Interfacing Analog and Digital I/O's

Theldon

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Join Date
Oct 2005
Location
Grande Prairie
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I am relatively new to the PLC, being fresh out of school. But I do not program them. I interface instrumentation to them. I work in the Oil and Gas industry, so I use a PLC to control valves (usually solenoid) or read sensors like temperature, level and pressure. But being relatively new to my job, I've been hooking these sensors and valves up to Analog and Digital I/O's.

I guess my question is, how do I know that I am interfacing it properly? I've done some paperwork with the AutoPilot RTU, and the PLC 5 by Allen Bradley.

A decent example would be I have a level sensor that emits a current at 4-20mA depending on the level of the liquid in a tank. What do I connect this to, and how should the connections be in order to properly interface this sensor to the PLC so that a user can see the level of the tank, or create some logic so that when the tank is full, an action can be taken.

Thanks.
 
confused

Hi,

I don't understand your question completly, do you want an elektrical drawing or do you want a PLC-programming example?

Rudi
 
A decent example would be I have a level sensor that emits a current at 4-20mA depending on the level of the liquid in a tank. What do I connect this to, and how should the connections be in order to properly interface this sensor to the PLC so that a user can see the level of the tank, or create some logic so that when the tank is full, an action can be taken.
The physical aspect will be basically the same with any plc, the input will connect to an ANALOG card. The device may use 2, 3 or 4 wire but the plc will only use 2 of them in most cases, this picture shows so Allen Bradley SLC analog modules.
analogwiring.jpg


The best thing to do is obtain the manual for any devices involved. To program to use the analog is another issue. This is a basic example for PID involving analog input and output.
http://domino.automation.rockwell.com/applications/kb/RAKB.nsf/0/A02E853A8B1B42EE85256D35005B6D2E?OpenDocument

In your tank level application the 4-20ma depending on resolution will have a numeric value in the plc...ie 12bit resolution the numeric values are 0-4095. You can use this number..example; tank gets to a value of 4000 you could use a Greater than or Equal to instruction to stop filling. If the tank level needs displayed you may want to convert that number to represent gallons, height etc etc.
 
Last edited:
Thanks, thats about what I'm looking for. Is it really that easy? Is there anyway to check to make sure everything will work, like a simulator?
 
Theldon,

At our plant (chemical) we let the wiring do by an external firm and when they have finished the job we do I/O tests. Each single wire is checked according the elektrical drawings. After that we do I/O tests on the level of PLC. To make sure that all the signals or comming in correctly ( Polarity for mA signals eso). So, at the stage, we also can look for bugs in the software according I/O.
I know this is not done in every company because is it time consuming and it costs alot of money. But on the other hand we have a clean start-up.

Rudi
 
Theldon said:
Thanks, thats about what I'm looking for. Is it really that easy? Is there anyway to check to make sure everything will work, like a simulator?
That depends, if the systems are predominantly AB there is RSEmulate http://www.software.rockwell.com/rslogixemulate/.

Easy is a relative term, this does require being familiar with the process and learning to program. You can start here http://www.plcs.net/contents.shtml

If you want an inexpensive way to learn programming with a simulator option then visit http://www.thelearningpit.com/. It offers a training program/simulator based on Allen Bradley's RSlogix500.
 

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