kolyur
Lifetime Supporting Member + Moderator
I'm really stumped on this one. I have a Turck linear transducer connected to a 1762-IF4 analog module on a MicroLogix 1400, configured for 4-20mA. The run is about 25 feet of 2-conductor shielded twisted pair (Belden 9463). I grounded the shield drain wire on the panel side only. Looking at the sampled value in RSLogix, the signal is very stable (+/- a few counts) until I turn on any of the 480VAC motors, at which point the noise increases to a few hundred counts. Portions of the 4-20 cable run along 480V cables so I pulled them out--no improvement.
On a whim I disconnected the analog cable shield from ground and the noise immediately disappeared, even with many motors running. I don't have a lot of experience with noise on analog signals but this is completely opposite of what I would expect to see. Any suggestions?
On a whim I disconnected the analog cable shield from ground and the noise immediately disappeared, even with many motors running. I don't have a lot of experience with noise on analog signals but this is completely opposite of what I would expect to see. Any suggestions?