Two Problems, if anyone can check, I did it right or not?

arnabbanik · Feb 24, 2014

Hi All,
I have done two questions, which I have doubts whether it's right or not. I am still new so there might be other ways to do it.

I would appreciate if you can let me know if I did it right or not.

Thanks,
Arnab

bernie_carlton · Feb 25, 2014

1c. This is a standard way of wiring a Stop type input. If the circuit were to be interrupted the motor should stop. Thus the motor should only run if there is a connection to the input.

2. Is the branch symbol around the output an accidental mistake? It would not be allowed in most PLC programs.

arnabbanik · Feb 25, 2014

Thanks

bernie_carlton said:
1c. This is a standard way of wiring a Stop type input. If the circuit were to be interrupted the motor should stop. Thus the motor should only run if there is a connection to the input.

2. Is the branch symbol around the output an accidental mistake? It would not be allowed in most PLC programs.

Hi,
thanks a lot for the reply.

So for 1(c), should I use XIC instead? are the other one's right?

2.I forgot to remove the branch, it shouldn't be there,accidental mistake. But is the logic right for the given question. Does my solution hold?

RoTaTech · Feb 25, 2014

1c should be a XIC.
The switch is ON/closed when you DON'T push it, so the PLC "sees" that on state with a XIC, and keeps the output on.

arnabbanik · Feb 25, 2014

RoTaTech said:
1c should be a XIC.
The switch is ON/closed when you DON'T push it, so the PLC "sees" that on state with a XIC, and keeps the output on.

Thanks , got it !

Can anyone provide any comments for that one please?

bernie_carlton · Feb 25, 2014

Ignoring the branch you answer to #2 appears to be correct. Analyzing the picture the switch in its NORMAL position is energizing Solenoid B. If actuated it would energize Solenoid A.

As a single NO contact is being supplied from this switch then your logic seems correct.

When I program I nickname the inputs with the state when the input circuit is turner ON. This then totally ignores the NO/NC state of contacts because all the PLC 'knows' is the status of the input.

For wiring type 'a' in your question I would lable the input 'Sol_A_On_Request' or something to that effect depending on the limits of the naming.

I also label outputs for the action which takes place when the output is turned ON. If your picture were more complete it would label Sol A whith the machine action which takes place when it is energized. Let's imagine it said 'Cylinder 1 Extend'. Therefore I would label O:2/0 as 'Cyl_1_Extend'. let's also say that O:2/1 said 'Cylinder 1 Retract'.

Now, imagining both labels are in place 'read' the first rung.
"If 'Sol_A_On_Request' is ON (say 'is ON' because of the XIC contact) then actuate 'Cyl_1_Extend'.

The second rung would inherit the labels and would read "If 'Sol_A_On_Request' is OFF then actuate 'Cylinder_1_Retract'"

When inputs, outputs, internal contacts etc. are properly labeled then the logic rungs when 'read' with an eye to XIC/XIO, branches then they read like a logical sentence.

arnabbanik · Feb 25, 2014

bernie_carlton said:
Ignoring the branch you answer to #2 appears to be correct. Analyzing the picture the switch in its NORMAL position is energizing Solenoid B. If actuated it would energize Solenoid A.

As a single NO contact is being supplied from this switch then your logic seems correct.

When I program I nickname the inputs with the state when the input circuit is turner ON. This then totally ignores the NO/NC state of contacts because all the PLC 'knows' is the status of the input.

For wiring type 'a' in your question I would lable the input 'Sol_A_On_Request' or something to that effect depending on the limits of the naming.

I also label outputs for the action which takes place when the output is turned ON. If your picture were more complete it would label Sol A whith the machine action which takes place when it is energized. Let's imagine it said 'Cylinder 1 Extend'. Therefore I would label O:2/0 as 'Cyl_1_Extend'. let's also say that O:2/1 said 'Cylinder 1 Retract'.

Now, imagining both labels are in place 'read' the first rung.
"If 'Sol_A_On_Request' is ON (say 'is ON' because of the XIC contact) then actuate 'Cyl_1_Extend'.

The second rung would inherit the labels and would read "If 'Sol_A_On_Request' is OFF then actuate 'Cylinder_1_Retract'"

When inputs, outputs, internal contacts etc. are properly labeled then the logic rungs when 'read' with an eye to XIC/XIO, branches then they read like a logical sentence.

Thanks a lot for your explanation, appreciate it !

Lancie1 · Feb 25, 2014

Can anyone provide any comments for that one please?

1-c. [FONT=Arial,Arial]Stop a motor from running when a pushbutton is pressed. The input field device is a normally closed pushbutton.
I will use XIO, because the circuit needs to open to work.
[/FONT]

A closed physical STOP button will produce a "1" at your PLC input. What instruction will tell the PLC that a "1" is at that input?

XIO Examine If Open will return a "0" or FALSE for a closed switch because XIO always produces the "Opposite" of the bit it is looking at. So in this case, the XIO would be looking at a "1" and will produce the Opposite or "0", which would mean an Open Circuit with the STOP NOT PRESSED. You want a "1", so use a XIC, which always produces the SAME as the Input being examined. "What you C (see at the input) is what you get with an XIC"!

This little aggravation with a STOP button is probably the most commom error among beginning students. Get this right in your head, and you will be able to answer a lot of questions correctly from here to the end of your PLC studies.

arnabbanik · Feb 25, 2014

Lancie1 said:
A closed physical STOP button will produce a "1" at your PLC input. What instruction will tell the PLC that a "1" is at that input?

XIO Examine If Open will return a "0" or FALSE for a closed switch because XIO always produces the "Opposite" of the bit it is looking at. So in this case, the XIO would be looking at a "1" and will produce the Opposite or "0", which would mean an Open Circuit with the STOP NOT PRESSED. You want a "1", so use a XIC, which always produces the SAME as the Input being examined. "What you C (see at the input) is what you get with an XIC"!

This little aggravation with a STOP button is probably the most commom error among beginning students. Get this right in your head, and you will be able to answer a lot of questions correctly from here to the end of your PLC studies.

That's a good way to remember it !
So according to that logic, will the answers be :-

a. XIO
b. XIO
c. XIC
d. XIC

Lancie1 · Feb 25, 2014

Yes, you have the Problem 1 answers correct.

arnabbanik · Feb 25, 2014

Lancie1 said:
Yes, you have the Problem 1 answers correct.

Sorry to bother with the same question again but I just wanna make this clear in my head.I have been thinking about this yet , I am confused.

1. So if I have a physical device initially open or "0" (false) then use XIO and vice versa?

2. Can you also explain the questions part b ( 1b.) please. I think that will allow me to see the difference and how and when to use what instruction.

3. Are these mnemonics specific to kind of input device we use?

I understand this is a very common problem , but any help from anyone is highly appreciated.

Thank you all.

Lancie1 · Feb 26, 2014

1. So if I have a physical device initially open or "0" (false) then use XIO and vice versa?

That question cannot be answered until you know WHAT the PLC intruction for that address should DO. If your physical device is a START pushbutton, then you would want your PLC instruction to produce a TRUE or "1" when the pushbutton closes or goes TRUE. So then you would use a Examine if Closed (XIC), because it always returns the SAME as the physical device (which is TRUE when closed).

2. Can you also explain the questions part b ( 1b.) please. I think that will allow me to see the difference and how and when to use what instruction.

b. [FONT=Arial,Arial][FONT=Arial,Arial]When a pushbutton is pressed, it operates a solenoid. The input field device is a normally open pushbutton. [/FONT]
[/FONT]

So when the input fied device (not the PLC instruction) goes from normally open to closed (button is pressed), then we want the solenoid to go TRUE or ON. What instruction produces the SAME TRUE output as the field device does in this case? What you C is what you get with the XIC. When the XIC sees the TRUE at the input terminal, then it produces a TRUE output which then turns on the solenoid.

3. Are these mnemonics specific to kind of input device we use?

Yes, and No. Within the same Allen Bradley PLC brand, for digital ON/OFF inputs you would always use the same XIC or XIO instructions. (For analog inputs in any brand of PLC, that is a different ballgame). But for other brand and models of PLCs, the digital input mnemonics are completely different (but mostly the same symbols | | and |/| are used).

arnabbanik · Feb 26, 2014

Lancie1 said:
That question cannot be answered until you know WHAT the PLC intruction for that address should DO. If your physical device is a START pushbutton, then you would want your PLC instruction to produce a TRUE or "1" when the pushbutton closes or goes TRUE. So then you would use a Examine if Closed (XIC), because it always returns the SAME as the physical device (which is TRUE when closed).

So when the input fied device (not the PLC instruction) goes from normally open to closed (button is pressed), then we want the solenoid to go TRUE or ON. What instruction produces the SAME TRUE output as the field device does in this case? What you C is what you get with the XIC. When the XIC sees the TRUE at the input terminal, then it produces a TRUE output which then turns on the solenoid.

Yes, and No. Within the same Allen Bradley PLC brand, for digital ON/OFF inputs you would always use the same XIC or XIO instructions. (For analog inputs in any brand of PLC, that is a different ballgame). But for other brand and models of PLCs, the digital input mnemonics are completely different (but mostly the same symbols | | and |/| are used).

Thank you Lancie !

So , if I want to "see" the same o/p as the i/p I should use "XIC" and if I want the opposite then "XIO" .

e.g., in question
"Turn on a light when a conveyor motor is running in reverse. The input field device is a set of contacts on the conveyor start relay that close when the motor is running forward and open when it is running in reverse."

Because I want the lights to be turned "ON" when physical contacts open, I should use XIO.

Is my understanding correct?

Again, I appreciate your patience and taking the time to explain things.

bernie_carlton · Feb 26, 2014

"Turn on a light when a conveyor motor is running in reverse. The input field device is a set of contacts on the conveyor start relay that close when the motor is running forward and open when it is running in reverse."

Because I want the lights to be turned "ON" when physical contacts open, I should use XIO.

Is my understanding correct?

If you use the naming style that I outlined then the choice of XIC or XIO is obvious. The input, when ON, is running in forward. Name the input "Run_In_Forward"

Now the logical statement for the rung would be:

If "Run_In_Forward" is OFF then turn ON "Reverse_Light"

The "OFF" implies XIO.

Lancie1 · Feb 26, 2014

Is my understanding correct?

Yes, and consider seriously what Bernie said about naming your instruction addresses.

Two Problems, if anyone can check, I did it right or not?

arnabbanik

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