Time Delay for Alarm in Ladder Logic

Hi guys,

I'm new to PLC programming and I'm trying to figure out the best way to take a program originally written in FIX 7.0 and translate it to ladder logic (specifically, a Function Block Diagram in ISaGRAF from ICLinks). The problem I'm facing is that the original FIX program relies on functionality that doesn't have a direct correlation in ladder logic.

So here's the basic gist of the program I'm trying to work on: there is an alarm that activates when a well pump is in local control; unfortunately there is no actual "local control bit" so the program implies local control by looking at a "pump running bit" (feedback from the remote site) and a "pump command bit" (command from master controller); if the pump is running and there's no run command after ten seconds, then it must be in local mode and conversely, if the pump isn't running after ten seconds but it's been given a run command, it's also in local mode...now I know this assumes a lot (namely, no failures) but it's an existing system that we're working with, so we're just trying to interface with it at this point.

The functionality I'm trying to figure out is, how would one tackle the challenge of creating the time delay functionality in ladder logic (which is continually running the rungs)? The main problem I'm noticing is that the original program uses indefinite wait and goto commands, which are not supported in any of the languages ISaGRAF has (IEC 61131). I'm working on the problem here (I'm thinking it's going to be a solution comprised of NO, NC and COS contacts) but I'd love to see what the more experienced programmers would do. Any help is greatly appreciated and thanks for taking the time to reply.

Here's what I've come up with so far:



The problem is that once the counter reaches ten, any change in logic will result in immediate alarm tripping; for example, on the top, if the pump is running and there is a pump command present, no alarm...but if the pump command goes away, it immediately trips...what I need is a ten second delay for that situation as well. Will continue to work on this, but thought I'd show what I had so far...seems to be close.

Try this:

ASF said:

Try this:

Click to expand...

Hey, thanks for the suggestion! I was actually one step behind you on this; I had the two contacts through an AND block that fed the counter, but when I looked at your response I realized that putting contacts in series is an automatic AND and my code was redundant. Cheers!