GE GFK-0787B user manual Communications Between PLCs, Global Data Redundancy

4

Communications Between PLCs

Data is transferred between the PLCs in the system using Genius global data. Two busses are used to transfer duplicate data. While the system is operating, they transfer global data automatically. This global data includes two types of information:

HApplication program global data from %G memory. The GMR software automatically copies this data into %R memory before sending it.

HAdditional %R data used by the GMR software.

Each scan of the Genius bus, a PLC takes the application program global data it has copied into %R memory, plus its own additional %R data, and broadcasts it on the bus.

During the same Genius bus scan, when the other PLCs have their turn on the bus, they send global data in the same way. When a PLC receives Global Data, it copies that portion of the data that is intended for application program use into %GA, %GB, or %GC memory (see the Programming chapter for details). The following diagram summarizes the transfer of GMR global data.

Sending PLC

Receiving PLCs

%G	%R
Memory	Memory
	GMR
Application	Global Data
Global	Application
Data
	Global
	Data

Genius Bus

'

%R Memory

GMR

Global Data

Application

Global

Data

%GA, GB. or GC

Memory

Application

Global

Data

Global Data Redundancy

During normal GMR operation, each PLC receives two sets of global data from each of the other PLCs (one set over each of the two busses mentioned above). The system defaults to use the data from the first bus (bus a) unless that bus has failed, in which case the data from the second bus (bus b) will be used). If a PLC loses communications with another PLC on both busses, the global data from that device is held at its last state. The GMR software places a fault in the PLC fault table when communications are lost. See the chapter on Diagnostics for more information.

In addition, the GMR software maintains status flags that can be monitored by the application program to check the state of communications between PLCs. These are described in the chapter on Programming.

Entering, Clearing, or Setting Global Data

The application program can read or transmit Global Data as required. Refer to the

Programming chapter for details.

In addition, the application program can use the PLC OK flag to clear or preset the data as required. This is also described in the Programming chapter.

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	User's Manual ± March 1995