I have 3 way valve with a 4-20 ma positioner in it. the problem is this. i have a 3 way valve over my generator system that is giving output to the local panel 4-20ma having plc card 331-1kf02-0ab0. there is also auxiliary system for that generator having the same card which have a function block to display the percentage of the valve there. also the same 331-1kf02-0ab0 card is installed. i want to know is it possible to connect both cards in series with the positioner?
It's possible and sometimes it works OK. Other times it doesn't and then you'll need a signal splitter that isolates the input from the outputs.
It depends on the isolation of the analog inputs and the level of any common mode voltage between the analog input ground and the power supply ground and/or the source ground. If there's more common mode than the cards can reject, then you'll get either an on-scale offset, an offscale offset or in the worst case, smoke and a failed input card.
I think most re-transmit AO's on positioners are loop powered, and therefore likely fairly well isolated. Not sure how much resistance those loop powered outputs can drive, but it's likely something more than 500 ohms.
Give it try and see.