Determining I/O for PLC Program

Hello,

I need to determine analog and digital I/O for a new ladder logic program I am going to be creating. Are there guidelines? How does one go about this task? I have googled this and found some advice but it is a bit limited. Thank you for your advice.

The IO is more to do with the equipment connected.... have you got a list of components for whatever you are controlling ??

... and don't forget to include some spares. There's always a good chance someone is going to think "oh hang on, we forgot about sensor XYZ that we want to get a reading from."

I normally try to include 20% spare I/O, but if that means an extra card with nothing connected then I just make sure there's space in the rack for that card in the future, if it's required.

And if your rack will be completely full or you're at the maximum number of I/O cards just with your existing program, then it may be worth considering a the next system up.

Hi. Thank you for your advice. According to the given specs I am controlling 7 gates for a water irrigation project with 3 different types of gates that includes actuators, limit switches, pushbuttons, etc and just trying to sort it out. Some of the I/O is given in the spec but is vague. I guess since this is a quote I don't want to underestimate what is really going to happen.

You need to get the specs clarified before you go further. Getting into a project that you don't understand is a recipe for disaster.

Which excellent consultant drafted up your spec? Welcome to the world of automation. .. You'll see some real shockers, trust me.

A vague spec isn't always a bad thing provided there are no overarching clauses that make you responsible for getting THEIR equipment to work. It gives you some leeway to design a system that will work well.

I would look at the manuals for the equipment they are specifying and decide for yourself what IO you need to be able to make this work smoothly, over and above what they have specified.

Just make sure to detail any extras in your quote, or provide a conforming quote and a non-conforming quote and detail why you think they should accept the non conforming one. Otherwise someone else will just price for what has been asked and the client will likely just select on price.

I've awarded tenders before to the 2nd or 3rd most expensive contractor purely because I saw they had thought it out, picked up on some things I hadn't thought about, and I knew that I wouldn't end up with a dozen variation orders half way through the job!

Hello Saffa,

Thank you for your encouragement. I do agree I need to know more about how these 3 different types of gates for water management for a river interact before creating my ladder logic and HMI code. From the spec there are level and flow meters for these gates with alarms attached. I assume I need to ask how each gate depends on each other? I have never done this before so am a bit overwhelmed. Hopefully by asking the correct questions and gaining understanding this will be achieved. Any further advice is appreciated.

dskplc said:

Hello Saffa,

Thank you for your encouragement. I do agree I need to know more about how these 3 different types of gates for water management for a river interact before creating my ladder logic and HMI code. From the spec there are level and flow meters for these gates with alarms attached. I assume I need to ask how each gate depends on each other? I have never done this before so am a bit overwhelmed. Hopefully by asking the correct questions and gaining understanding this will be achieved. Any further advice is appreciated.

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Get a GA of the project (and any process diagrams they might have), write all the sensors and actuators on it then get them to tell you how it works. PLC coding is usually split between function and faults - they will be able to tell you about the function but you might need to think more about the faults. If there is a process drawing that is often a good place to start for an HMI layout. If there isn't then it might be an idea to sketch something during your meeting.

Sounds like you've been dropped in it, keep telling yourself it's the best way to learn...

Are you a junior engineer with a someone mentoring you on this, or have you just picked up a job that was a little more then expected?

Because you could get yourself into a bit of hot water if you balls this up badly enough. Don't want to scare you or anything, but if you don't have someone else at your company who is making sure you are on the right path then speak out!

PM me if you're really stuck. Don't mind giving out a little free assistance on the phone. But drawings would be required at a minimum

But basically they should provide you with drawings at the very least and preferably a specification that clearly states how devices should interact, I.e. - "when the irrigation call signal goes high, flow control valve FCV501 opens until the flow rate as measured by flow meter FIT302 reaches Channel 1 Flow Setpoint ".

Cheers,

Steve