How to catch short momentary alarms?

I've got a situation where certain alarm conditions trip out equipment and the underlying device is either a relay or timer contact that resets as soon as the equipment trips. The integrator says the alarms signal is too short for his plc to "catch". We're using multiple AB-SLC-5/05 with attached panelmates in the field and Intellution HMI as the central SCADA package.

Is the integrator correct in his claim that the plc isn't capable of remembering a very short momentary input? I'd hate to think I'm going to need to go around and put seal-in relays all over the place.

The plc has a "scan time". That is the inputs are updated every so many milliseconds. This time is dependent on the speed of the processor and the size of the program. If your alarm is shorter than this scan time, you may not see it.

You should be able to put a mechanical device that latches between the alarm device and the plc input to hold the input. You would then reset the latch with an output from the plc.

Just how long is the alarm there for if it's on for less than the scan time then it could miss it, but your scan time should not be that long this biggest prog we have on site (also on a 5/05) has a scan time of 37ms..

Hope this helps

As the others have indicated, if the duration of the alarm signal is less than one PLC scan, then it could easily be missed. But that would have to be a fast-acting signal.

I wonder if what your integrator meant was that the alarm signal was too fast for his SCADA to catch? Most SCADA systems can't update all their tags every PLC scan. It may be possible for you to program software latches in the PLC to maintain the alarm signals long enough for the SCADA to sense them.

Steve is right about putting the "seal-in" relays in your PLC program and not externally. It is highly unlikely that the relay contact is faster than your scan, but it's possible. Another trick that you can use with most PLCs is an immediate instruction for the input. That might help you catch the input, since it forces the logic to read the status as the rung is executecd, and not at teh beginning of the scan.

from what you said in original post, I beleive your Ladder logic is seeing the input, executing reset action, but the SCADA comm is too slow to identify what cause the reset.

If so, just add a rung with all outputs that can cause the reset OR'ed
to start a new timer (a few sec delay) which latches it self on once started.

Then use this timer's TT bit to latch all those outputs so he will capture them. Adjust the time as required.

A point about Imediate Instruction use suggested by Tom J.

Imediate instruction by itself will not improve the capture of input
changes occuring more freq. than the Scan time. the logic is still
only acting upon the input status that has occured once per scan.

***Example:
If normal scan caught Input high, the imediate instr later in the
program might reset it if found as a zero.
However, using it multiple places in the program to latch a B3 bit
then move the existing logic rung to the end of program and use the
B bit as the input condition, and a final rung to reset the latched
B3 bit would probably accomplish what I think Tom wassuggesting in
his post

****Standard Use of Instruction Example:

Imeadiate Instruction is useful for updating data for use in logic
following after the instruction within the same program Scan. when
status or control outputs modified by logic prior to (often a
latched bit) may or maynot be unlatched near the end of the scan
based on this INPUT and other status conditions modified somewhere in between during this single scan.