Communication system

Details Communication system Communication system

2000-02-11

2001-10-09

Eng, David Y. (Department: 2155)

Electrical computers and digital processing systems: multicomput

Master/slave computer controlling

C714S018000

Reexamination Certificate

active

06301610

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to communications and, more particularly, to a communication system between a pair of communication devices. The invention also relates to a communication system between a pair of programmable logic controllers.
2. Background Information
It is known to transfer a collection of information from a master device to a slave device, and/or to transfer another collection of information from the slave device to the master device.
Examples of such master and slave devices include programmable logic controllers (PLCs) which are well-known in the art. PLCs are utilized to solve ladder logic which is represented by a power rail, a plurality of inputs or functional devices or contacts, a plurality of outputs or results devices or coils, and a neutral rail. The ladder logic includes a plurality of individual ladder diagrams, segments or rungs.
In addition to contacts and coils, data values in PLCs are often organized in a series of holding registers (e.g., 16-bit words). In tun, various “special functions” are employed in order to provide mathematical or logical manipulation of the holding registers, or to send and/or receive some of the holding registers via serial communication (e.g., RS-232, communication network). For example, a block move special function may be employed to send or receive a collection of holding registers to or from another device (e.g., another PLC, another controller, an input device, an output device).
A master PLC, for example, may transfer a collection of data, such as controls, to a slave PLC, and the slave PLC may transfer a collection of data, such as indications or display information, to the master PLC. One disadvantage of these forms of communication is that, typically, most of the information being sent is “static” rather than “dynamic”. In other words, between successive communications, only the dynamic data has changed. Thus, a significant portion of the bandwidth of the communication link between the master and slave devices may typically be wasted.
It is also known to transfer data between a first device and one or more second devices over a communication network within a specific time period. See, for example, U.S. Pat. Nos. 5,142,623; and 5,864,680.
In broadcast multi-cast communication networks, the messages are not addressed to any specific station. Each station receives all of the data transmitted and traditionally takes what it needs. This also requires processing of the message by the central processing unit of the receiving station. Typically, to reduce the burden on the system, data is only broadcast by exception (i.e., only when the value of a piece of data changes is a corresponding message sent). Thus, for instance, in a distributed process control system, where various functions are being performed by a number of stations on a network, data is only broadcast when the value of a variable changes. Other deterministic systems, such as automatic teller machines linked to a central unit by a network, also only send data when there is a particular transaction to process. However, such systems can become overburdened if there is a large disturbance in the process being controlled, or if a large number of transactions occur simultaneously. See, for example, U.S. Pat. No. 5,864,680.
In known transfers of collections of data between corresponding PLCs, it is believed that all of such data is transferred between the PLCs regardless whether any or all of that data has changed. For example, a large number of holding registers may be periodically sent even though only one holding register or bit, if any, has changed since the previous communication.
Although the prior art shows various types of communication systems, there is room for improvement.
SUMMARY OF THE INVENTION
In accordance with the invention, values are communicated between first and second communication devices. One of the devices determines the values along with a count of any of the values which changed after a previous determination of the values. That device alternatively selects for communication in a message, as a function of the count, either all of the determined values, or those values which changed since the previous determination of the values.
As one aspect of the invention, a communication system comprises a first communication device, a second communication device, and a communication channel between first and second communication means. The first communication device includes: first memory means for storing first information; first processor means (a) for repetitively determining a plurality of values as at least some of the first information, (b) for determining a count of any of the values which changed after a previous determination of the values, and (c) for alternatively selecting for communication in a message, as a function of the count, either (i) all of the values, or (ii) the any of the values; and the first communication means for communicating the message. The second communication device includes: second memory means for storing second information; the second communication means for communicating the message; and second processor means (a) for alternatively updating the stored second information with either (i) all of the values, or (ii) the any of the values of the message, and (b) for processing at least some of the stored second information of the second memory means.
As a refinement, the values are a plurality of first values and the message is a first message. The second processor means includes: means for determining a plurality of second values; means for determining whether any of the second values has changed; means for alternatively selecting for communication with a second message either (i) all of the second values, or (ii) such any of the second values; and means for processing at least some of the stored second information. The first processor means includes: means for alternatively updating the stored first information with either (i) all of the second values, or (ii) such any of the second values; and means for processing at least some of the stored first information.
Preferably, the second processor means further includes means for determining whether greater than a predetermined count of the second values has changed, and means for informing the first processor means that greater than the predetermined count of the second values has changed. As a further refinement, the first processor means includes means for requesting all of the second values after being informing by the second processor means that greater than the predetermined count of the second values has changed, and the second processor means further includes means for sending the first processor means all of the second values in a subsequent second message.
As another aspect of the invention, a communication system comprises a first programmable logic controller, a second programmable logic controller, and means for communicating information between first and second communication means. The first programmable logic controller includes: first memory means for storing first information; first processor means (a) for repetitively determining a plurality of values as at least some of the first information, (b) for determining which ones of the values changed after a previous determination of the values, and (c) for selecting for communication in a message the ones of the values which changed; and the first communication means for communicating the message. The second programmable logic controller includes: second memory means for storing second information; the second communication means for communicating the message; and second processor means (a) for updating the stored second information with the ones of the values which changed, and (b) for processing at least some of the stored second information of the second memory means.
Preferably, the first processor means includes means for alternatively selecting for communication with the message either (i) all of the values, or (ii) the ones of the values which chan

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