In summary I need to know if the 4-20ma input to the Mitsubishi FX2n 4AD AI card can be powered from the the device rather than the Fx2n 4AD. We have tried connecting 2 analogues from an Elpro 105U-1 Transceiver acting as a receiver. if we used the Elpro +24 power supply, then one channel at a time seemed to work ok. but as soon both were connected the readings were not correct and seemed to 'sharing' current across both channels. It turned out that both the the Elpro and Input card were supplying power. We have tried connecting the analogue from the COM terminal and the AO1/AO2 terminals on the Elpro unit and using the loop power from the 4AD. now the signals seem to be isolated, but the readings are not correct, much lower than input values.

Our next idea is to try and use the loop power from Elpro unit if we can stop the power from the FX2n 4aD module being fed into the loop.

 

We are planning to use output loop powered isolators.

1. We know the internal loop powered analog is correct from the Elpro units

2. The isolator will supply passive signal to PLC meaning we still use the loop power provided by the plc.

3. If the HMI value is still wrong then the fault lies with the PLC inputs, if not then the COM on Elpro units at fault.

Does this make sense to anyone?