I have a strange problem in reading 4-20mA sensor connected to AI module. 

My CPU is 1215Cconnected with 3 no of AI 8X13bit module. I had connected 11 sensors of 4-20mA in 11 channels of two AI modules. 3 SIEMENS Mag flow meter included in this 11. All the 8 sensors including the flow meters are working fine without any problems. But this 3 Temperature sensors 2 wire type of the 6 sensors are showing normal at time and all of a sudden one sensor starts to show different reading and become normal and other sensor starts to react the same way like the first.

I tried to change cable of one sensor to verify whether the problem is with the sensor cable, but even after changing the cable, the same thing happens.

I am in a doubt whether the current consumption of these modules can vary and cause these variations in the sensor readout. Also one strange thing is that if i disconnect one of this sensor, the other two sensor values becomes unstable. Whether over current consumption can cause this problem?

Any suggestions or ideas will be helpful.

Hello Jay,

at the AI-module 8x13bit can disconnected sensors (open inputs)cause problems at the other channels.

Can you tell us a little bit more

• schematic/wiring diagram of your PLC
• have all channels the same configuration?
• are there any wirings to connect ht AI+ and AI-  when the channels are not used?
• are these 6 sensors are all the same?
• do you test a not working sensor at a other "working" channel?

These are my first questions....

regards
CaOd

• schematic/wiring diagram of your PLC.I can provide you soon. It's all two wires, so just 24V supply to sensor and output connected to AI+ of PLC and AI- connected with 24V-. It's identical for all sensors and only flow meter differs.
• have all channels the same configuration? Yes all are same configuration in hardware 4-20mA and same scaling in software
• are there any wirings to connect ht AI+ and AI-  when the channels are not used? The first module is completely occupied in which all three fluctuating sensors are present. The second module the unused channels are left open without any connection. Unused channels i setup as 0-20mA
• are these 6 sensors are all the same? Yes it's all from E+H
• do you test a not working sensor at a other "working" channel? I am yet to check that, but three sensors and one sensor disconnection having impact on other two, is it normal?

Any of the three sensors disconnected will make the other two fluctuating. But one of the sensor has higher reading. But I am just thinking if any one sensor is having an issue, why the other two particular sensors are getting this kind of issue. I need to check the wiring again for sure. I used ferrule for all connections. I will counter check again if any of those ferrules are not crimped well. 

You mean one sensor can affect the other sensor values like this is normal?

Hello Jay,

are the sensors from E+H types that could configured?

regards
CaOd

Hello Jay,

could be there a problem with the power supply of your modules?

regards
CaOd