dlorentz

Thanks for all your help Guys. Daba, that was a very good explanation of rung execution. It helped me understand what was happening. Also thanks JRoss, that helps me understand what they are trying to accomplish Daren

I have read that back when PLCs were older, it was bad form to use an output coil to drive an input on that same rung. it would produce a verify error if the input was executed after the coil was executed. We do it all the time on Holding Contacts and there is no problem, but here in this case, would it produce timing issues or un-repeatable results? This logic is used to trigger a Master Fuel Trip on a boiler. All main shutdowns ie Low level, Low Fuelgas pressure, run through a rung like this. It looks like they want to make sure the MFT relay energizes before the First Out, but I don't see how that happens.

I was wondering if somebody can help me with this small piece of logic. It was written by a company that we are getting a piece of equipment for, and I need to understand how it operates. Specifically what happens when VITAL POWER drops out. What is the execution order of the rung branches. Will both B3[62].0 and B3[50].15 latch, or will the latching of B3[50].15 stop the latching of B3[62].0? Then what happens to the middle branch B3[63}.0? does it energize. The picture doesn't show properly, but there are 2 latching coils at the end of this logic (B3[62].0 and B3[50].15) and one regular coil B3[63}.0. I thought this was badly written logic, but considering that these pieces of equipment operate all over the world, and I am a relative newbie at this, there must be more to the story. Daren logic question.bmp