CTU RSLogix 5

When using a CTU in RSLogix 5 how does it affect the MCR? In the literature it appeared that there is some sort of warning about using a CTU. I do not have the book with me to quote from. It may have to do with entering out of spec counter values or jumpering the counter with other logic in the program but for some reason I am not understanding the why of it. I have used counters in other programs with no issues, is there something specific or unique to RSLogix 5?

The counter will not necessarily affect the MCR. The MCR however can affect the CTU. The MCR "zone" has the ability to make a rung false regardless of the logic on that rung.

When your first MCR (MCRs are used in pairs) rung is true, all logic within that zone will operate unaffected by the MCR. Everything operates normally.

When your first MCR rung is false, all logic within that zone will be scanned as false, even if the logic is actually true it will be executed as false.

A commom mistake many people make with the MCR is to assume that it resets timers & counters and turns bit style outputs off. Really all the MCR does is make the rung false.

So what does a CTU do when it is false? It freezes at it's current value. It won't increment either as that would require a true rung.

So, just remember that an MCR makes the rung false. The output does whatever it normally does when it is false.

OG

** edit **

I believe you are referring to the message on page 43 of the PLC-5 Instruction Set Reference Manual which warns you to place critical counters outside the MCR since it may not present an accurate count inside an MCR.

Link to PLC-5 Instruction Set Reference Manual on the internets