New member first question

[DMTJAGER]

Hello all new member here. Have upgraded to supporting member status as I will likely be her asking help quite often.

My situation is I just last week was transferred to a electrical maintenance job of a cold reduction steel process mill that is heavily dominated by automation, PLCs and VFD's. While I have a VERY basic understanding of PLC's my last training was a little over 10 years ago. Per my current trainers 95% of my trouble shooting issues will problems control power issues and all either in the PLC's or VFD's but majority PLC problems.

To be absolutely clear I will Not ever need to write or enter PLC programs I will strictly only need to navigate my way through the PLC program to trouble shoot why what ever it is stopped working. My job will be trouble shooting and solving problems 90% are controlled by PLC's.

My current experience electrically speaking has been on traditional industrial electrical power and control systems as it applies to timers, relays, rectifiers to control the speed of three phase motor operation.

I feel I will be best served if I should progress in my training and ask advice based on classifying my self as a PLC novice. Yes I have a basic understanding of PLC's and Ladder logic but thats about it.

The training I will be getting in my new department will be mostly OTJ and instruction by the two most knowledgeable and capable electricians in my department. I will be receiving NO form of organized class room instruction or formal training from my employer to any degree, so that is why I am here.



I am going to supplement my training at work with training and studying on my own time at home.

For my very first three questions I need someone to explain to me in as simple and complete of terms as possible the difference between:

Examine On

Examine Off

I feel there is no point in proceeding with my training in PLC's any farther until I absolutely understand those two terms and the difference between those two terms.

Second question is I need someone to please explain to me the difference between N/O N/C VS Examine on and Examine off.

Once I absolutely understand those two questions I can proceed with my PLC education.

Lastly can someone PLEASE recommend me the best affordable PLC training program for at home learning for a novice wishing to become proficient at trouble shooting PLC's I do not need to be able to program PLC's.

Greatest of thanks as I can not overstate the importance of this to me.

 

[I_Automation]

PLC ladder logic was designed so that a person with that can read a ladder logic schematic can follow the same type of BASIC instructions in the PLC.


Examine ON theoretically is the same as a N.O. relay contact. If the relay is ON there will be current flow to the next component. In a PLC if the tag (can be an INPUT, OUTPUT, MEMORY BIT, MESSAGED BIT, and more) is ON then the instruction is TRUE and the next "component', actually INSTRUCTION, can be checked or processed. If the rung finishes TRUE, which can be through various branches sometimes, the output instruction is made TRUE - turn on a BIT, OUTPUT, enable a timer or counter, etc.


The EXAMINE IF OPEN is similar to a N.C. relay contact. It will be TRUE if the associated INPUT, OUTPUT, BIT, etc. is NOT ON, Just as a N.C. contact sends current through when the relay is not on.


For starters go to YouTube & search for RSLogix. There is a video on how to download the free version of RSLogix and after you install that search YouTube for PLC, there are many videos covering everything from the basics for a beginner to advanced methods.

 

[parky]

Examine on (XIC)is a normally open contact (without the /) i.e. ----| |------ this means just like a relay when the contact closes the state is true, the other is just the opposite Examine off (XIO) is a normally closed contact (in PLC terms) or
------|/|---- in other words if the PLC input is off i.e. no signal then the logic is true, I hate AB's way of describing it most PLC manufacturers use N/O or NC
So XIC is N/O & XIO is N/C.
So in the following logic

------| |---------|/|------------( )

The first contact is XIC or N/O so when it is true the power (if you like to call it that) flows in that contact i.e. if it was an input to the PLC it goes true when switched on, the second contact XIO or N/C is true when it is off.
the output coil will only energise when the first contact is on and the second contact is off. just like real relay logic.
Just think of those as relay contacts off two relays so when the first relay is energised the contact closes i.e. it is on(N/O but closes XIC), the second one If the relay is not energised as it is a N/C contact (XIO) then it is also true, however, if the second relay is turned on then that contact opens.
Think of it as IF Relay 1 is on AND NOT relay 2 (off) then the rung of logic is true so energises the output coil ( ).
Perhaps look at the learn PLC's on this forum it will give you a starting point.
Other things to consider is that a PLC originally was to simulate standard relay logic, however, this has now changed & does far more, you need to get your head round how the memory is organised for example there are digital inputs & outputs, digital bits (used internally sometimes called flags these can be set to 1 like an output coil but can also be used as a contact for example in AB terms B3:0/0 is an internal bit, this bit can be used as a relay that has infinite contacts, there are also other memory areas for words (integer) reals (floating point) these can hold values i.e. a number like 1234 or 123.456, older models of PLC's had fixed memory areas but the trend now is to make them dynamic these do not use physical addresses but use symbols.
Of course there are timers & counters, loads of functions like maths or conversion not to mention analogues etc.

 

[widelto]

Hi, this video ( https://www.youtube.com/watch?v=A4kg0Jb5Bgw ) is dedicated to RSlogix 500 , but the final concept is exactly the same.
So, I hope that you can understand the concept behind XIO and XIC, understand that they do not mean NO and NC contacts respectively as many people believe for the very first time.

http://www.plctalk.net/qanda/showthread.php?t=61692&highlight=green+power+flow

 

[jaden]

If you already understand relay normally open and normally closed operation, just ignore the XIO and XIC designations, and go by the symbol.

 

[RVaughan]

Watch all eleven of these videos:
Ron Beaufort PLC Boot Camp Intro

This will clear up your XIC/XIO ideas and give you a good start in troubleshooting thought.

 

[drbitboy]

Watch all eleven of these videos:
Ron Beaufort PLC Boot Camp Intro

This will clear up your XIC/XIO ideas and give you a good start in troubleshooting thought.

^ THIS +1e12!


If you watch all eleven (it takes just under 2h), I'll be surprised if you have any questions remaining.

 

[willxfmr]

Watch all eleven of these videos:
Ron Beaufort PLC Boot Camp Intro

This will clear up your XIC/XIO ideas and give you a good start in troubleshooting thought.

Best advise you're going to get. Those videos are simply the best at explaining the basic operation of AB ladder logic.

Bubba.

 

[drbitboy]

I still think OP's best bet bar none will be to watch Ron's videos, but I do want to mention one more thing:

Consider a pair of XICs in series at the start of a rung:

          the_1st_bit            the_2nd_bit
[COLOR=Blue][COLOR=Black]|---[/COLOR][B]TRUE[/B][/COLOR]------] [----------[COLOR=Red][B]A[/B][/COLOR]---------] [---------[COLOR=DarkGreen][B]B[/B][/COLOR]-----...

Now it's been a while since I watched them, so maybe Ron's videos cover this later on (although #10 does not*), but that first XIC is a binary AND between two things: (1) the state (truth?) of the incoming rung; and (2) the result of the "Look for a 1 in boolean tag [the_1st_bit]" request made of the processor.

I.e. writing this in a procedural type of language, the rung state of the output from the first XIC, i.e. A, will be a logical/boolean AND of the truth of two statements, specifically

[COLOR=Red][B]A[/B][/COLOR] = [B][COLOR=blue]TRUE[/COLOR] [/B]AND (the_1st_bit's value is a 1)

That initial input rung is TRUE because it is directly "attached" to the V+ "rail;" please note that, while I used the terms "attached" and "rail" there, I put them in quotes because, as Ron points out, there is no actual physical "rail" or V+ voltage to be "attached" to.

Similarly, the rung state of the output from the second XIC, i.e. B, will be a logical/boolean AND of the truth of two statements, specifically

[B][COLOR=DarkGreen]B[/COLOR][/B] = [COLOR=red][B]A[/B][/COLOR] AND (the_2nd_bit's value is a 1)

Maybe this is obvious to everyone out there, but I know that, until recently, I thought of XICs as unary instructions i.e. operating by reading a single input, and yet had no trouble reading and understanding them, but they are in fact binary operators: because the second input to the instruction is the input rung state itself.

I am not saying that this is in any was revalatory, just that ladder logic is so intuitive that this dependence on the input rung seems to be rarely mentioned yet universally understood.

* Video #10 could cover it, but Ron wisely focuses each video on one thing in particular, and the point of video #10 is so foundational that to bring this binary nature of XICs in would risk failing to transmit a more important concept.

 

[dogleg43]

Forget XIO and XIC. That is specificaly and only Allen Bradley nomenclature. Any other ladder logic I can think of uses NO and NC.

A-B speak for NC is XIO
A-B speak for NO is XIC

—-|/|—- is NC (XIO)
—-| |—- is NO (XIC)

 

[pturmel]

because the second input to the instruction is the input rung state itself.

I tend to think of the rung condition as the first operand. Or accumulator, if you will, as that is what's really going on. In C or C++ it would be the &= operator.

 

[Corsair]

Years ago a well-intentioned individual at Allen Bradley attempted to come up with terms for the contacts that would more correctly describe what they do rather than using the classical relay terms 'Normally Open' and 'Normally Closed'. His or her correct thinking was that PLC programming IS NOT a relay wiring schematic - it is a programming language. Electricians that have an understanding of relay schematics appear to have a head-start at understanding PLC programming although they tend to get in trouble because they do not understand the implications of the PLC scan.

As with most things Allen Bradley the terms have gone on to hold a near-religious significance with plenty of megabytes devoted to questioning, attacking, and defending them. When I do training I call them Normally Open and Normally Closed. My expectation is that anyone bright enough to understand PLCs will realize that I am using the terms in a PLC context and that they need to be understood in that context. I personally do not have any troubles switching contexts from PLC programming to relay schematics.

I suspect the suggested references are quite good as I've come to trust the people that are recommending them. My personal training attempt is the PLC class outline at corsairhmi.com.

Another general comment is that with good HMI diagnostics you usually should not have to resort to the PLC programming laptop to troubleshoot a problem. Some things like monitoring the status of bits in an interlock string can be accomplished with the HMI. You may then even have a happy day where the operator figures out something by himself and takes care of it without calling maintenance. Everytime that you are called think of what could be done to avoid that call.

I wish you well. 35+ years ago when I discovered PLCs my father wondered if I was getting into something that would go obsolete. There's more work today than ever. Lots of it is dealing with code from programmers who aren't and never will be. Get into it with a strong emphasis on the basics and avoid the brand-specific quicksand that seeks to trap you.

 

[drbitboy]

I tend to think of the rung condition as the first operand. Or accumulator, if you will, as that is what's really going on. In C or C++ it would be the &= operator.

Good point.

 

[parky]

In the very early 80's I was tasked with replacing a relay logic system with a PLC, however, nobody knew how the machine worked, we only knew that it was a cutting press with about 9 different patterns so basically the head would traverse left to right & cut at intervals, the head could rotate i.e. cut then rotate left cut rotate right etc. the panel was a mass of relays we had a very long fold out paper with the ladder logic relay drawing, so we decided to replicate what was on the ladder drawing, this was about 30ft long about A4 width. the only down side was to reduce the number of relays many contacts were in the cold side of the coils (most PLC's will not allow that). We replicated it, it worked so left it alone did not try to improve it or modify it in any way, so YES a PLC originally was a replacement for relay logic.
Ladder has two inferred power rails just as a relay system has so this **** about XIC/XIO is just another way of saying N/O N/C, well in my book & that has over 40 years experience.
Just think of digital inputs as contacts it's either on or off, how you use them in the program is just N/O or N/C.
The PLC digital input is just a simulated relay contact in effect, the only difference is it is a relay with hundreds of contacts either N/O or N/C, Digital bits known as internal bits are effectively relay coils with again as many N/O or N/C contacts as you require the fact that you can turn them on or off by dropping the coil in the ladder or change it's state via coms from an HMI etc. does not matter.
There is a trend to move to FBD or ST etc. this does not have the inferred power rails but works in the same way, so if it is in ladder or ST or FBD what it actually is
LOAD I0.0 means it is effectively connected to the assumed power rail, an OR around that would mean it is effectively also connected to the assumed power rail, contacts after that are then either ANDed or ORED as required.
Many PLC programs can be displayed in ladder or Mnemonic list (AKA STL)
The other functions like AND LOAD, OR LOAD, MPP, MPS, A(, OR(, determine if the "contacts" are connected to the assumed power rail or not. I have no idea why AB decided to re-invent what was an already accepted description.
Now I will wait for the no. no. no. it's not like that.
PLC's have moved on & have many more powerful instructions but it all boils down to bits in a pattern, either a "1" or a "0".
I have been programming for 40 + years in many languages and designed & repaired logic boards (before microprocessors where they used ALU's etc. to do the maths) even on AB I drop what I consider a N/O or N/C contact it works for me.

 

[VAN]

Per my current trainers 95% of my trouble shooting issues will problems control power issues and all either in the PLC's or VFD's but majority PLC problems.

To be absolutely clear I will Not ever need to write or enter PLC programs I will strictly only need to navigate my way through the PLC program to trouble shoot why what ever it is stopped working. My job will be trouble shooting and solving problems 90% are controlled by PLC's.

I don't know if this is useful to you or not, but I'd setup some kind of engineering screen that is just on the network, like engineering laptop, create a new FTME or FTSE and make a screen and shortcuts. (or even SCADA if you have that already).

Every time you get called for a "PLC" problem and you find the issue start making a "interlocks" style screen. So, if you get called about why isn't whatever running, pull up the screens and you can see it has an alarm, broken sensor, or "is running".