GE GFK-0787B user manual Programming for Global Data, Global Data for the Application Program

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Programming for Global Data

In a Series 90-70 PLC/Genius system, Global Data is data that is automatically broadcast by a PLC bus controller, each bus scan.

The GMR software uses this Global Data capability as the vehicle for exchanging system information between the PLCs. Each PLC provides 8 words of system data to the other PLCs as Global Data. Because Global Data messages can be up to 64 words in length, up to 56 additional words of Global Data capacity are available for use by the application program. Since each PLC can broadcast just one Global Data message per bus scan, the system Global Data and the application Global Data are a sent in the same message.

Global Data for the Application Program

The application program can send Global Data by placing it into %G memory, as detailed below. Each PLC uses %G0001 through %G0896 to send ªapplicationº Global Data. It is not necessary to use all of the references.

The application program can read Global Data received from the other PLCs from %GA, %GB, and %GC memory. In addition, each PLC can also read a copy of its own Global Data. As explained in the PLC chapter of this book, each PLC actually receives two sets of Global Data from each of the other PLCs. It gives preference to Global Data from the bus designated bus a. If that data isn't available, a PLC uses Global Data received from the bus designated bus b. Under ordinary circumstances, these two sets of data would match. The use of two busses provides redundant operation in case a bus or bus controller is not available.

The incoming Global Data is data that can be read in %GA, %GB, or %GC memory, therefore, is the Global Data received on bus a if that data is available. Otherwise, it is the Global Data received on bus b.

All PLCs	%G0001 ±%G0896	Global data to be transmitted.
PLC A	%GA0001±%G A0896	Copy of transmitted global data.
	%GB0001±%GB0896	Data received from PLC B
	%GC0001±%GC0896	Data received from PLC C
PLC B	%GA0001±%G A0896	Data received from PLC A
	%GB0001±%GB0896	Copy of transmitted global data.
	%GC0001±%GC0896	Data received from PLC C

PLC C

%GA0001±%G A0896	Data received from PLC A
%GB0001±%GB0896	Data received from PLC B
%GC0001±%GC0896	Copy of transmitted global data.

GFK-0787B Chapter 7 Programming Information 7-27