plc inputs

mir

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• Apr 11, 2014

• #1

How will the control program of a PLC respond if two input devices are connected in parallel to the same input terminal?
thanks guys appreciate the help

 

duckman

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• Apr 11, 2014

• #2

if they are in parallel, which ever one is ON the input is going to be ON, no different then having 2 light switches in Parallel. If in series both would need to be on for the input to go on/high/1

 

mir

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• Apr 11, 2014

• #3

im doing my plc online & answered that which ever1 is triggered first would trigger the input but was incorrect
the response i was given was if there are 2 incoming signals,what would happen?the inputs would...the process would result in??

 

duckman

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• Apr 11, 2014

• #4

without knowing the program or process we have no idea what might happen, but if the the inputs are digital and for what ever reason you had 2 inputs in parralell and 1 or 2 where turned on, the input would stay ON untill both were turned OFF. There must be something in the program that they are asking about?

 

duckman

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• Apr 11, 2014

• #5

Hi Mir, here is a great simulater. Has the ability to write programs and then put them into the same screen to watch how things work. I copied this from a previous thread.
LogixPro is a good PLC simulator for Allen Bradley RSLogix 500 software on a SLC 5/02 or similar. You can find information (and also the program) at thelearningpit.com. This program does show the "physical setup" side-by-side with the ladder program. There are also many good student problems with scenarios built in to the simulator program.

http://www.thelearningpit.com/
Last edited by Lancie1; January 14th, 2014 at 01:52 AM.

 

Jeebs

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• Apr 11, 2014

• #6

Code:

Input  Switch 1  Switch 2
  0       0         0
  1       1         0
  1       0         1
  1       1         1

The input would be TRUE as long as either switches/input devices are TRUE.
The input would be FALSE only if both switches/input devices are FALSE.

 

Mihir_007

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• Apr 11, 2014

• #7

Jeebs said:

Code:

Input  Switch 1  Switch 2
  0       0         0
  1       1         0
  1       0         1
  1       1         1

The input would be TRUE as long as either switches/input devices are TRUE.
The input would be FALSE only if both switches/input devices are FALSE.

Click to expand...

Simply an "OR" gate.
:site:

 

duckman

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• Apr 11, 2014

• #8

Hi Mir, keep in mind that there are a few versions of what is termed ON=high/true/ON/1. OFF= Low/false/OFF/0. As Jeebs has shown on the above truth table it is no different then having 2 light switches in parallel, turning 1 or 2 ON turns the input ON.
UNLESS they have XOF (examine OFF or N/C contact) as the contacts for the input PROGRAM then of course it would all be back to front and you would be turning the program/process OFF. That's why you need to tell the whole story of what is asked or required.

 

Mickey

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• Apr 11, 2014

• #9

Jeebs, your truth table needs a little work. See picture..

 

PLC Pie Guy

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• Apr 11, 2014

• #10

Correct me if I am wrong but his truth table is the same, except he put the input column first. What is his mistake?

 

boneless

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• Apr 11, 2014

• #11

Could the question be relating to an analog signal?

What happens when you wire two analog signals in parallel to an input?

 

Nesa09

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• Jun 28, 2014

• #12

I was marked incorrect with an answer of "The input would be true as long as either inputs are true, however the input would be false only if both of the inputs are false."

Any help on this?

 

bernie_carlton

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• Jun 28, 2014

• #13

Ask the questioner - if possible - "why do you consider this answer wrong? What would you consider a more correct answer?"

Maybe post back here the answer you receive - if any.

 

danatomega

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• Jun 28, 2014

• #14

Nesa09, was that the answer you gave or a response indicating what the correct answer should be?

If it was the answer you provided, I would be very interested what the questioner would consider more correct...

 

Lancie1

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• Jun 28, 2014

• #15

"The input would be true as long as either inputs are true,
however the input would be false only if both of the inputs are false."

Click to expand...

The above is correct (except you should have said "either input device (or switch)" instead of "either inputs" and "both of the input devices or switches" intead of "both of the inputs". The "input" would refer to a terminal and the electronic input parts of the PLC, and the things that cause the input to react are external devices such as switches and sensors. That might be why it was marked wrong - you did not seem to understand the difference between external devices and the parts of the PLC and program.