Controlling Program Execution - GE PACSystems RXi User Manual

Controlling Program Execution

6.2
There are many ways in which program execution can be controlled to meet the system's
timing requirements. The PACSystems CPU instruction set contains several powerful control
functions that can be included in an application program to limit or change the way the CPU
executes the program and scans I/O. For details on using these functions, refer to the Proficy
Machine Edition online help.
The following is a partial list of commonly used methods:
■
The Jump (JUMPN) function can be used to cause program execution to move either
forward or backward in the logic. When a JUMPN function is active, the coils in the part of
the program that is skipped are left in their previous states (not executed with negative
power flow, as they are with a Master Control Relay). Jumps cannot span blocks.
■
The nested Master Control Relay (MCRN) function can be used to execute a portion of
the program logic with negative power flow. Logic is executed in a forward direction and
coils in that part of the program are executed with negative power flow. Master Control
Relay functions can be nested to 255 levels deep.
■
The Suspend I/O function can be used to stop both the input scan and output scan for
one sweep. I/O can be updated, as necessary, during the logic execution through the use
of DO I/O instructions.
■
The Service Request function can be used to suspend or change the time allotted to the
window portions of the sweep.
■
Program logic can be structured so that blocks are called more or less frequently,
depending on their importance and on timing constraints. The CALL function can be used
to cause program execution to go to a specific block. Conditional logic placed before the
Call function controls the circumstances under which the CPU executes the block logic.
After the block execution is finished, program execution resumes at the point in the logic
directly after the CALL instruction.
GFK-2816F August 2015
Chapter 6. Program Organization
115