Deboucing switch inputs in an IDEC PLC

Good Morning all.

I am have a PLC that takes 4 inputs from 4 alarm panels. The idea is that when the idividual alarm panel sees a fault, it provides an input to the IDEC PLC which then sounds an alarm etc. What would be a good software debounce suggestion for each input?

thanks All

Vic

You could build some code into your plc to include a timer so the signal from your alarm panel must be true for at least 1second or whatever is sufficent.

What kind of signal are you getting from the alarm panel?

Hi there Digitalfist. I am hoping that the alarm panel will give me a continuous "no fault" 24 volt DC output so I can build an "examine on" rung for each input. However, all four alarm panels are different so I need to be able to change the input function according to the panel.

Big,

I think "debounce" is not quite the correct term here. De-bouncing usually refers to adding a very short time delay for the switch contacts to settle (1 second or less). For alarms, I have found through hard experience that it is best to use a much longer delay time, 4 to 6 seconds, depending on how critical the alarm. I would call this "alarm delay time", not de-bounce time.

I am talking here about alarms that human ears will have to hear, for days without end. Most plant operators get very annoyed at listening to loud horns and buzzers. More than once, I have seen an operator knock an alarm horn off the wall. Think about this as you design your alarms. There are many things that a PLC programmer can do to make necessary alarms easier on the nerves. Why stress out your operator? Would YOU want to work where you know that a loud irritating noise may blast your ears at any moment?

1. Wait as long as possible before turning on the dammed racket.

2. Do not ever use a continuous-on horn. ALWAYS pulse the alarm, 0.6 seconds on, 3 or 4 seconds off. That OFF time reduces the stress level from 100 down to about 30. Try it, you will hear and feel a difference right away. The result is the same: you have told someone about a problem. It is unlikely anybody will react any faster simply because you used a continuous, distracting, blaring noise.

3. Turn the dammed thing off as soon as possible. Always provide a Silence or Acknowledge button. In many cases you can also program an Auto-Silence function that shuts off the horn after, say 2 or 3 minutes. If no one has heard it by then, you are probably out of luck anyway, depending on how critical the alarm.

4. Instead of audible alarms, look for ways for the program to take automatic action to correct the alarm condition. Only turn on an audible alarm if there is nothing ELSE that can be done without human intervention. When the operator hears this alarm, what actions will he be able to perform? Why cannot my program perform those actions also? Is it a dumb program, or what?