Control logix/Logix V16/First out

I tried to search the forumn for topics related to "first Out" alarmingm but didnt have much luck.
I noticed V16 of Logix5000 can somewhat support "first Out" alarming? Has anyone used this, or had any luck with it?
I have a number of digital inputs, through a few 1756-IV16 cards, that need to be monitored. The customer only tells me they want local "first out" annunciation. From what ive read, the rolling timestamp will only update per I/O card, as a whole (ie. I cant use the time stamp for two different inputs on the same card, to determine which input actually tripped first.)
I see Knowledgbase has a sample program for 5 inputs on a Logix500 platform.
Can anyone shed light on how this can be done on a Logix5000 platform. Or if anyone has any luck with the new alarm features of Logix5000 V16 - and if its a true "first out"?

Thanks
NH

nhicks,

Although this won't help you with RS5000, there is an example with RS500 in the thread i'm linking to. Near the end of this thread Ron Beaufort speaks about the pitfalls of the first out concept.


http://www.plctalk.net/qanda/showthread.php?t=35449

Heres the logic i use, its negative logic based. Once the bits are mapped, its fast. You can mask in and out the alarms you want, so setting up multiple sections of a plant from one main latch file is also simple. Overkill for 5 alarms, but it works

Ghetto,

If you have 2 or more PLC scans to determine your First Out, then it should work just dandy. I find that most systems have 5 or 6 scans before relays drop out, or another switch changes state, so a First Out WILL work well. Maybe not 100%, but good enough to make it worthwhile.

Yes, if more then one input changes on a single scan, then you will not be able to find the proper firstout. It will flag the first firstout it finds. It more used for a trouble shooting tool then any thing

Ghetto,
Am I understanding you corrcetly, that it is not possible to catch "first Out" on the same card, if more than one input changes?

If the application is critical try using 1756-IB16ISOE or 1756-IH16ISOE cards.

from nhicks:

Am I understanding you correctly, that it is not possible to catch "First Out" on the same card, if more than one input changes?

Click to expand...

actually it IS “possible” ... it’s just not 100% GUARANTEED RELIABLE ...



the simple reason is that the individual input devices built into the card do NOT all share exactly the same electrical characteristics ... specifically, some may react faster or slower than the others ... so if two or more inputs electrically turn OFF at the same time, there’s no way to guarantee that the PLC will see - and record - the bit status correctly ...



but don’t take my word for it ... experiment and prove it to yourself ... jumper a bunch of inputs together and then turn them all OFF at the same time - and have the PLC record the bit pattern as soon as the input data changes ... you’ll find that USUALLY all of the inputs will operate simultaneously - but NOT always ... it’s a random thing ...



the major idea is that a PLC-based “first out” tester MIGHT be fully adequate for your needs ... if it is, then party on ... on the other hand, you need to be aware that such an arrangement might occasionally give incorrect results ... just be aware of that potential limitation - and take action accordingly ...