Adding I/O to your PLC

Hey, maybe this is a little too obvious for most of you, but I had a problem today where I needed an extra digital output in my PLC and I had none free. I couldn't wait nor afford to buy an additional I/O module, so I came with this idea ( Nothing revolutionary but...)

As most relays have 4 NO/NC contacts, I used 2 of them connected in series as the scheme in the pic shows and with only 2 digital outputs controlling them I can manage 3 physical outputs congrats

The disadvantage is that you cannot have 2 outputs ON at the same time, but maybe this is not a problem for your application, as it was not in mine.

A little dumb? Well, maybe...but it saved my day!

Enjoy...

Just be sure to document what you did and why. Next person that comes behind you to do any programming related to the outputs will be confused otherwise.

I would suggest that you put the addition of an output card on your list of things to do.

TWO WORDS....POOR PRACTICE.

I'm assuming that the PLC outputs are digital and your relays must be mechanical. One failure of a relay brings down all three outputs. Also very difficult to troubleshoot let alone understand.

In three words...don't do it. Or if you've done it. Undo it.

Thanks for your comments...

I know it's not a good reliable solution. I know that as NICK B said it can be considered as Poor Practice (It's almost like going back to hard-wired control panels), but what would you have done? Can you share some other alternative?

I did this because as I said I needed to leave the equipment working yesterday and I couldn't wait to buy an extra module (things usually take between 2-4 weeks to arrive to where I work), so that was a solution I came up with in despair. I know it's not the smartest thing I've donde so far, but it is a solution and that is what we get paid for.

So, while I wait for the extra I/Os to come, this will be working ok, and everyone in maintenance is aware that if something on those 3 outputs fail, the first thing to do is to check those 2 relays.

Thanks again, but honestly, I'd like to hear what other options I had in a case like this.

I didn't really follow your drawing but I understand that you have basically used the fact that both outputs are on as an 'extra' condition with which to control something.

As others have commented, this is going to be quite difficult to fault find on so I hope you have made sure the drawings and software are well documented.

I understand your reasoning behind it (i.e. unable to get an output card quickly enough etc) and I think it was very resourceful of you.

Between your solution or the production line being off due to a lack of an output then I would choose your solution every time.

As has already been mentioned though, I would retrofit a new output card as soon as possible and put the configuration back to how it should be.

;-)

Good work around but like others have said, make a point of getting a new O/P card installed... what you going to do if you need 4 outputs ?

Just make sure you leave adequate information for others that may have to do work on the equipment !!

Regards

JV

Spookily I have just used this method. I had 8 PLC outputs that had to control 12 contactors.