REF I/O Refresh - This instruction interrupts the execution of the program, write outputs, service comms, read inputs, then resumes the execution of the program. IOM Immediate Output with Mask - This instruction interrupts the execution of the program and updates the outputs, then resumes the program execution. IIM Immediate Input with Mask - This instruction interrupts the execution of the program and updates the inputs, then resumes the program execution.

It is intended to help capture specific events when troubleshooting and debugging. SUS Suspend - This instruction causes the PLC to enter Suspend mode and stores the ID in S:7. TND Temporary End - Acts just like the END instruction at the bottom of the routine but can be placed higher up, conditionally suspending the execution of the code after the TND. MCR Master Control Reset - All non-retentive outputs in the MCR zone will be turned off. Note that you can nest up to 8 subroutines with the Micrologix PLC unless you are using the STI subroutine, DII subroutine, I/O event-driven interrupt subroutine, or user fault routine, then you can only nest 3 subroutines. The RET Return instruction will immediately return to the JSR instruction that called it without finishing executing the code in its subroutine. SBR Subroutine and RET Return - The SBR Subroutine instruction serves no purpose and is always evaluated as true. Multiple JMP Jumps can use the same LBL Label. A JMP instruction will skip the execution of rungs until it encounters its associated LBL label. JMP Jump and LBL Label - These two instructions allow you to skip over rungs. Therefore, the RET is unnecessary and can only add confusion in how the program is scanned. If you look in the RsLogix 500 help file, it states 'Without a RET instruction, the END statement (always present in the subroutine) automatically returns program execution to the JSR instruction in your calling ladder program.'.

I've had a few people say that you should have a RET 'just in case'. Note that when using the JSR as done above the RET return instruction is not necessary.

Rslogix 500 programming examples pdf