Understanding How The Controller Interprets Mcps - Allen-Bradley PLC-5 User Manual

16-2 Using Main Control Programs
Understanding How the
Controller Interprets MCPs
Publication 1785-UM012D-EN-P - July 2005
The MCPs are scheduled to execute in the order in which you specify on the
Crocessor Configuration screen. You can configure:
an I/O image update and housekeeping after each MCP is completed
(default parameter), or
the controller to skip the I/O scan and run the next MCP
After the last MCP is completed, all MCPs are then repeated in the same order.
Note that the watchdog setpoint covers one scan of all MCPs. The figure
below shows how the controller interprets MCPs when an I/O image update
is specified to occur after each MCP is completed.
MCP A
I/O image update
and
housekeeping
I/O pre-scan performed on
transition from program to
run mode
By disabling I/O scans between MCPs, you can gain 2-3 ms of program-scan
time per disabled I/O scan. The controller updates your I/O when it reaches
the next I/O scan command, which can be:
an enabled I/O scan between MCPs, and/or
the end of a pass through the entire MCP list.
The controller always performs an I/O scan after a pass through the MCP list.
The follwing figure shows how the controller skips I/O scans and moves to
the next MCP.
MCP A
MCP B
I/O image update
and
housekeeping
I/O pre-scan performed on
transition from program to
run mode
MCP B
I/O image update
I/O image update
and
housekeeping
Important: Mode changes to program or
remote program can occur between the
execution of MCPs.
MCP C
MCP D
I/O image update
I/O image update
and
housekeeping
Important: Mode changes to program or
remote program can occur between the
execution of MCPs.
MCP P
and
housekeeping
MCP xx
MCP xx
and
housekeeping