AD042

[Guest Tham]

I have connected a PH meter 4-20mA to input 2 of the AD42. The AD is working fine. Then I put in another PH signal 4-20mA to input 1. Strange things happened. The mA at input 2 goes from 15mA to 19mA. The transmitter which is integrated with the PH meter goes over ranged and refused to work. Once the input 1 was taken off, the system returns to normal. In the mean time, input 1 was registered on the PLC as normal. Only input 2 faces the problem. I then abandon the input 1 and did a serial loop for both input 1 and 2 using one signal source from input 2. Everything was fine. Please help if you have similar experience. I am really confused.

[KinK 0]

Ok I am not sure because I cant find any decent info on that unit. check to see if the commons for the inputs are linked. If they are it could be the the 2 ccts are interacting with one another expecialy if the power supply for the 2 transmitters is different. you could try a diode and see if that helps. it depends on the cct for the input of the plc and the output from the transmitters Peace

[Jay Anthony 118]

If the source of power for both transmitters is not the same source, then this could happen. If the source of power is the same, try referencing the shields to LG of the PLC power supply. Since the module is not an isolated input module, the other solution is to use external isolation devices to fully separate the signals.