Siemens S7-200 analog problems

[KJLRSA]

Hi All

I currentely have a CPU226 with a EM231 (6ES72310HC220XA0). We are using channel A and B for a 4-20mA Var transducer input. We have a HMI connected to the CPU and this is displaying the scaled VAR value. We are having an issue when the 4-20mA signal is sent to the card, we are having wild fluctuations on the reading. It reads the correct value for a few seconds and then drops to bottom of scal for a few milli seconds and then goes to top of scale for a few milli seconds.

Cables used are shielded and earthed. Analog card and CPU using the same power supply.

This is playing havoc with our control system. Has anyone encountered this problem on S7-200 and how can we fix.

Thanks in advance.

 

[Skogsgurra]

I had a similar problem a couple of years ago. Not on the S7-200, but on the Logo! - which is also in the "Micro Automation" family.

It turned out that the input amplifier of the analogue section (an LM358) was unstable in the lower voltage range (because of capacitive loading at the output).

That problem has been corrected and the new devices do not show the problem. It is possible that you have the same thing in the S7-200, which - after all - is a "family member".

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[controlsdude]

Have you done the obvious thing and put a meter in series to see the 4-20mA signal vary correctly on the meter?

1st Check to see if the sensor is sending the correct signal back?

2nd Signal correct at the sensor, Signal Correct at the PLC analog input?

3rd Is the PLC input analog card setup correctly?

4th Did you check with your local distributor on what Gunner was saying about the hardware issue?

 

[KJLRSA]

Thanks All

I was on site yesterday trying all suggestions. It turns out the temp supply we were using was a ungrounded UPS supply. We connected to a VT supply and the problem was sorted. There appeared to be common mode voltage fluctuations due to differing voltage sources.

For once an easy fix with Siemens...Doesnt normally happen.