Scan-dependent Logic

If you use controller-scoped tags and program a lower priority task so that one instruction is dependent on another instruction that occurs elsewhere in your program, a task interrupt and switchover can disrupt your programming. The disruption can occur because the higher priority task can interrupt the lower priority task and then a switchover can occur before the lower priority task is completed.

When the lower priority task is executed from the beginning by the new primary controller after the switchover, the dependent instruction can fail to execute at the most recent value or state. For example, if a higher priority task interrupts the logic as shown below, the value of scan_count.ACC is sent to the secondary controller at the end of the program in the higher priority task. If a switchover occurs before the primary controller completes the EQ instruction, the new primary controller starts its execution at the beginning of the program and the EQ instruction misses the last value of scan_count.ACC. As a result, any programming that uses the Scan_Count_Light tag can also execute by using incorrect data.

The following logic uses a Counter Instruction (CTU) to count each scan of the program. An Equal To (EQ) instruction uses the accumulated scan-count value as a reference to turn on an indicator when the thousandth scan is complete.