Isolated discrete inputs / outputs. is that similar to sourcing / sinking?

Just a little confused about this.

I have this modicon plc TSK DEY 64D2K that has 64 isolated inputs, and i want to replace the plc with Flex I/O. Do I choose sourcing or sinking?






Thanks in advance

No, the "sourcing" and "sinking" relates to if the PLC switches the positive or the negative (for outputs) or is connected to a positive or a negative common (for inputs).

"Sourcing" means that the electrical current flows away from the input or output.
"Sinking" means that the electrical current flows towards the input or output.

Sinking inputs and sourcing outputs is the normal way of doing it these days (some PLC manufactures do not have the option for sourcing inputs and sinking outputs).
In the olden days (the 80s and before) it was the opposite, since the electrical circuitry can be a little bit simpler for sourcing inputs and sinking outputs. As far as I remember, especially TTL level circuitry was usually with sourcing inputs and sinking outputs.
That some vendors still offer the possibility for sourcing inputs and sinking outputs must be for when you have to migrate an ancient control system.

Thanks Jesper. I remember the source / sink bit now.

What about isolated / non-isolated inputs /outputs then?

Isolated with respect to digital inputs and outputs can mean two things.

1. (isolation to the backplane) There is some kind of circuitry (optocouplers), so that if there is an electrical error, it will not propagate to other channels or the entire PLC. So you will not blow up the entire PLC if someone connects 440V to an input for example. All "regular" PLC IO have this.

2. (isolation between channles) The various cannels on a module have a common voltage on one side (non-isolated). Or have completely individual connections to the PLC (isolated).

JesperMP said:

Isolated with respect to digital inputs and outputs can mean two things.

1. (isolation to the backplane) There is some kind of circuitry (optocouplers), so that if there is an electrical error, it will not propagate to other channels or the entire PLC. So you will not blow up the entire PLC if someone connects 440V to an input for example. All "regular" PLC IO have this.

2. (isolation between channles) The various cannels on a module have a common voltage on one side (non-isolated). Or have completely individual connections to the PLC (isolated).

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That's great thanks Jesper.

So if a plc has isolated inputs (2nd example) wired to it and you were to replace the plc and the input card was non-isolated this could cause problems as the wiring would need to be different?

It could potentially create problems. You have to investigate why the inputs were isolated in the first place.
At the very least you will probably have to change some wiring.

Ok. thanks for your help

JesperMP said:

Sinking inputs and sourcing outputs is the normal way of doing it these days (some PLC manufactures do not have the option for sourcing inputs and sinking outputs).

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I didn't think there was a 'normal way' of doing things Still thought it was manufacturer dependent.

We use mainly Siemens PLCs (sourcing) and Motoman robots (sinking) so we used to stock both PNP and NPN sensors. This became an issues due to techs just knowing they needed an M12 sensor. After replacing a few sensors due to them not working out of the box they would the controls team for help. After this happened a few times we stared only stocking PNP sensors and just added PNP/NPN converters to the senors on the robots.

Maybe Japan still prefers sinking systems...

JOLTRON said:

Maybe Japan still prefers sinking systems...

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In my limited experience, Yaskawa Motoman has been the main thing I've seen with Sourcing Inputs and Sinking Outputs.

That and automotive stuff. Because the frame exists everywhere at common potential and it makes for less wires for switches.