I am doing a lot of Machine Monitoring and I want to pose a question is it actually easier 'install-wise' to use an Analog input card for reading Digital Signals? Especially off of a PLC card where 24v is 'ON' my Analog input card would read 10v and I would interpret this as being on. So please correct me if I am wrong in the picture below the Analog input card does not need a GND connected because it is already sharing a common ground with the PLC. With a Digital Card, I would need a separate reference ground for this application.

I would to hear your thoughts on this. Bit new to reading directly from I/O boards.

 

Really, AI has not much to do with physical connections.

 

Hi IJC,
It depends on the length of the 0V cable/distance between the I/O module and the PLC.
What is the 'rd' cabling length between AIN0 and the PLC.

BTW: Some casual readers of your post, appear to be confused by the fact that the AI term refers to the AIN0 and the AIN1 inputs and NOT Artificial Intelligence

E

 

Indeed it did cause confusion. AI for Analog Input numbers zero and one would be written AI#0 and AI#1 with no possible confusion. Also, many analog input modules have an isolated input return common to avoid digital system noise and spikes.
And a caution is that with some PLC types, analog value reading, followed by an evaluation, is not as fast as reading a digital input.
Is the actual intent to verify that the green stack light has been powered?? If it is only to verify that the pwoer to the light is on, there is a much simpler scheme.

 

Indeed it did cause confusion. AI for Analog Input numbers zero and one would be written AI#0 and AI#1 with no possible confusion. Also, many analog input modules have an isolated input return common to avoid digital system noise and spikes.
And a caution is that with some PLC types, analog value reading, followed by an evaluation, is not as fast as reading a digital input.
Is the actual intent to verify that the green stack light has been powered?? If it is only to verify that the pwoer to the light is on, there is a much simpler scheme.

Hi Bill, sorry for the confusion. Can I see your recommendations for a similar scheme? Thank you for the feedback so far.

 

Unless you are seeking a numerical result of reading the green light stack output, a single ladder line with a N.O. contact having the same address as the one that controls the light, followed by the D.o. with the address you picked to activate when the green light came on. The very simplest of ladder logic.

 

I am doing a lot of Machine Monitoring and I want to pose a question is it actually easier 'install-wise' to use an Analog input card for reading Digital Signals? Especially off of a PLC card where 24v is 'ON' my Analog input card would read 10v and I would interpret this as being on. So please correct me if I am wrong in the picture below the Analog input card does not need a GND connected because it is already sharing a common ground with the PLC. With a Digital Card, I would need a separate reference ground for this application.

I would to hear your thoughts on this. Bit new to reading directly from I/O boards.
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The issue here isn’t that you couldn’t make it “work”, it’s that you are taking a digital signal and handling it with an analog input for the sake of saving one wire. Doing something that ignores best practices evolved over a long time for trivial reasons—and without even being sure of the implications is an error.

Whoever comes after you to maintain and grow this thing you are building is likely to have a few choice nicknames for you when they discover you’ve done this because they need to handle new digital signals from whatever is replacing the old machine and they find its all connected to analog and won’t work properly.

That person stands a good chance of being you, in fact.

It’s best to add a little cost to ensure that you are not second-guessing what workaround might be able to be used in the future. Your one wire savings might just cost far more than that in time and/or money when new things need to be accommodated.

 

To explain additionally, to use an analog input as a program input from a program output indicates that you are not able to program a PLC effectively or efficiently.
Using an output as a contact in another line is a standard way to implement PLC logic.
The one possible exception will be if that output shown as a stack light is actually some sort of analog sensor, hidden in the drawing for some secret purpose. But I doubt that is the case.

 

It just came to me that possibly the TS has no digital inputs available. But even then, using a digital output contact in another rung is a legal ladder logic operation for most PLC brands. It may not be accepted for some brands of PLC, possibly.