Analog Output Redundancy

[mvlrao]

I am using control logix 1756-OF8 analog output module for my application. I need to connect 2 analog output channels to a single field equipment(I/P converter) . Is there any external device to handle both channels(4-20 mA) and give one channel 4-20 mA output.

 

[mgvol]

I'm not familiar with such a device that would look at 2 A.I. and produce 1 common A.O. without summing the 2 A.I. signals. The least expensive way would probably involve isolating the 2 A.I. signals with a relay (one input on N.C. terminals; the other on N.O. terminals). You could then wite logic in the PLC looking for an under-range condition (loss of signal). You could then use an output to switch the relay.

 

[Tom Jenkins]

I'm curious. Why you think you need two analog outputs signals to a single field device? Wouldn't it be simpler to have two data registers in the PLC, and use internal logic to determine which one you want to use? Then the value in this register could be moved to the analog output.