GE GFK-0787B user manual Estimating CPU Sweep Time, Sweep Time Contribution of Genius I/O and GBCs

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Estimating CPU Sweep Time

The GMR system software runs on Series 90-70 CPU788 or CPU789 PLCs. It produces a ªbaseº CPU sweep time that becomes a part of the overall sweep time of the CPU with a ladder logic application program in it. This base sweep time should be taken into consideration during the application program design and development.

Base sweep time depends on GMR configuration parameters such as Input and Output table sizes. Typical base sweep times for 788 and 789 CPUs are shown below. In this example, there are six Bus Controllers in each PLC,

with table sizes of Voted %I = 64 Voted %AI = 64 Logical %Q = 64

with table sizes of Voted %I = 256 Voted %AI = 256 Logical %Q = 256

Base Sweeptime= 79 Milliseconds Base Sweeptime = 88 Milliseconds

The base sweep time for your system could be less or more depending on the table sizes you configure. Also, base sweep time varies by $ 10mS during single sweeps when the GMR system software performs diagnostics on the CPU subsystem and I/O subsystems.

Sweep Time Contribution of Genius I/O and GBCs

The contribution of Genius I/O and Genius Bus Controllers to the sweep time of the PLC CPU is similar to that of Series 90-70 I/O. There is an overhead for the I/O scan, a per Bus Controller sweep time impact, a per scan segment sweep time impact; and a transfer time (per word) sweep time impact for all I/O data.

The potential Bus Controller sweep time impact on the CPU has three parts:

1.Time to open the system communications window, added only once when the first intelligent option module (such as a Bus Controller) is placed in the system.

2.Time needed to poll each Bus Controller for background messages (datagrams). This must be added for every Bus Controller in the system.

3.Time needed for the CPU to scan the Bus Controller.

For detailed information about estimating CPU sweep time, refer to the Series 90-70 PLC Reference Manual (GFK-0265).

Important Note

In the section on Sweep Time Impact, the Series 90-70 PLC Reference Manual describes the technique of eliminating the first and second parts of the Bus Controller's sweep time contribution by closing the system communications window (setting its time to 0).

This technique should NOT be used in a GMR system.

4-6	GeniustModular Redundancy Flexible Triple Modular Redundant (TMR) System	GFK-0787B
	User's Manual ± March 1995