No trouble with electrical schematics but lost in cnc ladders

Hi, new member looking for help trying to understand why it is I have no problems going through hard wired electrical schematics, but struggle when I try to understand plc ladders. I just cant seem to follow them, especially cnc onboard ladders. I have no trouble checking I/O status indicators on non cnc plcs, but cant understand those programs either. Anyway, welcome any help or suggestions as to where I should start.

Thanks

Welcome to the PLCTalk forum community !

First, start by describing the make and model of the CNCs you're working with. If you're familiar with a particular type of PLC controller whose syntax and editor you are more comfortable with, describe that and people might be able to explain some of the differences.

In general when somebody posts "I don't understand PLC controllers" we just point them to the "Learn PLCs" link at the top; the Forum is just the popular community part of this website, and the basic tutorials about relay ladder logic and instructions and addressing and execution are really excellent.

Ummmmmmm . . .
On many schematics, the output devices are shown on the print . . .
The relay contacts that activate them are shown on the print . . .
And the relay coils themselves are also shown on the print along with what makes them pull in.

Frequently the coils and associated contacts are in different places - sometimes even different pages in the print set.

In the PLC logic the output devices are not shown. In the schematic the PLC is a 'black box' with the inputs coming in and the outputs going to the output device. But the program is lots and lots of relay contacts and logic blocks.

In my case, I can work forwards - find the input device then which input number it goes into the PLC black box then find the input in the logic program and chase it along. Or I can work backwards - find the output device and which output number turns it on or off on the schematic then find that output in the logic and chase it backwards.

I think the real difficulty is that the PLC not only does relay logic - or the equivalent in something that looks like a programming language . . . it also does things like string calculations, arithmetic, etc. etc. etc. and those things do not translate into electrical schematic diagrams. They are 'black boxes' within the program that also control relay coils, have contacts etc. etc.

It takes time and study . . . and Ken has the right idea.
Poet.

@Noob, you're not the first one to feel that. It doesn't get easier, you simply get better at it.

KingPresident said:

@Noob, you're not the first one to feel that. It doesn't get easier, you simply get better at it.

Click to expand...

That's what I was afraid of... lol

Mad_Poet said:

Ummmmmmm . . .
On many schematics, the output devices are shown on the print . . .
The relay contacts that activate them are shown on the print . . .
And the relay coils themselves are also shown on the print along with what makes them pull in.

Frequently the coils and associated contacts are in different places - sometimes even different pages in the print set.

In the PLC logic the output devices are not shown. In the schematic the PLC is a 'black box' with the inputs coming in and the outputs going to the output device. But the program is lots and lots of relay contacts and logic blocks.

In my case, I can work forwards - find the input device then which input number it goes into the PLC black box then find the input in the logic program and chase it along. Or I can work backwards - find the output device and which output number turns it on or off on the schematic then find that output in the logic and chase it backwards.

I think the real difficulty is that the PLC not only does relay logic - or the equivalent in something that looks like a programming language . . . it also does things like string calculations, arithmetic, etc. etc. etc. and those things do not translate into electrical schematic diagrams. They are 'black boxes' within the program that also control relay coils, have contacts etc. etc.

It takes time and study . . . and Ken has the right idea.
Poet.

Click to expand...

Exactly all this and then some. Thanks for the input.

Ken Roach said:

Welcome to the PLCTalk forum community !

First, start by describing the make and model of the CNCs you're working with. If you're familiar with a particular type of PLC controller whose syntax and editor you are more comfortable with, describe that and people might be able to explain some of the differences.

In general when somebody posts "I don't understand PLC controllers" we just point them to the "Learn PLCs" link at the top; the Forum is just the popular community part of this website, and the basic tutorials about relay ladder logic and instructions and addressing and execution are really excellent.

Click to expand...

Thanks, and yes started looking at those basic tutorials already and ran into something already that was confusing to me. In the tutorial "the level application " starts by saying the sensors are both normally closed but yet they are not drawn that way in the ladder example, which is contrary to what I am used to.

Also to give you a specific from your previous comments, Like a fanuc onboard ladder from say a 15t control. I get confused looking at those as they seem to almost show the opposite of what I would expect to see most times with regards to status.

Anyhow advice taken, will continue beating my head down the logic path... Thanks

KingPresident said:

@Noob, you're not the first one to feel that. It doesn't get easier, you simply get better at it.

Click to expand...

Haha! Now quit trying to scare him. It definitely gets way easier!

I'll put my 50 cents in on this one. Stupid inflation. First of all, watch these 11 videos:

https://www.youtube.com/watch?v=zlsJxSK8tPE

Second, you need to realize that, though they appear similar, ladder logic programs are in fact NOT equivalent to hand-drawn schematics. Hand drawn schematics follow one of two form factors (in general). Either everything is shown in the "standard state" where none of the contacts are actuated, and everything is in its normal, untouched position or they are drawn in an actuated position, where everything is "on" or the switch is in a specific position.

Ladder logic is NOT like this. The contacts are looking for a specific value in a memory address throughout a scan cycle. This can be synchronous or asynchronous, meaning outputs immediately energize if a rung is true for asynchronous, or outputs do not energize until all of the logic has been executed for synchronous. The very first thing you should do is find out what kind of scan cycle your controller has. This is PARAMOUNT and in my eyes the single most important thing to learn first.

Now, lets look at your example of the ladder logic using an NO contact, -| |-. This means that as the rung is being evaluated, left to right, top to bottom, if the value assigned to that instruction is a 1 (or ON, or true), then the rung is true to that point and will continue to be evaluated. So, it does not matter if you are using NC or NO sensors. The important thing of note is the STATE you wish that sensor to be in to make the rung true. You can use an NO contact or an NC contact and get the exact same logical effect.

Watch those videos, start looking at ladder logic in those terms, and for pete sake learn the scan cycle of your PLC. This will help you understand what is going on.