Output from 2 pulses on input

Got a tricky one that I can get my head around - I am interfacing a micrologix 1400 with a system that only has a single output for two different alarms - One alarm is a latching output and the other is pulsed.

At this point, I only need to capture the second (pulsed) alarm.

The timing of the outputs on the system I'm interfacing can be adjusted, so I am thinking I set the first alarm to its minimum of 1 second, but can't get my head around how to write the logic for ignoring the first pulse and only have the line come true if it receives a second pulse.



Hope that makes sense.
Any suggestions?

Use a counter with a preset of 2?

I knew there would be an easy answer.
Thanks. It's been a long day.

You will however need some way to also detect when the pulsing stops. Maybe a XIO on the input driving a 5 second timer, to reset your counter?

What happens if both alarms are active? Hopefully the one you're interested in over rides the first, always on output!

Did something similar many years ago, needed somewhere in the region of 10 different codes from one PLC to another, there was only one spare output so a signal was sent either 1 to 10 on a mark/space ratio, the sending plc would then wait x seconds before sending another signal, so count the pulses within a given time assume 12 seconds at a pulse every second at the 12 seconds after the first pulse store the count & reset it in actual fact we stored the counts into a shift register it was actually a fault code from one PLC to another and store the last 10. We actually reduced the mark/space ratio & the send time until it went unstable then increased it until stable, cannot remember the exact timing but it turned out to be pretty quick.

parky said:

Did something similar many years ago, ... We actually reduced the mark/space ratio & the send time until it went unstable then increased it until stable, cannot remember the exact timing but it turned out to be pretty quick.

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IO-232? What about parity?

This was in the early 70's, on a PLC with no real way to communicate between them, the operation was quite slow compared to 232 and very simple no problems. you have to remember that in those days PLC's did not have the communication facilities of modern ones. Also did a system where we output an 8 bit parallel data from a Symax 300 to a Tralfa robot via an 8080 processor board, the 8 bit data used 7 bits for the data & 1 bit as a strobe, we had ordered a communications processor but the lead time from America was some 12 weeks. We called the unit PSICON pretty crude "parallel to serial converter"
All I remember is having to slow the 232 communications to 150 baud the robot would not reliably communicate any higher, just goes to show the tech at the time the robot memory was half a megabyte of bubble memory in a cabinet the size of a wardrobe.

parky said:

This was in the early 70's, ... half a megabyte of bubble memory in a cabinet the size of a wardrobe.

Click to expand...

Hilarious.


And to think that was high-tech at the time.


Thanks for sharing.