Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
TL-88-01
Z(~0979~
SECURE TRANSMISSION IN REMOTE CONTROL
AND SUPERVISION SYSTEM
DESCRIPTION
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention generally relates to
telecommunications systems and, more particularly,
to a method of secure transmission in remote -
control and supervision systems particularly
adapted for railroad trains.
Description of the Prior Art
End of train (EOT) devices are now in common
use on several major railroads. These devices are
typically mounted on the trailing coupler of the
last car in the train and are equipped with
pressure monitoring and telemetering circuitry. A
hose is connected between the train's brake pipe
and the EOT device so that the air pressure of the
brake pipe at the end of the train can be
monitored. The monitored pressure is transmitted
to a head of train (HOT) device mounted on the
console in the locomotive cab. The HOT device
provides the engineman with a display of brake pipe
pressure at the end of the train and, in response
to a command from the EOT device, signals the
engineman that an emergency condition exists such
as a sudden loss of air pressure or air pressure
falling below a predetermined level.
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The current EOT transmissions are one way;
that is, the transmissions are from the end of the
train to the head of the train only. It has been
proposed to provide two way transmission in EOT
systems so that command and control transmissions
can be made from the head of the train to the end
of the train. One application for such a
transmission would be for the remote control of the
emergency application of brakes at the end of the
train. Currently, emergency application of brakes
is initiated at the locomotive and progresses along
the brake pipe to the end of the train. This
process can take considerable time for long trains,
and if there is a restriction in the brake pipe,
the brakes beyond the restriction may not be
actuated. By initiating the emergency braking
process at the end of the train as well as the head
of the train, the process not only can be
considerably shortened, but more importantly, the
brakes will be applied at the end of the train even
if there is a restriction along the length of the
brake pipe. Thus, a fail/safe emergency operation
of the brakes is provided.
U.S. Patent No. 4,641,892 to Schmid discloses
an emergency brake system which enables the
engineman in the locomotive cab to remotely
initiate emergency braking at the end of the train
or at any intermediate car of the train. Schmid
merely discloses a telemetry link from the
locomotive to a remote radio receiver for
transmitting an emergency brake command. This
system does not recognize the potential problems of
such a system which may be caused by accidental or
improper transmission to the receiver by a
~ransmitter other than that in the locomotive.
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In end of train systems, it is necessary to
key receivers on a train to respond to transmitters
on that particular train and not to other
transmitters which may be on trains in the
vicinity, whether in a train yard or on the road.
In current EOT systems, this is typically done by
dialing in a number in the HOT receiver in the
locomotive cab that corresponds to the transmitted
identification (ID) number of the EOT unit mounted
at the end of the train. This "keying" operation
is important to assure the validity of data
transmitted to the engineman in the locomotive, but
it becomes critical when the emergency application
of brakes is concerned. Further, such emergency
control must be safeguarded against accidental
actuation caused, for example, when an engineman in
another locomotive dials in the wrong ID or even
sabotage which might occur by the intentional
transmission by an off-train transmitter of an
emergency brake command.
U.S. Patents No. 3,273,145 to Joy et al., No.
3,380,399 to Southard et al., 3,639,755 to Wrege,
No. 3,699,522 to Haner, Jr., Nos. 4,553,723 and
4,582,280 to Nicols et al., No. 4,723,737 to
Mimoun, and No. 4,735,383 to Corrie all disclose
various train communication systems. The Joy et
al. and Mimoun systems involve no security in
transmission. Southard et al. disclose a remote
control and supervision system for master and slave
locomotives in a train which uses addressing and
multiple transmission techniques to insure the
integrity of the communication link. The Wrege and
Haner, Jr. systems use an addressing scheme to
provide some level of security in the transmission.
The two Nichols et al. patents are both directed to
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TL-88-01 2~0~79~
an air brake operating system which includes
apparatus for verifying the establishment of a
communication link by signalling through the
mechanical coupling in the train. Corrie discloses
a procedure for transmission and acknowledgment of
vital control information.
SUMMARY OF TH~ INVENTION
It is therefore and object of the present
invention to provide a transmission protocol for
use in transmitting commands to an end of train
(EOT) device for controlling a train's brake
system.
It is another object of this invention to
provide a technique for the secure transmission of
control and command signals from the head of a
train to the end of the train, or to intermediate
positions of the train, to control various
functions, including brakes, lights and the like.
It is a further object of the invention to
provide a transmission protocol for the secure
remote control of emergency and testing operations,
particularly for the control and testing of
railroad brake systems.
According to the invention, the Head of Train
(HOT) device, typically located in the locomotive
cab, is provided with a thumbwheel switch or
similar input device for inputting numbers which
identify an End of Train (EOT) device. In the
simplest implementation of the invention, the EOT
device will respond to any HOT that dials its
number and it will do whatever it is told to do.
To prevent dangerous commands from being executed
in error, either several repetitions of the command
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must be sent to the EOT device by the HOT device within 9790
a predetermined period of time before the command, or
the command takes the form of a very large predefined
number or a combination of the two. This is the minimum
security provided by the invention but has the advantage
of being simple while limiting the possibility of an erroneous
e~ecution of a command.
The next level of ~ecurity provided by an alternate
e~bodiment of the invention requires only a minimum increase
in circuit complexity yet provides security against all
except very sophisticated and knowledgeable n saboteurs n
with proper equipment. In this embodiment, a memory i6
added to the EOT device and a unique ID number is ascigned
to the HOT device. The HOT unique ID is communicated
to the EOT device during an arming æequence, and the unique
ID i 6 imbedded in the memory of the EOT. Once armed,
with the Hot unique ID stored in the memory, the EOT device
will respond only to that particular HOT device. It will
be observed that this level of security is not mutually
exclusive with the basic implementation of the invention,
and the two procedures can be combined to provide an even
greater level of security.
In a further embodiment of the invention, security
is increased by replacing the unique ID of the HOT device
with a "pa~word". The "pas6word" can consict of a very
large (e.g. 32 bits) number that has the characteristic
of being the product of two unique prime numbers. The
choice of 32 bitC is arbitrary and could be larger or
smaller. The added ~ecurity of this embodiment i8 provided
3~ by another characteristic of the "password" which is that
it i~ a difficult mathematical task to derive the two
unique prime
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TL-88-01 2(J~
numbers. The system of this embodiment works as
follows: First, during the arming sequence, the
unique ID ti.e.~ the "pas~wordn) iB ~ent
from the HOT to the EOT which stores the ID in its
memory. After that, the EOT responds only to
instructions that carry either the "password" or
the two unique prime numbers associated with the
"password". For non-critical commands, like
turning the lights on and off or other routine
commands, the "password" is sent. For a
critical command, the unique pair of prime numbers
is sent by the HOT device. Then the EOT device
multiplies these numbers and compares the product
to the "password" stored in the memory.
Alternatively, the password may be any
predetermined number which is transmitted by the
HOT device scrambled in a predefined manner. The
EOT device, using a "descrambling" algorithm,
descrambles the transmitted number and compares it
to the "password" stored in the memory.
BRIEP DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, aspects and
advantages of the invention will be better
understood from the following detailed description
of a preferred embodiment of the invention with
reference to the drawings, in which:
Figure 1 is a block diagram of an end-of-train
(EOT) device;
Figure 2 is a block diagram of a head-of-train
(HOT) device;
Figure 2A is a block diagram of an existing
HOT device showing a retrofit to provide
bidirectional communication;
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l'L-8~-01 2(~0979~
Figure 3 is a flow diagram illustrating the
procedure for keying a HOT to an EOT in a first
embodiment of the invention;
Figure 4A is a flow diagram illustrating the
- 5 command procedure according to a first variation of
the first embodiment of the invention;
: Figure 4B is a flow diagram illustrating the
command procedure according to a second variation ;
of the first embodiment of the invention;
Figure 4C is a flow diagram illustrating the
command procedure according to a third variation,
combining the first and second variations, of the
first embodiment of the invention;
Figure S is a flow diagram illustrating the
arming procedure at the HO~ according to a second
embodiment of the invention;
Figure 5A is a flow diagram of a modification
of the process shown in Figure 5 where a two-man
arming process is implemented;
Figure 6 is a flow diagram illustrating the
arming procedure at the EOT according to the second .
embodiment of the invention;
Figure 6A is a flow diagram of a modification
of the process shown in Figure 6 where a two-man
arming process is implemented;
Figure 7 is a flow diagram illustrating the
logic of the program for transmitting an emergency
brake command from the HOT according to the second
embodiment of the invention;
Figure 8 is a flow diagram illustrating the
logic of the program for executing an emergency
brake command at the EOT according to the second
embodiment of the invention;
Figure 8A is a flow diagram of a modification
35 of the process shown in Figure 8 where a two-man
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TL-88-01 2(1~/9 7 9~
arming process is implemented;
Figure 9 is a flow diagram illustrating the
arming procedure at the HOT according to a third
embodiment of the invention;
- 5 Figure 10 is a flow diagram illustrating the
arming procedure at the EOT according to the third
embodiment of the invention;
Figure 11 is a flow diagram illustrating the
logic of the program for transmitting an emergency
brake command from the HOT according to the third
embodiment of the invention; and
Figure 12 is a flow diagram illustrating the
logic of the program for executing an emergency
brake command at the EOT according to the third
embodiment of the invention.
DETAILED DESCRIPTION OF A PREFERRED
EMBODIMENT OF THE INVENTION
~ eferring now to the drawings, and more
particularly to Figure 1, there is shown a block
diagram of an EOT device which may be used in the
practice of the invention. The EOT device includes
a pressure transducer 20 which monitors the
pressure of the train brake pipe 21 and generates
an electrical signal that is sampled and converted
to a digital number. The digital number is input
to a microprocessor driven circuit 22 which
processes the digital number and stores the number
for transmission to a HOT device. Periodically, or
if the digital number changes by more than one
unit, the microprocessor 22 controls a radio
transceiver 24 to transmit the pressure data to the
HOT device. The EOT devices currently in service
comprise these three basic components, except that
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TL-88-01 Z(~(~5, ~
the transceiver 24 is simply a transmitter since
the communication between the EOT device and HOT
device is one way. In addition, the EQT is
typically provided with a test switch 23 and a
display 25 which allows pressure to be displayed on
command.
As mentioned, it is desireable to transmit to
the EOT from the HOT in order to remotely control
various functions, including the emergency
application of brakes. Therefore, the transceiver
24 not only transmits data to the HOT, it also
receives commands from the HOT. The microprocessor
driven circuit thus processes the commands and, in
the case of an emergency brake command, generates a
signal to a valve actuator 26 that drives a valve
28 connected to the brake pipe 21. ~s will be
explained in more detail hereinafter, the
implementation of the second and third embodiments
of the invention requires the addition of a memory
29. The memory may be implemented with an EPROM
(electronically programmable read only memory) or
similar device. It should be noted that the memory
need not be a nonvolatile memory since the EOT is
operated on battery power thereby inherently
providing battery backup. In addition, the EOT may
be provided with an arm enable switch 27 and an
armed light 30, both of which are optional. The
arm enable switch 27 would be used in the practice
of certain embodiments to be described in more
detail below.
Figure 2 shows in block diagram form the HOT
device, which in many respects is similar in
overall ~esign to the EOT device. More
particularly, the HOT device includes a
microprocessor driven circuit 31 which receives
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2~10~7~
input8 from a thumbwheel dial switch 32, an emergency
brake switch 33 and other command or function switches
34. The microprocessor driven circuit 31 is provided
with an EPROM 3S, similar to EPROM 29. The microprocessor
driven circuit 31 tran~mits commands to and receives data
from the EOT via a transceiver 36. Data received from
the EOT device and other information useful to the engineman
i6 displayed on a suitable di~play device 38 such as a
CRT (cathode ray tube), LED (light emitting display),
LCD (liquid crystal display) or other such display device.
Similar to the EOT device, the HOT device may be optionally
provided with an arm switch 37.
Figure 2A shows a conventional HOT device with
a retrofit to provide the function~ of the HOT device
shown in Figure 2. In Figure 2A, like reference numerals
de6ignate identical or similar components as shown in
Figure 2. More specifically, the HOT device is illustrated
in the figure above the dotted line and the retrofit component ,
below the dotted line. The HOT device is already equipped
with a microprocessor driven circuit 31a and in the retrofit,
this is supplemented by a logic unit 31b. The HOT device
is also equipped with an RF receiver 36a. The tran~mit
function is accomplished by the addition of a transmitter
36b coupled to the antenna by a transmit/receive 36c.
In operation, the microprocessor driven circuit 31a initiates
a test by simply outputting a "test" command logic unit
3~b, When the EOT receives a "te~t" transmis~ion, it
responds with an acknowledge transmission. When the micro-
proce6sor 31a receives this acknowledge transmission from
the EOT, it knows the ~ystem i6 operational. Otherwise,
the logic unit 31b repetitively performs the dedicated
functions described below under the control of the crystal
oscillator 39.
In the first embodiment of the invention, the
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TL~88-01
11 2~
arming procedure at the HOT, as illustrated in
Figure 3, is very simple. Basically, all that may
be done is to read the dialed number of the EOT
device at function block 40. This number is
entered by the engineman using the thumbwheel dial
switch 32~ In order to do this, the engineman must
know the number of the EOT that has been installed
at the end of his train by railroad personnel.
This is normally communicated to the engineman via
voice radio. This procedure is no different than
that already in use.
For additional security, the arming procedure
can be done in a manner that requires the
coordinated action by two individuals. The purpose
of this procedure is to discourage or prevent
malicious and/or erroneous arming by the engineman
at the HOT. For example, if the engineman were to
accidentally dial in the wrong EOT number at the
HOT and if the EOT having that number were within
radio range, the wrong EOT would be erroneously
armed.
In this two-man procedure, the EOT with the
arm enable switch 27 shown in Figure 1 and the HOT
is provided with arm switch 37 shown in Figures 2
or 2A. The EOT can only be armed for a period of
time, say ten seconds, after its arm enable switch
27 is pressed. The engineman then must press the
arm switch 37 on the HOT within this ten second
time period. This two-man arming procedure may be
used in any of several embodiments to be described
or, alternatively, the arming procedure can be
carried out by the engineman alone.
Although the arming procedure may be
conventional, the command procedure initiated at
the HOT is entirely new. The first variation is
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TL-88-01 12 2(~09~90
shown in Figure 4A. In decision block ~2, at test
is made to determine if the engineman has actuated
switch 34, shown in Figures 2 or 2A. If not, the
program loops waiting for an input from emergency
brake switch 34. When an input is received from
switch 34, the HOT transmits the emergency brake
command together with the EOT number a
predetermined number of times, as indicated in
function block 44. At the EOT, the command
execution procedure, as shown in Figure 4A,
monitors received transmissions for the EOT number
in decision block 46, and when the EOT number for
that particular EOT device is detected, the decoded
transmission is checked in decision block 48 to
determine if it includes the emergency brake
command. Here it should be mentioned that it is
quite possible that other commands, such as
operating lights and the like, could be
implemented. But for purposes of this description,
we are only concerned with the emergency brake
command. If the emergency brake command is
detected, then in function block 50 the number of
times the command is detected is counted. A test
is made in decision block 52 to determine if a
predetermined period of time has passed after
beginning the count of emergency brake commands.
If so, the count is reset and the process exits in
function block 54. Otherwise, a test in made in
decision block 56 to determine if the count equals
a predetermined number. Note that this number may
be a number less than the predetermined number of
times the command is transmitted by the ~OT device
but in any case should be a sufficiently large
number to insure reliable reception of the command.
If the count equals the predetermined number, the
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TL-88-01 2(3~979~
13
brakes are applied in function block 57; otherwise,
the program loops back to decision block 52 to
again test the timeout period.
In a second variation of the first embodiment,
the command procedure at the HOT is illustrated in
Figure 4B as simply transmitting the emergency
brake command with the EOT number and some
predefined number, at function block 58. This
predefined number may be built into the EOT device
and read from a table in the HOT device or, as will
be described in more detail with respect to the
second and third embodiments, it may be loaded into
memory in the EOT device during the arming
procedure. The command execution procedure at the
EOT, as illustrated in Figure 4B, first tests a
received transmission to determine if the EOT
number for that particular EOT device has been
received, as indicated in decision block 60. When
the EOT number is detected, a test is then made in
decision block 62 to determine if an emergency
brake command ha~ been decoded. If so, a further
test i8 made in decision block 64 for the
predetermined number. If the number is not
detected, then the process exists without applying
the brakes, but if the number is detected, the
emergency brake procedure is initiated in function
block 65.
The third variation of this first procedure is
illustrated in Figure 4C and comprises a
combination of the first and second variations to
further improve the security of the procedure.
When an input is received from switch 34, the HOT
transmits the emergency brake command together with
the EOT number and the predefined number a
predetermined number of times, as indicated in
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- 14
function block 66. At the EOT, the command
execution procedure, as shown in Figure 4C,
monitors received transmissions for the EOT number
in decision block 67, and when the EOT number for
that particular EOT device is detected, the decoded
transmission is checked in decision block 68 to
determine if it includes the emergency brake
command. If the emergency brake command is
detected, a test is made in decision block 69 for
the predefined number. If the predefined number is
not detected, the process exits. However, if the
predefined large number is detected, then in
function block 70 the number of times the command
is detected is counted. A test is made in decision
block 72 to determine if a predetermined period of
time has passed after beginnins the count of
emergency brake commands. If so, the count is
reset and the process exits in function block 74.
Otherwise, a test in made in decision block 76 to
determine if the count equals a predetermined
number. If the count equals the predetermined
number, the brakes are applied in function block
77; otherwise, the program loops back to decision
block 72 to ayain test the timeout period.
In the second embodiment of the invention, the
arming procedure is an active procedure. At the
HOT device, the dialed number of the EOT is read at
function block 80 as before, but in addition, the
HOT transmits the EOT number with a uni~ue ID
number of the HOT device to the EOT, as indicated
in function block 82 in Figure 5. At the EOT, the
procedure is shown in Figure 6 as first testing for
the EOT number in decision block 84. When the EOT
number for that particular EOT device is detected,
a check is made in decision block 86 to determine
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TL-88-01 zn~7~
if the EOT is already armed. If it is, control
goes to other processes supported by the EOT.
However, if it is not yet armed, then a test is
made in decision block 87 for the unique HOT ID
number. When the HOT ID number is detected, then
it is stored in memory in function block 88, and
the process exits. Once armed, the EOT will
respond only to the HOT which transmits both its
EOT number and the unique ID number of the HOT.
As mentioned, the memory may be implemented as
an EPROM so that the EOT can be reset by clearing
the address space in the EPROM for the HOT ID
number and resetting a flag indicating that the EOT
has been disarmed. This would typically be done by
maintenance personnel in the yard prior to
installing the EOT on a train. In certain EOT
devices, access to the reset switch is only by
partial disassembly of the device; that is, by
separating the transceiver electronics package from
a battery module. Such partial disassembly can not
be done while the EOT is mounted on a coupler.
As previously mentioned, the arming procedure
may be a two-man operation requiring the
cooperation of railroad personnel at the end of the
train. In this case, the EOT and HOT are provided
with arm enable switch 27 (shown in Figure 1) and
arm switch 37 (shown in Figures 2 and 2A),
respectively. In this case, the arming procedure
shown in Figures 5 and 6 may be modified as shown
in Figures 5A and 6A. At the HOT, reading the
dialed number of the EOT may be dispensed with and
instead the arm switch 27 on the HOT is monitored,
as indicated by decision block 81. When the arm
switch 27 is pressed, the arm command is
transmitted with a unique HOT number to the EOT at
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16
function block 82a. In Figure 6A, the EOT detects
the arm command in decision block 83 and then
determines in decision block 85 whether the arm
command has been received within the time period
allowed for arming; e.g. ten seconds. If not, the
process exits; otherwise, a test is made in
decision block 86 to determine if the EOT is
already armed. If it is, the process exits;
otherwise~ the test is made in decision block 87 to
determine if the a unique HOT number has been
received. If not, the process exits without arming
the EOT, but otherwise, the unique HOT number is
stored in memory in function block 88. Thereafter,
the unique HOT number is used as a password for
controlling operation of functions at the EOT.
The command procedure at the HOT as
illustrated in Figure 7 first tests for an input
from switch 34 in decision block 90. When an input
from switch 34 is detected, the HOT transmits the
emergency brake command together with the EOT
number and the unique HOT ID number at function
block 92. In the case where the two man arming
procedure is implemented, it is not necessary to
transmit the EOT number.
At the EOT, the procedure as illustrated in
Pigure 8 is to first test for the EOT number of
that particular EOT device, as indicated in
decision block 94. When that number is detected, a
test is then made in decision block 96 for the
unique HOT ID number. If the unique HOT ID number
is not detected, control passes to other processes
supported by the EOT that may not require the HOT
ID. If the HOT ID number is detected, then a test
is made in decision block 98 to determine if the
emergency brake command has been decoded. If not,
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TL-88-01
17
the process exits; otherwise, the emergency brake
operation is initiated in function block 99.
Alternatively, when the two-man arming process
is employed, the process shown in Figure 8A may be
employed. In this case, it is not necessary to
transmit the EOT number. Instead, a test is made
in decision block 96 for the unique HOT number.
When that number is detected, a test is made in
decision block 98 for the emergency brake command.
If the emergency brake command is not detected,
control passes to other processes. If, however,
the emergency brake command is detected, the brake
process is initiated in function block 99.
It will be observed that the process described
with respect to Figures 5 to 8A inclusive represent
a variation of the procedure described with respect
to Figure 4B. As i~ the Figure 4B procedure, this
procedure could be combined with the procedure
illustrated in Figure 4A wherein the command is
transmitted a predetermined number of times before
the emergency brake operation i~ initiated.
Turning now to Figure 9, the arming procedure
for the third embodiment is illustrated. At the
HOT device, the dialed number of the EOT is read at
function block 100 as before, but in addition, the
~OT transmits the EOT number with a number which is
the product of two prime numbers, as indicated in
function block 102. This number is the "password".
At the EOT, the procedure is shown in Figure 10 as
first testing for the EOT number in decision block
104. When the EOT number for that particular EOT
device is ~etected, a check is made in decision
block 106 to determine if the EOT is already armed.
If it is, control goes to other processes supported
by the EOT. ~owever, if it is not yet armed, then
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TL-88-01 Z~0~7~
18
a test is made in decision block 107 to determine
if a "password" has been received.
Note here again that the arming procedure can
be a two-man operation as previously described. In
that case, it would be necessary for the arm switch
37 on the HOT be pressed within a predetermined
time period after the arm enable switch 27 on the
EOT is pressed before the process could proceed to
decision block 107. Failure to press the arm
switch 37 within the predetermined time period will
result in the process exiting without arming the
EOT. Assuming, however, that the process
continues, then when the password is received, then
it is stored in memory in function block 108, and
the process exits.
Once armed, the EOT will respond only to the
HOT which transmits both its EOT number and either
the password or the two prime numbers. The
password i8 effective for ordinary commands, while
the two prime numbers is required for the emergency
brake command. The command procedure at the HOT as
illustrated in Figure 11 first tests for an input
from switch 34 in decision block 110. When an
input from switch 34 is detected, the HOT transmits
the emergency brake command together with the EOT
number and the two prime numbers at function block
112. This is the process for critical commands,
such as emergency application of the brakes. For
non-critical commands, an alternative process is to
transmit the "password", i.e., the product of the
two prime numbers, rather than the two prime
numbers.
At the EOT, the procedure as illustrated in
Figure 12 is to first test for the EOT number of
that particular EOT device, as indicated in
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19
decision block 114. When that number is detected,
a test is then made in decision block 115 to
determine if the two prime numbers have been
received. If the two prime numbers are not
detected, a test is made in decision block 116 to
determine if the "password" is detected. If so,
control passes to other processes supported by the
EOT that may not require the security afforded by
the invention for critical commands. Note that by
using the "password", a certain level of security
is still provided for non-critical commands. If
neither of the two prime numbers or the password
are detected, the control returns to monitoring for
the EOT number.
If the two prime numbers are detected, then
they are multiplied in function block 117, and a
test is made in decision block 118 to determine if
the product equals the number stored in memory. If
not, the process exists; otherwise, a test is made
in decision block 119 to determine if the emergency
brake command has been decoded. If not, the
process exits: o~herwi~e, the emergency brake
operation is initiated in function block 120.
Again, the process described with respect to
Figures 9 to 12 inclusive may be considered a
further refinement on the process disclosed in
Figure 4B. Therefore, this process could also be
combined with the process shown in Figure 4A to
further increase the security of the system.
As was previously mentioned, the EOT may be
disarmed by maintenance personnel by resetting the
EPROM memory which stores the predefined number or
"password" and/or the EOT number. Vigarming can
be done by ~imply disconnecting the EOT battery
power~
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TL-88-01 ~ 0 9 7 9V
:
Alternatively, disarming can also be accomplished
by a command sent by the HOT which armed the EOT.
While the invention has been described in
terms of a specific preferred embodiment, those
skilled in the art will recognize that the
invention may be practiced with modification within
the spirit and scope of the appended claims.
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