3) indicate the nature of
the fault. If no fault has occurred (XO = 1~, then
the th7rd bit is indicative of the hold last state
value and the fourth bit is indicative of the default
value.
The microcontroller 90 (Fig. 3) determines how
much infoLmation is to be received from the
communications section lll by the number of pulses
per frame contained in the control signal, SIG, which
is sent to the communications section lll. The
microcontroller reads the state signal on line M
immediately after it puts a rising edge of SIG on the
D line. Thus, the number of pulses per frame in the
control signal and the number of status bits read
back per frame are the same. Normally, the
microcontroller puts out two pulses per frame and
reads back XO and Xl. If XO indicates a fault, the
7'74~
21-IYP-2470
-21-
microcontroller then shifts to four pulses per frame
so that it can read a fault message contained in the
X2 and X3 bits. In ~he absence of a fault, the
four-pulse mode may also be used to read and write to
the HLS latch 123 and the default latch 12q. In such
~ase, the third and fourth pulses of SIG either set
or reset the HLS and default latches, 123 and 124
respectively, and X2 and X3 of the state signal
indicates the status of these two latches.
The control and sensing section 113 of Fig. 4
includes switch logic circuitry 133, compar~to~
cir~:lJi~ry 135, and a gate drive circuit 137. The
switch logic circuitry 133 receives the ON/OFF signal
producsd by the communications section 111 and,
depending on the status of other input signals,
provides a corresponding gate signal, via the gate
drive circuit 137, to the gate terminal of a power
switching device. The power switching device is
preferably an insulated gate transistor which will be
more fully discussed hereinbelow.
Among the other signals provided to switch logic
circuit 133 are signals representative of the power
supply voltage level and the temperature of the power
switching device. Signals representing line and load
voltage and load current are provided as inputs to
the comparator circuit 135. The comparator circuitry
135 develops a set of signals which indicates the
level of load cl~rrent with respect to a pre-selected
low limit, an intermediate limit, and a high limit.
The comparator circuitry 135 also provides a signal
indicative of the level of load voltage with respect
to the line voltage level, and, for ac, a signal
indicative of the ac zero crossing. All of thcse
signals are provided as inputs to the switch logic
f~
21-IYP-2470
-22-
circuit 133 via a five conductor bus 136. ~n
additional input ~o switch logic circuit 133,
denominated ac/dc, is provided for pre-selecting
operation in either the ac mode or the dc mode.
The switch logic circuit 133 provides the set of
diagnostic signals supplied to state encoder 125 via
~he six conductor bus 115. This set of diagnostic
signals is derived from the voltage and current level
signals provided by comparator circuitry 135 and from
the temperature and supply voltage signals. The six
- diagnostic signals may be used, for example~ ~-
indicate: 1) that there is an open or disconnected
load; 2) that load is in exce~s of a first high limit
value requiring an immediate protective response: 3)
a load current in excess of a second high limit value
requiring a protective response only if the surrent
remains above the limit for some pre-selected time
period; 4) that load voltage has, or has not, been
applied; 5) the relative level of the supply voltage;
and 6) the relati~e temperature of the power
switching device.
~ arious input/output switching circuits may be
provided to be controlled by the gate signal
emanating from the control and sensing section 113.
For example, switching means comprising field effect
transistors or silicon controlled rectifiers (SCRs)
may be used as the input/output switching circuits.
A preferred switching circuit will, in any case,
include a shunt current path including means for
providing a signal indicative of the current to a
connected load. The switching circuits most
preferred, however, make use of an insulated gate
transistor, or IGT.
The IGT, in general, is a power semiconductor
7~
21-IYP-2470
-23-
device which may be gated both into and out of
conduction. That is, the I&T may be both turned on
and ~urned of~ th!ough its gate ~erminal. Some
versions of the IGT include a cu~rent emulation
section which is a section of the IGT provided to
carry a proportional fcaction of the total IGT
current. The emula~ion section i5 advantageous in
that it can be used to monitor the total current
without resort to large power dissipating shunt
resistors for current sensing. A single gate signal
controls current flow both both in the main sectio~
of an IGT and in its emulation section. The
insulated gate transistor is described (albeit under
a different name) in an article by B. J. Baliga et
al., entitled ~The Insulated Gate Rectifier ~IGR): A
New Power Switching Device", IEDM ~2 (December 1982),
pages 264 - 267. An IGT having an emulation section
is the subject of a Canadian patent applicati~n,
Serial Number 461,634, of c~mm~n assignee with
the present invention and which was filed on
Aug. 23, 1984. Figs. 7A -7C show various
input/ output switching circuits using IGTs which may
be used in used in the I/O system disclosed herein.
In the dc source circuit of Fig. 7A, the gate
signal is applied to the gate terminal 140 of a
P-channel IGT 141 having an emitter 142 for a main
cuerent section and an emitter 143 for an emulation
current section. The positive side of the dc power
soucce is connected directly to the the main emitter
30 142, and. through burden resistor 145, to the emitter
143 of the emulation section. The collector of the
IGT device is connected externally to one end of the
parallel combination of a free-wheeling diode
l~P~7~ 3
21-IYP-2470
-24-
147 and and pre-load resistor 148. The opposing end
of the combination of diode 147 and pre-load resistor
14B is returned to the negative side of the dc power
source. The junction of IGT 141 and the
diode-preload resistor combination pro~ides the
IN/OUT terminal 149. Although, in actual use, both
an input device and a load would not be connected at
the same time, a load 150 is shown between IN/OUT
terminal 149 and the load (i.e., outuut) return
termina~ 152, and an input device 153 is shown
between the IN/OUT terminal 149 and ~he ir.put retuLn
terminal 155. Return terminals 155 and 15Z are
electrically common, respectively, with the positive
and negative lines of of the dc power source.
Pre-load resisto~ 148 is relatively high in ohmic
value and burden resistor 145 is of relatively low
ohmic value as are the corresponding pre-load and
burden resis~ors used in the circuits of figs. 7B and
7C. For example, for a 120 volt source, pre-load
resistor 148 may be on the order of 20K ohms and
burden resistor 145 may be on the order of ten ohms.
When the circuit of Fig. 7A is operated as an
output, load current is controlled by turning the IGT
141 on and off at appropriate times. Load current
passes from the power source, through the IGT 141 and
the load 150, and back to the source. Load current
monitoring i6 facilitated by 'che IGT emulation
section which provides a load current indicative
signal at the junction between burden resistor 145
'O and emitter 143. A lQad voltage signal, conicming
that load voltage is indeed applied, is taken from
the junction of the pre-load resistor 148 and the
collector of IGT 141. A line voltage signal is taken
from the opposite end of the pre-load resistor 148.
7~
21-IYP-2470
-25-
The free-wheeling diode 147 is provided as a shunt
for reverse currents from inductive loads.
When the circuit of Fig. 7A is operated as an
input, the IGT is held in an off state. The state of
input device 153 ~open or closed) is then detected by
monitoring the voltage developed across the pre-load
resistor 14~. This status signal is monitored via
the load voltage line.
The dc sink input/output circuitry of F1g. 7B
contains the same operative elements as does the
source ciecuitry of Fig. 7A, but in a somewha~
different configuration. When this circuitry is
operated as an output, the load 157 is connected
between the IN~OUT terminal 158 and the load return
te~minal 159. The IGT 161 is switched on or off to
control the load current. Notable, however, is the
fact that IGT 161 is an N-channel IGT. The collector
terminal is connected to one end of the parallel
combination of a free-wheeling diode 165 and pre-load
resistor 167. This combination is in parallel with
the terminals 159 and 158 to which the load 157 is
connected. A burden resistor 168 is serially
connected between the emulation section emitter and
the negative side of the dc power source. The main
section emitter is tied directly to the negative side
of the dc power source. An IGT current signal,
indicative of load current, is taken from the
junction of the burden resistor 16~ and the emulation
section emitter 163. The load voltage 6ignal is
taken from the IN/OUT terminal 158 and the line
voltage signal is taken from the positive side of the
dc power source which is also connected to input
return terminal 160. As with the dc source
ciccuitry, discussed above, when the input/output
~7'748
21-IYP-2470
-26-
circuitry is used as an input, the IGT 161 is held
off and the state of the input device 170 is sensed
by the voltage developed across th~ pre-load resistor
167. This status signal is l.ransmitted via the load
voltage line.
In Fig. 7C, illustrating an ac input/output
circuit, parallel P and N channel IGTs, 175 and 176
respectively, are used. The IGT gate signal is
applied to a gate control circuit 178 which provides
two simultaneous gate control signals (of opposite
polarity) for controllinq (i.e.~ turning n~ ~n~
IGTs 175 and 176. The emulation section of IGT 175
is provided with series connected burden resistor 180
and the emulating section of IGT 176 is provided with
series connected burden resistor 181. An IGT current
signal, indicative of the load current in the IGTs,
is provided by comparing the signals developed across
the two burden resistocs 180 and 181 in differential
c~mp~rator 183. A transient voltage suppressor 185
20 i6 connected in parallel with the main section of the
IGTs and between the IN/OUT terminal 186 and the
input device device return terminal 187. The return
terminal 187 is also electrically common with one
side of the ac line. A pre-load resistor 189 is
connected between the IN/OUT terminal 186 and the
load return terminal 190. This latter terminal, 190
is connected to the other side of the ac line.
When the circuitry of Fig. 7C is operative as an
output, gate control 178, in response to an IGT gate
30 signal, commands the IGTs 175 and 1~6 to
simultaneously be either on or off, thereby switching
the load current on or off. The load 191 is
connected between the IN/OUT terminal 186 and the
load return terminal 190. When operated as an input,
~2~7748
21-IYP-2470
-27_
load 191 is not connected, and an input switching
device 192 is connected between the IN/OUT terminal
186 and the return terminal 187. The IGTs 175 and
176 are held in the off state and ~he state (i.e.,
S the status) of the input switching device 192 is
determined by the presence or absence of voltage on
the load voltage line; the presence of vol~age
indicating a closed input switch.
Referring to Fig. 8, showing the control and
sensing section in greater detail, the ON/OFF signal
from the communications section is apnlied tc one
input of NAND gate 195, to inverter 196, and to the
ceset inputs of flip-flops 19~ and 199. The other
input of NAND gate 195 receives the output signal of
NAND gate 201. The first input of NAND gate 201 is
supplied with a signal which is either high or low,
depending on whether the output ciccuit is to be
operated as an ac output or as a dc output. It will
be reco~nized that this signal may be provided by a
switch or wiring jumper appropriately connecting the
ac~dc select line to a high or low reference value.
The remaining input of NAND gate 201 receives a
signal from zero crossing detector 202, through
inverter 201a, to indicate those instances in which
the ac line voltage (for ac output circuits) is
within a certain range of zero voltage. Thus, in the
case of an ac output, NAND gate l9S passes the ON/OFF
signal only during a zero crossing of the ac line
voltage. Zero crossing detector 202 may be any one
of a number of conventional circuits providing a
signal indicating that the ac input signal is within
some range of a zero crossing. For a dc output, the
state of NAND gate 201 allows the ON~OFF signal to be
passed by NAND gate 195. The ON/OFF signal from NAND
~2~774B
21-IYP-2470
-28-
gate 195 is applied to the set inpu~ of flip-flop
203. The Q output of flip-flop 203 is applied as one
of the three inputs to AND gate 205, the output of
which serves as the IGT gate signal.
The remaining two inputs to AND gate 205 are
supplied by the Q outputs of flip-flops 198 and 1~9.
Flip-flops 198 and 199 are both reset when the ON/OFF
signal goes to the off state. Flip-flop 198 receives
a set signal from comparator 207 whenever the IGr
cucrent exceeds a pre-selected value. Thus, a signal
indicative of IGT current is applied to the in~erti~
input of comparator 207 while a reference voltage
representing an excessive level of IGT current is
applied to its non-inverting input. For example, the
ceference voltage may have a value corresponding to
30 amps of current. Similarly, flip-flop 199
receives a signal on its set terminal from power
supply monitor 209. Power supply monitor 209 may be
any one of a number of well-known means providing a
signal indicative of whether the dc power supply
voltage is above or below some pre-selected value.
operatively, therefore, a low supply voltage o~ an
excessively high IGT current will inhibit AND gate
205. This forces the IGT ~which is connected to the
output of AND 205) to an off state in which it
remains until the fault condition is cleared.
The Q output of flip-flop 198 is provided for
use as an overcurrent shutdown signal and is one of
the six switch state signal6 provided to conductor
30 bus 115 (Fig. 4). The Q output of flip-floP 199, in
addition to going to AND gate 205, is also applied as
one input to logic gate 210. The signal from power
supply monitor 209 is applied to the remaining input
of logic gate 210 so that its output signal is
~ 77~8
21-IYP-2470
-29-
indicative of the status of the dc power supply.
This output signal is also one of ~he six switch
state signals.
Flip-flop 203 receives a rese~ signal from the
output of NAN~ gate 212. Of the two inputs to NAND
gate 212, the first is the inverted ON/OFF signal
from inverter 156 and the second input is from NAND
gate 211. The ac/dc selection signal is provided tc
one input of NAND gate 213 and the output of
comparator 214, ~hrough inverter 201b, is provided to
th^ O~h2r input. Comparator 214 is a monitor
COmpaIatOr for IGT current and has the IGT current
signal applied to its inverting input. A reference
voltage corresponding to a relatively low, minimal
IGT current value te.g.. 0.05 amps) is applied to the
non-inverting input of comparator 214. This
combination, comprising N~ND gate 212, inverter 196,
NAND gate 213, and comparator 214, is operative
through flip-flop 203 to prevent the IGT from being
switched (in an ac mode of operation) unless the IGT
load current is less than the reference value.
The IGT current signal is also applied to the
non-inverting input of comparator 215 wherein it is
compared with an intermediate reference current
value. The intermediate reference current value
(e.g., corresponding to two amperes) is applied to
the invecting i~put of compacator 215. However, also
connected to the non-inverting input of comparator
215 is a time delay network comprising resistor 216
and capacitor 220. The combination of resistor 216
and capacitor 220 causes the voltage at the
non-inverting input of comparator 215 to be delayed
with respect to the I~T current. Thus. only if the
IGT current exceeds the reference value for an
1~ 7748
21-IYP-2470
-30-
extended period of time will the output of comparator
215 be affected. If the overcurrent is merely of
short duration, then no change of state of compara~or
215 occurs. Both the output of comparator 215 and
the output of comparator 214 are p~ovi ded as switch
state signals. These signais serve as diagnostic
signals and indicate, respectively, whether the IGT
current is above or below the intermediate reference
value and whether it is ab~ve or below the low
reference value so that corrective action can be
initiated by the microcontroller if necessary.
In case the IGT current exceeds the intermediate
reference value, corrective action is taken only if
the overcurrent is of sufficient magnitude and time
duration to trip comparator 215. That is, the load
current may exceed the intermediate reference value
for some time before correclive action is ta~en. It
is preferable, in some instances, tn eliminate the
time delay network (i.e., resistor 216 and cap~titor
220) and caLry oUt the time delay function by
software routines implemented in the
microcontroller. Comparison of the IGT, or load
current, with the low, or minimal value, reference
allows the generation of a diagnostic signal (e.g.,
0.05A) that is indicative of whether a load i5
connected, or if connected, whether it is open. The
Q output of flip-flop 217 is a diagnostic switch
state signal indicative of whether or or not voltage
is present at the connected load. The set input
terminal of flip-flop 217 is connected to the output
of NAND gat.e 218. NAND 218 receives the inverted ac
zeco crossing signa] from inverter 219 on its first
input terminal and receives the output of comparator
221 on its remaining input terminal. Comparator 221
12~7~8
21-IYP-2470
-31-
compares the line and load voltages to provide a
logic signal which indicates whether the load voltage
is greater or less than a pre-selected percentage of
the line voltage. For example, the output signal may
be indicative of whether the load voltage is greater
or less than 70 percent of the line voltage. The
line and load voltages are applied, respectively,
through input resistors 223 and 224 to the input
terminals of comparator 221. Functiorally, NAND gate
218 prevents a change of state of the output of
f 1 ip-f]~p ~l7 ~henever the ac line voltage is within
a certain range of zero volts. In effect, therefore,
decisions regarding the status of the load voltage
are not made whenever the ac line voltage is near a
zero c~ossing.
Flip-flop 217 is reset by the output from NAND
gate 226. The first input of NAND gate 226 is
provided with the inverted zero crossing signal from
inverter 219 and the second input is provided with
the output from the comparator 221 after it is
inverted by inverter 227.
The remaining switch state signal is provided by
temperature monitor 229 and îs indicative of the
relative temperature of the IGT (or IGTs in the case
of an ac output) switching device. The temperature
monitor 229 is preferably a simple P-N junction
temperature detector 229 which i6 in good thermal
communication with the IGT. The temperature detector
229 may be selected, for example, to provide an
indication that the IGT temperature has exceeded
150 C.
Fig. 9, comprising Figs. 9A-9C, illustrates an
embodiment of the communications section (111 of Fig.
4) in greater detail. The output signals from timer
~77~8
21-IYP-2470
-32-
117 are derived from an RC timing networ~ comprised
of resistor 300 and timing capacitor 301. Resistor
300 and capacito~ 301 are connected in series between
a positive voltage source +V and a common circuit
point. The junction between the resistor 300 and
capacitor 301 is connected to the inverting input of
LOS comparator 303 and to the non-inverting inputs of
SYNC and CLOC~ comparators, 304 and 305,
respectively. Resistors 308-312 comprise a voltage
divider network in which the resistors are serially
connected between +V and the common circuit point.
Each junction between the resistors 308-312 of the
divider network thus provides a voltage reference.
The highest reference voltage, taken from the
junction between resistors 308 and 309, is applied to
the non-inverting input of comparator 303. The other
voltage reference values, in descending order of
voltage level, are correspondingly applied to the
inverting inputs of sync comparator 304 and clock
comparatoL 305, and to the non-inverting input of
control comparator 314.
The collector terminal of transistor 315 is
connected through collector resistor 316 ~o timing
comparator 301, the other end sf which is connected
to the emitter of tran~istor 315. The on-off state
of transistor 315 controls the charge-dischaege cycle
of capacitor 301 and is itself, in turn, controlled
by the Q output from flip-flop 317. A resi6tor 318
is connected between the base terminal of transistor
315 and the Q output of flip-flop 317. The reset
terminal of flip-flop 317 receives the output signal
from control comparator 314. Control comparator 314
continuously compares the vGltage across the timing
capacitor 301 (applied to the inverting input of
21-IYP-2470
-33-
compaIator 314) with the reference voltage from the
junction o~ resistors 311 and 312.
In considering operation of timer 1~7, it may be
a:;sumed initially that the Q output of f lip-flop 317
is at a low level, holding transistor 315 off so that
capacitor 301 is charged to some level of voltage
such that the output of control comparator 314 is
low. Under these conditions, a rising edge of a
pulse applied to the clock input of flip-flop 317
through buffer amplifier 320 causes a high level to
appear at the Q output. This turns transistor 315
on, discharging timing capacitor 301. With the
discharge of capacitor 301, the CLK signal output of
comparator 305 is forced to a low level. The output
of comparator 304, if not already low, is also forced
to low and the output of LOS comparator 303 is forced
high if it is not already in that state.
The discharge of capacitor 301 is detected by
comp~rator 314 whose output goes high, resetting
flip-flop 317. The Q output of flip-flop 317 then
goes low, turning transistor 315 off, thus allowing
the capacitor 301 to begin recharging. Once the
recharged voltage is sufficiently high, the clock
comparator 305 is triggered, producing a high level
CLK signal. If capacitor 301 is allowed to continue
to charge, some voltage level will be reached which
will trigger, first the SYNC comparator 304, and then
the LOS comparator 303. The SYNC comparator 304 is
thus triggered by a "missing pulse" and the LOS
comparator .s triggered by a loss of SIG las~ing for
a~proximately 2.5 T as has been described.
In Fig. 9B the SIG and CLK signals are applied
to output data filter 119 which includes flip-flops
325 and 326, exclusive N02 gate 329, NAND gate 328,
7~8
21-IYP-2470
-34-
inverte~ 330, and transmission gates 331 and 332.
The SIG and CLK pulses are applied, respectively, to
the D and C inputs of flip-flop 325 which operates to
retain, at its Q output, the high or low state of the
immediately previous SIG pulse so that the values of
the first two pulses of a frame are compared. When
the clock pulse appears, ~he SIG value is either high
or low depending on whether the pulse value is 75
percent or 25 percent duty cycle. For a 25 percent
duty cycle pulse, the Q output of flip-flop 325 is
forc~d low: f~l a 75 pQr-ent duty cycle pulse, the Q
output is high. Thus, there is in effect a sampling
of the SIG value at each occurrence of the ~lock
- pulse. The Q output value from flip-flop 325 is
applied to one output of exclusive NOR gate 329 and
the SIG value is applied to its other input. Thus,
the current pulse value and the previous pulse values
are compared in exclusive NOR 329 whose output is at
a high level whenever the inputs are the same.
The output from exclusive NOR 3Z9 is applied as
one input to NAND gate 3Z8 which receives count
pulses S0 and Sl, respectively, on its other two
inputs. The values of S0, S0, Sl and Sl, taken
together, indicate which pulse in a frame is being
received. ~herefore, if the first two pulse values
of a frame are the same and if it is the second pulse
that is being received, the output of NAND gate 328
assume6 a logical zero value. At all other times and
under other conditions, the output of NAND gate 328
is a logical one.
A logical zero at the output of NAND gate 328
thus indicates agreement between the first two pulses
of a frame and a valid condition for updating the Q
output of flip-flop 326. To that end, the output
7~
21-IYP-2470
-35-
from NAND gate 328 is applied in parallel to the
input of inverter 330 and opposing control terminals
of transmission gates 331 and 332. A logical zero at
the output of NAND gate 328 causes transmission gate
33Z to be tuLned off and transmission gate 331 to be
turned on passing the control signal SIG to the D
input of flip-flop 326. The occurrence of a clock
pulse then clocks the new value through to the output
of flip-flop 326.
On the other hand, if there is a lack of
red~r.d~nc, .r. thc fi~ t .wo æulses of a frame, the
output of NAND gate 328 is a logical one, causing
transmission gate 331 to be held off and transmission
gate 332 to be held on. Under these conditions, the
15 output of flip-flop 326 is fed back through gate 332
causing flip-flop 326 to hold the previous output
state. The Q output of flip-flop 326 therefore
represents a filtered version of the on-off signal
which is then passed to output selector 120.
In addition to the filtered on-off signal,
output selector 120 receives the LOS signal and the
hold last state and complementary default signals,
HLS and DEF respectively. The function ~f output
seleccor 120 (which includes NOR gates 335-337 and OR
gate 338), is to select a desired value for the
output O~/OFF signal in the event of a loss of
communications between an I/O point and the
microcontroller, i.e., a 106s of the control signal
SIG. Should such a loss in communications occur, the
output selector 120 provides an output ON/OFF signal
which is either the last transmitted value of SIG or
a default value, depending on the signals HLS and DEF
supplied as control inputs to the selector 120.
The HLS and DEF signals are generated by the
21-IYP-2470
-36-
hold-last-state latch 123 and ehe default latch 124,
respectively. These latches are substantially
identical~ but respond to different pulses in a
control signal frame. The HLS latch 123 includes
5 NAND gate 340, tcansmission gates 342 and 343,
inverter 344, and flip-flop 345; the default latch
124 (Fig. 9C) includes NAND gate 348, transmis~ion
gates 349 and 350, inverter 352, and flip-flop 353.
Since the ciccuit confi3uration and operation of
these two laeches is substantially identical, only
~he ~S latch '23 requires any detailed explanation.
The HLS latch 123 responds to the third pulse in
a control signal frame (i.e., it responds to high
level S0 and Sl pulses from two bit counter 121) in a
manner that allows the latch output to be updated.
The SO and Sl pulses are applied as inputs to ~AND
gate 340 whose output controls transmission gates 342
and 343. The output of NAND gate 340 is applied to a
first set of opposing control terminals of
20 transmission gates 342 and 343 and to the inverter
344. The output of the invecter 344 is applied to a
second set of opposing contcol terminals of
transmission gates 342 and 343. Thus, in operation,
transmission gate 343 is turned on and transmission
gate 342 is tucned off by the occurrence of a third
pulse in the control signal frame. Since the control
signal is applied as the input to transmission gate
343, the signal is passed thcough to the D input of
flip-flop 345, thereby updating the HLS signal which
is taken from the Q output of flip-flop 345. The HLS
output is also fed back to the input of transmission
gate 342 so that, in the absence of a third pulse in
a control signal frame, the HLS value remains
latched. The clock signal is applied to the CLOC~
1~77~8
21-IYP-2~70
-37-
input of flip-flop 345. The output of the HLS latch
123 is supplied to the output selector 120.
By comparison~ the default latch 124 operates in
substantially the same manner but responds to the
fourth pulse in a frame. That is, the default latch
cesponds to the SO and Sl pulses of a control signal
frame. Notable, however, is the fact that the output
of the default latch 124 is taken from the Q output
of flip-flop 353 so that the complementary signal DEF
is supplied to the output selector 120.
In nnr~al operations, the output selector 120
functions to simply invert and pass the control
signal from flip-flop 326 which signal ~hen becomes
the on-off output signal applied-to the control and
sensing section 113 (Fig. 4). However, upon loss of
communications between the I/O point and the micro
controller (i.e., a loss of the control signal SIG3,
the output ON/OFF signal is forced to a
predetermined, desired state determined by the LOS
and HLS signals. These latter signals are both
applied as inputs to the output selector 120. In the
event there is a loss of communications, the output
selector 120 either holds the last s~ate or selects a
default state, depending on which has been
pre-selected. The pre-selection is made to force the
I/O point to a preferred, safe state should there be
a communications 10s6.
The LOS and HLS signals are inputs to NOR gate
335 whose output is one input to NOR gate 337. The
second input to NOR gate 337 is the signal from the Q
output of flip-flop 326. Thus, NOR gate 335 controls
NOR gate 337 so that if either LOS or HLS are at a
high level, NOR gate 337 simply inverts the control
signal from flip-flop 326. On ~he other hand, if LOS
1~7~4~
21-IYP-2470
-3~
is low (loss of communications) and HLS is also low,
the output of NOR t3ate 335 is high, holding ~he
output of NOR gate 337 at a low level.
The LOS, HLS, and DEF signals are applied to NOR
gate 336 whose output, along with the output from NOR
gate 337, aee applied as inputs to NOR gate 338. The
output of OR gate 338 is the control ON/OFF signal.
Thus, with a loss of communications (LOS low) and no
command to hold the last state (HLS low), the output
ON~OFF signal from OR gate 338 is selected to be the
default signal, DEF (i.e , n~F becomes invected by OR
gate 336). The operation is such, therefore, that if
there is a loss of communications and the hold last
state is not selected, a default condition is
selected. Whether the last state is held if the
default condition is selected is, of course,
controllable by appropriately setting the HLS latch
123 and the default latch lZ4.
The foregoing describes the forward pa~h through
the control and communications section 111 in
detail. The return of encoded diagnostic
information, is, as has been discussed above, through
state latch 125 and one of four data selection 129.
The encoding of the information is discussed in
detail in connection with Fig. 10; however, at this
point it is sufficient to note that the inputs,
X0-X3, I.o ~;tate latch lZ5 are encoded to contain the
diagnostic and other information to be ceturned to
the miceoconteoller 90 of Fig. 3. The state latch
125 may be a commercially available device such as
the Model MC14174, available fLom Motorola Inc. The
encoded information, X0-X3, is latched into the state
latch 125 on the rising edge of the SYNC signal which
is also supplied to the state encoder 125. Thus, a
:~2~7~
21-IYP-2470
_~9_
new set of data is latched in on each frame of the
control signal. This data forms a diagnostic signal
indi~tive of the operating parameters of the IJ0
point.
The data from state latch 125 is transmitted
bit-by-bit through one of four data selector 129 to
the microcontroller 90 through buffer amplifier 360.
The data selector 129 responds to the current value
from 2-bit counter 121 to cause the values of X0-X3
to be fed through in order. Thus, for example, as
the first pulse ir. a .rame i~ beLn~ received, the X0
bit of diagnostic data is simultaneously
transmi~ted. The data selector 129 may be a
commercially available device, such as the Model
MC14052 feom Motorola, Inc.
Fig. 10 illustrates a truth table for a state
encoder such as encoder 125 of Fig. 4. An encoder
in accordance with the truth table of Fig. 10 may
readily be implemented with standard combinational
logic elements by one of ordinary skill in the art.
Referring to Fig. 10, the inpu~ conditions are
listed horizontally across the top of the left-hand
portion of the table. Underneath, in columnar
fashion, are the possible value that each input may
take. In the table, ~'ones~ indicate that a value is
true (e.g., a high level signal), "zeroes" indicate
that a value i5 not true, and X's indicate "don't
cares" ti.e., may either be one or zero without
effect). The 4-bit output (X0-X3) of the state
encode. 125 is shown in the right-hand portion of the
table whe~ein X0-X3 a~e distributed horizontally
across four columns. Each horizontal row across the
four columns is thus a 4-bit word which uniquely
defines the state of the I/0 point. This 4-bit word
1~l'7 ~ ~
21-IYP-2470
-40-
is the diagnostic data which is returned to the
microcontroller 90 of Fig. 4 and ultimately to the
controller CPU (Fig. 1).
For example, in the truth table, the first row
shows a high level in the low voltage column while
the remaining columns ace indeterminate "don't care~
conditions. Under these circumstances the 4-bit
output is uniquely deteemined to be all zeroes. This
all zero 4-bit word signals a los~ of the l/O point
power supply. By further example, cow six shows that
t~le output is csmmanded on.~ bu~ that the output is in
a shorted condition. That is, a one appears in
column one under ON/OFF indicating that the I/O point
is to be turned on, while simultaneously, there is an
overcurrent indication in the overcurrent column
(col. 6). The 4-bit output word for this condition
is all zeroes except that X3 is at the one level.
Similarly, ~here is a set of fifteen unique 4-bit
words that define the various conditions of the IJO
point.
The foregoing describes features of an improved
input/output system having utility in connection with
programmable controllers. While the best mode
contemplated for carrying out the invention has been
described, it is undecstood that various other
modifications may be made therein by those of
ordinary skill in the art without departure ~rom the
inventive concepts inherent in the true invention.
Accordingly, it i6 intended by the following claims
to claim all modifications which falI within the true
spirit and scope of the invention.
