Events triggered by digital input in PLC

sameenkhan · Post by **sameenkhan** » 26 Apr 2010, 12:46

Hi Guys,

How ON & OFF events are triggered by digital input in the PLC??

On most sites, I have seen digital input change state from CLOSE to OPEN or OPEN to CLOSE and corresponding PLC events ON to OFF or OFF to ON respectively are reported...which normal and is understandable...

The big question comes, when PLC report ON to ON event & OFF to OFF event.. What is the main reason behind such events??

Regards,
Sameen

Post by **ali.abbas** » 09 Aug 2010, 09:45

Sameen,

Could you expand as to what you mean by ON and OFF events? Is this terminology being used by a specific make of PLC's?

My guess so far is that since PLC's have basically evolved from relay logic, ON and OFF might have to do something with energized or de-energized. Still, if you can just clarify a little, thatll be better.

Regards,
Ali Abbas

sameenkhan · Post by **sameenkhan** » 09 Aug 2010, 12:55

Hi Ali,
IO Module triggers ON state when voltage measured crosses ON threshold and same for OFF state..
It totally depends on the detection technique used...
OFF->ON event is reported by Input module when measured voltage crosses ON threshold i.e. measured voltage < 6Vdc (it may vary from PLC to PLC)
ON-> OFF event is reported when measured voltage crosses OFF threshold i.e. measured voltage > 17Vdc (it may vary from PLC to PLC)
For details on thresholds, see IEC 61131-2..

ON->ON event is reported when measured voltage goes above ON threshold but don't cross OFF threshold, meaning it remains in between ON & OFF thresholds, later it comes back and crosses ON threshold...it is also how system detects events... most system detect state change w.r.t time therefore ON-> ON event can be also translated as ON (past state) - > ON (present state)... same concept is also applicable to OFF->OFF event.

Such events are very useful in establishing field switch performance... In plant area, switches experience alot of vibration.. some time in case when PV (Process Variable) & SP of switch are very close than vibration of line & switch mounting plays are very vital role in chattering... Depending on nature of vibration, switch may chatter depending on quality of switch & sensing methodology.. now most process switch have mainly two resistances i.e. open switch resistance normally infinity & contact resistance depends on contact size & material.. during chattering you have neither open contact nor closed, meaning you experience contact operation at high frequency.. this phenomenon results in introduction of another resistance which lies in between.. this resistance drops voltage & that drop is measured by input module of PLC... but module does not declares it either ON or OFF state due to it undetermined nature.. that is when you see such ON-ON or OFF-OFF events...
I hope that you got the idea...

Regards,
Sameen

Post by **ali.abbas** » 14 Aug 2010, 11:49

Sameen,

Yeah Ive got the idea.This is quite an informative post, and well even though Ive come across the problem chattering quite a few times myself, I have to agree that I have never thought quite as deeply on these lines.
But to me it seems that you've just answered your own question

Regards,
Ali Abbas

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