First day with Allen Bradley PLC...

Hello folks,

Well, the first day of my PLC training has come and gone, and I absolutely loved it....of course it was with Allen Bradley trainers, but the training was generic enough, I should be OK with the others.
After the introduction, and some theory on I/O, each of us were given a simple diagram to make a program for: one n/c stop switch, one n/o start switch, and an output to make a light come on. Pretty simple stuff.
Of course, my mind still tries very hard to work in relay logic, as thats what its used to, so for my n/c stop switch, I originally entered an XIC symbol, since it was a n/c switch...would not work! So then, after it was explained to me, I switched it to a XIO, and it worked perfect. So, I still don't really understand why I wanted an "examine if open" symbol for a n/c switch.
Can somebody explain this in simple words for me....is it because I want the processor to get a "false" reading when the n/c stop switch opens? I don't want to learn by repetition, I want to understand it.
And last of all, I'm learning ob AB PLCs, but we don't use AB at work, and never have. We had Siemens, and now almost all Schnieders.....are they going to be easy to get on to if I'm comfortable with the AB equipment?

Thanks for any thoughts guys.

Rembrant

Here's a thread that explains it pretty good.

After you read through that, do a search for "xic xio" and you will find hundreds more.


Then, when you get done come back and recite what you learned.


Maybe you could answer one of the most difficult questions posted here:

Describe the design characteristics of typical programmable logic devices - Give Examples of the unitary, modular and rack-mounted methods of implementation.

Click to expand...

The very simplest explanation of --| |-- and --|/|-- is as follows...

These are program questions posed to the computer.

The --| |-- symbol is asking the computer, "Is this bit ON?"

The --|/|-- symbol is asking the computer, "Is this bit OFF?"

As you can see, the "Normally Closed" Symbol does NOT indicate the type of switch existing in the field. In fact, the program doesn't give a hang (or a HOOT!) about what kind of switches or contacts you have in the field.

All the program wants to know is...
"Is it ON? --| |--
or...
"Is it OFF? --|/|--

That's it! Nothing more! Nothing less!

You might have a Normally Closed contact in the field.
--| |-- is simply asking if the signal from that contact is coming in.
--|/|-- is simply asking if the signal from that contact is NOT coming in.

You might have a Normally Opened contact in the field.
--| |-- is simply asking if the signal from that contact is coming in.
--|/|-- is simply asking if the signal from that contact is NOT coming in.

Nothing more! Nothing less!

Thanks guys, I just printed the thread linked above, and I'm going to read over it. I guess I just have to get my brain away from the traditional wiring diagram, and think ladder logic, and PLC symbols. I guess it just seems wierd to think of just the input, and not the actual switch in the field....at least in my case.
I'll get back to ya tomorrow.

Rembrant

AB vs Siemens

My first experience with professional PLCs (Not the ones I made myself) was with Siemens PLC. However, lately I learned to use AB PLCs aswell. It was easy to switch over although it was a little frustrating at first. ( I would often write 'Q' instead of 'o' for output or 'T:0', say, instead of 'T:0/dn"). From whateve little experience I have I can tell you that there are quite a few differences between AB and Siemens PLC. AB PLCs are perhaps the easiest to learn and use. Siemens, on the other hand offers a much powerful instruction set. I personally believe it is easier to switch from Siemens to AB than from AB to Siemens. Don't take it for granted though. For I too am only a novice.

Well, pardner, the way I look at it is different than my ole buddy Terry (WELCOME BACK!) does. Like you, I think in terms of relay logic, and NO and NC contacts.

Where I reckon you were getting confused was looking at the relay analogy a little wrong. Like Terry says, the PLC doesn't care what type of field device you have. I look at the input to a PLC as a relay coil (which it kind of really is). If you have power at the input, it is putting power to the input's "relay coil". In your program, when the input is on any contacts change state - the NO contacts close and the NC contacts open - just like a relay's contacts.

PS - Phil, I really like the quick edit button!

Rembrant said:

Hello folks...


one n/c stop switch, one n/o start switch, and an output to make a light come on....


I originally entered an XIC symbol, since it was a n/c switch...would not work! So then, after it was explained to me, I switched it to a XIO, and it worked perfect.

Click to expand...

Is this a typo? If you have a NC stop switch and you want to deenergize a coil in a typical "seal-in" rung, you would want the XIC. If the stop button were a N.O. then you would want the XIO.

--|/|--  is XIO

--| |--  is XIC

Here is how I am picturing your rung in RSLogix500:

Yes, it was a typo, or should I say lack of knowledge.....and maybe part of my confusion. The rung I had to make looked exactly like the one you have pictured above, exactly!
Where I was screwed up was, with wiring diagrams on my mind, I saw the symbol -]/[- as a closed switch, therefore XIC, and -] [- as open, and therefore XIO.
OK, so if a N/C switch is sending a signal to a terminal labeled as XIC, then as the processor "scans" it, it sees it as a "true", and allows that rung to proceed? Then if the N/C switch in the field is activated, and opened, the processor sees that its opened, examines it, and labels it "false", therefor stopping the rung from proceeding? Does this sound even remotely correct?
thanks guys.

Rembrant

Rembrant said:

Does this sound even remotely correct?
thanks guys.

Click to expand...

Sounds like you're gettin it!

In my opinion, forget about the "examine" term, as that is what confuses me.
I think the easiest way to look at it is to ask: Is there voltage (24vdc 120vac or whatever type if input card you have) at the input terminal? If you want the instruction to be true with voltage use --| |-- . If you want the instruction to be true with NO voltage at the input terminal then use --|/|--.

You know that the stop button, in it's un-activated state will provide voltage to the input terminal, and you also know that the start PB will only provide voltage when it is activated.

It will only take a little practice and you will have it!

Ok, Its getting a lot clearer now, I just spent about an hour reading a PLC book, and I think my main problem was I was getting lost in the terminology. I was stuck on the "examine" part, or "closed" part of the instruction, and when I read the PLC book, they used "examine on", and "examine off", which is again, just different terminology. Thanks for your help, I just wanted to understand whats going on, and not just do it because somebody said it was right. Once I get a little further ahead, it will probably be like second nature to me.

Rembrant

Willingness to learn is a great thing!!

If you have mastered the "art" of understanding and troubleshooting relay based systems, you will definitely understand this stuff, you will wonder how the "old timers" did it without PLC's, just give it a little time, and dont forget to have fun!

Well, its funny, I never really thought of troubleshooting relay logic as an art....lol. I've been working on industrial equipment for years, all AC motor drives, traditional relays and contacors, but all the machines had specific microprocessors, which came with pre-programmed e-proms....all I ever had to do was enter the variables using the keypad (miserable 4 finger simultaneous button pushing...lol), and if there was ever software issues, I'd just order a new chip, or e-prom. And now, that I'm getting into PLCs, SSRs, and Servo drives, everything else is looking pretty antiquated now.
Thanks again, I'll be back as I get further involved. I'm scheduled for a Premium PL7-Pro course in September, so I hope to be well up to speed by then.

Later.