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Description  |
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TECHNICAL FIELD
The present invention relates, in general, to a communication system for a
plurality of programmable controllers and, more particularly, to a
communication system which duplicates the transmission of data or
information to and from a plurality of controllers via a plurality of
coaxial cables thus ensuring the integrity of the data or information
transmitted between same.
BACKGROUND ART
Numerous approaches are available to facilitate communication between
programmable controllers or the like within a communication network. Some
approaches utilize a communication topology wherein the programmable
controllers are interconnected in a network having a ring configuration.
With such a topology, if a fault occurs in a cable interconnecting the
programmable controllers, the direction of communication is reversed so
that communication is maintained via the other cables within the
communication ring. For example, if communication is occurring in a
clockwise direction between two programmable controllers in such a ring
configuration and a fault occurs in one of the cables interconnecting
same, communication automatically reverts to the counter-clockwise
direction causing the data or information to pass through the cables which
interconnect the other programmable controllers within the communication
ring.
Another approach which ensures the continuity of communication between
programmable controllers in a communication network involves the
utilization of a plurality of cables to interconnect the controllers. Such
approaches typically require the simultaneous communication over both the
cables and the comparison of the incoming signals at the receiving
programmable controller to determine which incoming signal should be
selected. In this case, each incoming signal is typically compared against
a preset value, and the signal which exceeds the preset value by a
predetermined amount is selected. Alternatively, the signals can be
compared against one another and the "stronger" signal selected. Since the
incoming signal is not analyzed as to data integrity and/or the presence
of noise, the possibility of forwarding faulty data or information to a
receiving programmable controller exists.
In view of the foregoing, it has become desirable to develop a
communication system which utilizes simultaneous communication over a
plurality of cables and which analyzes the integrity of the data or
information being received at the receiving programmable controller and
selects the cable based on the integrity of the data or information being
received, rather than incoming signal strength.
SUMMARY OF THE INVENTION
The present invention solves the problems associated with the prior art by
providing a dedicated modem for each programmable controller and each
processor within a programmable controller in the communication system.
The modems are utilized to interconnect the programmable controllers
and/or processors via pairs of coaxial cables. Each modem is provided with
three transceivers, one transceiver for each cable within a pair of cables
and one transceiver for interconnecting the modem to a programmable
controller or processor within a programmable controller. Each
transmission from a programmable controller or processor within a
programmable controller is simultaneously transmitted via its associated
modem over a pair of cables to all of the other programmable controllers
and processors within programmable controllers in the communication
system.
Each transmission is comprised of a series of flag characters at the
beginning of the transmission, the data or information being transmitted
and another series of flag characters at the end of the transmission. The
modem associated with each of the other programmable controllers or each
of the other processors within programmable controllers in the system
receive the transmitted data or information. The modems decode the signal
from each of the pair of cables, and pass the decoded signal, if it is
substantially defect-free, from the main cable to its associated
programmable controller or associated processor within a programmable
controller, via a channel selector. If, however, the signal on the main
cable is non-existent or includes a defect, the signal on the other cable
is passed to the receiving programmable controller or receiving processor
within a programmable controller via the channel selector. In this manner,
true redundancy exists with respect to communication between the modems,
the associated programmable controllers and the processors within
programmable controllers.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a communication network utilizing the
present invention and showing the interconnection of a plurality of
programmable controllers, each having at least one dedicated modem, via a
plurality of coaxial cables.
FIG. 2 is a schematic diagram of the transmit path through the modem
utilized in the communication network illustrated in FIG. 1.
FIG. 3 is a schematic diagram of the receive path through the modem
utilized in the communication network illustrated in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings where the illustrations are for the purpose
of describing the preferred embodiment of the present invention and are
not intended to limit the invention described herein, FIG. 1 is a
schematic diagram of a network 10 including a plurality of components
interconnected in a communication arrangement and utilizing the present
invention. The network 10 may include, for example, a programmable
controller 12; a programmable controller 14 having remote input/output
devices, shown generally by the numeral 16, connected thereto; and a
programmable controller 18 including redundant processors 20 and 22, each
having remote input/out devices, shown generally by the numeral 24,
connected thereto. Each of the foregoing components has a span redundancy
modem 26 associated therewith and the components are interconnected via
the modems 26 and coaxial cable sections. For example, programmable
controllers 14 and 12 are interconnected via their associated modems 26
and two coaxial cable sections, viz., cable sections 28 and 30. Similarly,
programmable controller 12 and processor 20 within programmable controller
18 are interconnected via their associated modems 26 and coaxial cable
sections 32 and 34. Lastly, processors 20 and 22 within programmable
controller 18 are interconnected via their associated modems 26 and
coaxial cable sections 36 and 38. Each span redundancy modem 26 is
connected to its associated programmable controller or processor within a
programmable controller by a cable section 40. Those programmable
controllers or processors within a programmable controller that utilize
remote input/output devices are connected to same by a cable section 42
and at least one span redundancy modem 26 associated with the remote
input/output devices.
The span redundancy modem 26 is a stand-alone unit having its own power
supply. In addition, the modem 26 has three transceivers, one transceiver
for each channel of communication, Channel A or Channel B, and one
transceiver for connecting the modem to its associated programmable
controller or processor within a programmable controller. In order to
accomplish the necessary communications within a network arrangement, each
span redundancy modem 26 is provided with three (3) 75 ohm BNC jack
connectors, one connector for attachment to the cable section for Channel
A, one connector for attachment to the cable section for Channel B, and
one connector for attachment to the cable section 40 which interconnects
the modem to its associated programmable controller or processor within a
programmable controller. For example, considering the span redundancy
modems 26 for programmable controllers 12 and 14, connector 50 on span
redundancy modem 26 associated with programmable controller 14 is attached
to one end of cable section 28 whose other end is attached to connector 52
on span redundancy modem 26 associated with programmable controller 12.
Similarly, connector 54 on span redundancy modem 26 associated with
programmable controller 14 is attached to one end of cable section 30
whose other end is attached to connector 56 on span redundancy modem 26
associated with programmable controller 12. Lastly, connector 58 on span
redundancy modem 26 for programmable controller 14 is attached to
controller 14 via its associated cable section 40. Similarly, connector 60
on span redundancy modem 26 for programmable controller 12 is attached to
controller 12 via its associated cable section 40. Similar connections are
provided between other span redundancy modems 26 and programmable
controllers or processors within programmable controllers in the network
10. Cable section 28 is utilized for transmission of Channel A between
span redundancy modems 26 for programmable controllers 12 and 14 and cable
section 30 is utilized for transmission of Channel B between same. As will
be described hereinafter, true redundancy exists in the foregoing network
communication arrangement inasmuch as the same data or information is
transmitted over Channels A and B to all of the programmable controllers
or processors within programmable controllers in the network 10.
As previously stated, each span redundancy modem 26 includes three
transceivers. Referring now to FIG. 2, the transmit path through a typical
modem 26 is illustrated. The transmit path includes a receiver 70
connected, via a cable section 40, to the programmable controller or
processor within a programmable controller associated with the particular
modem 26, a decoder 72 connected to the output of receiver 70, a
Manchester encoder 74 connected to the output of decoder 72 and
transmitters 76 and 78 connected to the output of Manchester encoder 74.
Transmitter 76 is associated with Channel A whereas transmitter 78 is
associated with Channel B. Referring to the previous discussion of the
interconnection of span redundancy modems 26 for programmable controllers
12 and 14, the output of transmitter 76 (Channel A) is transmitted over
cable section 28 whereas the output of transmitter 78 (Channel B) is
transmitted over cable section 30 when either of the span redundancy
modems 26 associated with the foregoing cable sections are in the transmit
mode.
Referring now to FIG. 3, the receive path through a span redundancy modem
26 is illustrated. When a span redundancy modem 26 associated with a pair
of cable sections is in the receive mode, both receiver 80 associated with
Channel A and receiver 82 associated with Channel B receive the same data
or information over their associated cable sections. This data or
information is decoded by Manchester decoder 84 which is connected to the
output of receiver 80 (Channel A) and by Manchester decoder 86 which is
connected to the output of receiver 82 (Channel B). An output (active
line.sub.-- A) of receiver 80 is connected to a flag detection timer 88
whose output is connected to a frame analyzer 90 for Channel A. Similarly,
an output (active line.sub.-- B) of receiver 82 is connected to a flag
detection timer 92 whose output is connected to a frame analyzer 94 for
Channel B. The output of Manchester decoder 84 is connected to a flag
detector 96 whose output is connected to frame analyzer 90. Similarly, the
output of Manchester decoder 86 is connected to a flag detector 98 whose
output is connected to frame analyzer 94. An output (R.sub.x Data.sub.--
A) of Manchester decoder 84 (Channel A) and an output (R.sub.x Data.sub.--
B) of Manchester decoder 86 (Channel B) are also connected to a channel
select circuit 100 which is also connected to the outputs of the frame
analyzers 90 and 94. The output of channel selector 100 is connected to an
encoder 102 whose output is connected to a transmitter 104 which is
connected to the programmable controller or processor within a
programmable controller to which the span redundancy modem is associated.
Transmissions over Channel A and Channel B are in the form of frames of
data or information each having a "flag" as an identifier at the start of
the frame. When in the transmit mode, a frame to be transmitted from a
programmable controller or a processor within a programmable controller is
decoded by decoder 72, encoded by Manchester encoder 74 and then
transmitted simultaneously via transmitter 76 (Channel A) and transmitter
78 (Channel B) within its associated span redundancy modem to all of the
other programmable controllers or processors within programmable
controllers in the network via the cable sections interconnecting same.
The receiver portions of the transceivers being utilized are disabled
during the foregoing transmission. The receiver 80 (Channel A) and
receiver 82 (Channel B) in the span redundancy modem associated with each
of the other programmable controllers or processors within programmable
controllers in the network receive the frames of data or information via
the pairs of cables interconnecting the modems, and decodes same by its
associated Manchester decoders 84 and 86, respectively. Each frame of data
or information is comprised of a series of flag characters, the data or
information being transmitted, and flag characters at the end of a frame.
Upon receiving a frame of data or information, receiver 80 (Channel A) and
receiver 82 "go active" causing their respective flag detection timers 88
and 92 to transmit a 40.mu. second timing pulse to frame analyzers 90 and
94, respectively. Flag detectors 96 and 98 review the decoded signals
produced by Manchester decoders 84 and 86, respectively, for the existence
of a flag character and transmit a signal to their respective frame
analyzers 90, 94 upon the occurrence of same. If frame analyzer 90
receives a signal from flag detector 96 indicating the existence of a flag
character while receiving the 40.mu. second timing pulse produced by flag
detection timer 88, frame analyzer 90 transmits a signal to channel
selector 100. Conversely, if frame analyzer 94 receives a signal from flag
detector 98 indicating the existence of a flag character while receiving
the 40.mu. second timing pulse produced by flag detection timer 92, frame
analyzer 94 transmits a signal to channel selector 100. As a result of the
foregoing process, the channel selector 100 determines which channel will
send the subject data or information to the associated programmable
controller or processor within a programmable controller. The selected
data or information is encoded by encoder 102 and transmitted, via
transmitter 104, to the associated programmable controller or processor
within a programmable controller. Thus, if there is a loss of data or
information on one of the channels, or some noise is present on a channel,
the channel selector 100 selects the other channel for transmission of the
data or information thereon to the associated programmable controller or
processor within a programmable controller. In essence, true redundancy
exists throughout the entire network communication arrangement.
Certain modifications and improvements will occur to those skilled in the
art upon reading the foregoing. It should be understood that all such
modifications and improvements have been deleted herein for the sake of
conciseness and readability, but are properly within the scope of the
following claims.
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