Wiring analog input module

master_blaster · Sep 17, 2017

I am trying to wire four analog signals to a analog input module AI 4xU/I 2-wire ST (6ES7134-6HD00-0BA1). When I turn on the first device (connected to channel 0), it gives me a reading on channel 0 which is correct, but at the same time it also produces a reading on channel 1, and 2 even though the devices connected to those 2 channels are switched off. The reading is not the same as it is on channel 0, but a lower value. All the devices have their own power supplies, and output an analog signal in the range of 0v...5v. I have tested the wire for shorts, but their is none. Have I connected them right?. I have attached my schematic.

spaderkung · Sep 17, 2017

Is the configuration Ok for the channels? Why are you grounding the - and are the 2 grounds you show different?

https://www.google.se/url?sa=t&sour...ggmMAA&usg=AFQjCNG-_vHSsw6h9AflmlVQHdcokxZ_Qw

master_blaster · Sep 17, 2017

Yes, the configurations are OK for the channels. Channels 0 and 1 measure 2 outputs (pins 7 and 8 - voltage) from the same device, and it is referenced to ground (see picture). Channels 2 and 3 are connected to 2 different devices. The output signals from these devices are referenced to ground, that is why I have grounded the -. Ground is same. it is just grouped separately.

spaderkung · Sep 17, 2017

You can disconnect wires from card and measure to eliminate card issue. I'm not sure about your ground.

master_blaster · Sep 17, 2017

The card itself is fine. I have tried multiple cards, and they all show the same behavior

keithkyll · Sep 17, 2017

The outputs of your Matsusada power supplies might be high impedance. You need the L2 buffer option as shown on the S series. Without that, they say you need a 10 Meg meter (typical Fluke DVM).
The output impedance on the RA/RB Series is 1K, and should work fine with the PLC. The Siemens module is 120K input impedance.
What model Matsusada are you using?

master_blaster · Sep 17, 2017

keithkyll said:
The outputs of your Matsusada power supplies might be high impedance. You need the L2 buffer option as shown on the S series. Without that, they say you need a 10 Meg meter (typical Fluke DVM).
The output impedance on the RA/RB Series is 1K, and should work fine with the PLC. The Siemens module is 120K input impedance.
What model Matsusada are you using?

Yes, it does have 1K output impedance. RB60-30

keithkyll · Sep 17, 2017

Then it's got to be grounding. Because supplies are referenced to chassis ground, testing should be easy. While you have condition, probe all grounds looking for voltage. Probe ground to ground.
If that doesn't do it, then I would say there is crosstalk from one supply to the others for an unknown reason. Add isolators. Try just one isolator on channel 1.
Have you tried to disconnect channel 0, and see if channel 1 affects channel 2?

master_blaster · Sep 17, 2017

keithkyll said:
Have you tried to disconnect channel 0, and see if channel 1 affects channel 2?

I am still in the process of debugging the problem. Haven't gone through all the possibilities yet

kalabdel · Sep 17, 2017

The ground should not be connected to any of the input, the ground should be connected to M .

danw · Sep 17, 2017

master_blaster said:
When I turn on the first device (connected to channel 0), it gives me a reading on channel 0 which is correct, but at the same time it also produces a reading on channel 1, and 2 even though the devices connected to those 2 channels are switched off. The reading is not the same as it is on channel 0, but a lower value.

This is the classic description of a ground loop.

The difference in ground potentials between the PLC and the two field devices is allowing current to flow which appears as an offset in the analog input channels, which share a common ground.

First thing to try is to disconnect the individual channel ground from GND ground to see if the ground loop if affected. You might get lucky.

You might get two channels to work but need isolation modules for the remaining channels. Or you might need isolation for all the channels to eliminate the ground loop.

master_blaster · Sep 17, 2017

kalabdel said:
The ground should not be connected to any of the input, the ground should be connected to M .

But the output from the device that I am trying to measure is referenced to ground

kalabdel · Sep 17, 2017

That's right. The sensors and the input module (M terminal) should be referenced to the same point.

I should rephrase : The power source to the devices and to the analog module should share the same reference and the output of the sensors connect directly to the inputs of the analog module.

cardosocea · Sep 18, 2017

Do you have a voltage between both grounds? More importantly, how are the devices powered and is there a difference between the grounds of the different power supplies (instruments and PLC card)?

master_blaster · Sep 23, 2017

Got the problem fixed. It did turn out to be a ground loop, which I wasn't aware off. Removed unnecessary grounds, and the problem disappeared. Thanks for all your help guys

Wiring analog input module

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