Using non-isolated Analog I/O on S7 1215C CPU

I'm working with a Siemens S7-1215 PLC for the first time. I typically use isolated analog I/O in my projects, but this particular CPU comes with two non-isolated 0-10VDC analog inputs and two non-isolated 0-20mA analog outputs. It has been a long time since I've worked with non-isolated signals, so I could use a refresher course.

I am interfacing with two Yaskawa V1000 drives and hope to use the PLC analog channels for drive speed reference and feedback. The PLC output can drive the VFD analog input and vice versa as the impedance requirements are satisfied. The analog outputs are all sourced, not loop powered. My question is, can I simply connect the drive analog O/I to the PLC analog I/O or do I need some type of signal isolator/conditioner?

Thanks in advance.

It depends on how much common mode voltage there is between the (-)/common terminals of the drive and the PLC.

Non-isolated I/O use the ground reference as the analog (-)/common.

If you run wires from the drive voltage output terminals to the PLC (or vice versa) and measure (-) to (+) with a floating battery powered voltmeter, that's the differential.

Say differential is 5.5Vdc across (+) and (-) {the real, expected output is 5.5Vdc).

Then measure from each leg to the common of the receiver input.

An isolated input would measure
0Vdc from drive (-)/common to PLC (-)/common.
5.5Vdc from (+) to (-)/common

A non-isolated input might measure
4.3Vdc from drive (-)/common to PLC (-)/common. (4.3V of common mode)
9.8Vdc from drive (-)/common to PLC (-)/common. (5.5Vdc differential + 4.3Vdc common mode)

If the common mode is low, a couple volts are so, you won't damage the I/O so connect both and see how it works.

If you have something over 5 volts common mode it's likely to saturate and drive the signal offscale.

If you have 10+ Volts (ac or dc), you need an isolator for each channel.

I had someone tell me that the common mode changed with the frequency of the VFD he worked with. I suspect that the VFD had grounding issues.

Thanks Danw for the detailed explanation. What you are saying about common mode voltage helps to clear the picture.

This is a new install; we are supplying the control panel and the customer is supplying the drive. I won't know about common mode until on site. I'm wondering if anything can be done (besides installing isolators) to prevent this issue ahead of time since we're dealing with new equipment. Also will this be a potential issue with the current loops as well?

Also will this be a potential issue with the current loops as well?

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Yes, unless the 4-20mA devices are isolated themselves.

RonJohn said:

I'm wondering if anything can be done (besides installing isolators) to prevent this issue ahead of time since we're dealing with new equipment.

Click to expand...

Well, maybe, but not likely.

You, like Siemens, can insist on "Equipotential Earthing" which means getting grounds at different geographical locations to a common potential. That can be engineered on big controls jobs (it is on DCS jobs) but it tends not to be the case with small project and upgrades. You get the ground potential you get.

Other than leaving some space on the DIN rail for a couple loop isolators I don't know what you can do in advance, other than demanding "equipotential earthing" with who-knows-what-results.

Thanks folks for the responses. We are going with isolators and I've learned a lesson. Better in the shop than out in the field, although there's still time!