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Chapter 2. Program Organization
2.2 Controlling Program Execution
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 Chapter 4.
The following is a partial list of the commonly used methods:
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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.
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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.
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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.
24
PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual
GFK-2950C
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