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4-6 Genius Modular Redundancy Flexible Triple Modular Redundant (TMR) System
User’s Manual – March 1995
GFK-0787B
Estimating CPU Sweep Time
The GMR system software runs on Series 90-70 CPU788 or CPU789 PLCs. It produces a
“base” 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 with table sizes of
Voted %I = 64 Voted %I = 256
Voted %AI = 64 Voted %AI = 256
Logical %Q = 64 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.