Chapter 2. Program Organization24 PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual GFK-2950C2.2 Controlling Program ExecutionThere are many ways in which program execution can be controlled to meet the system’s timingrequirements. The PACSystems CPU instruction set contains several powerful control functions thatcan be included in an application program to limit or change the way the CPU executes the programand scans I/O. For details on using these functions, refer to Chapter 4.The following is a partial list of the commonly used methods:▪ The Jump (JUMPN) function can be used to cause program execution to move either forward orbackward in the logic. When a JUMPN function is active, the coils in the part of the program thatis skipped are left in their previous states (not executed with negative power flow, as they arewith a Master Control Relay). Jumps cannot span blocks.▪ The nested Master Control Relay (MCRN) function can be used to execute a portion of theprogram logic with negative power flow. Logic is executed in a forward direction and coils in thatpart of the program are executed with negative power flow. Master Control Relay functions canbe nested to 255 levels deep.▪ The Suspend I/O function can be used to stop both the input scan and output scan for onesweep. I/O can be updated, as necessary, during the logic execution through the use of DO I/Oinstructions.▪ The Service Request function can be used to suspend or change the time allotted to the windowportions of the sweep.▪ Program logic can be structured so that blocks are called more or less frequently, depending ontheir importance and on timing constraints. The CALL function can be used to cause programexecution to go to a specific block. Conditional logic placed before the Call function controls thecircumstances under which the CPU executes the block logic. After the block execution isfinished, program execution resumes at the point in the logic directly after the CALL instruction.
