monkeyhead
Member
So I had a strange problem today that defied what I've learned about data communication.
basic setup is: Scanner -> PLC via RS-232 using only the Tx, Rx, and GND at 9600/8/None/1
Basically what I saw was that whenever any of the VFDs for the motors where running, my scanner read rates went to hell. But the RS-232 connection continued to work fine. The PLC read the bad-scan data with no problem.
I finally resolved the problem by connecting the shield for the RS-232 line to earth ground on both ends. I was grasping at straws by the point I tried this and just happened to get lucky.
This makes no sense to me. Doesn't this create a ground loop? I've always heard to only connect the shield to earth ground on one side to properly eliminate noise and never on both ends.
I almost have the feeling that the scanner which is on a seperate 120vac circuit isn't grounded properly and by hooking up the ground on both ends I was providing the scanner a proper ground but I really have no idea. I have a scope meter, but am pretty clueless with what I'm looking for as far as detecting line noise.
Any info you kind fellers have on the subject would be greatly appreciated.
basic setup is: Scanner -> PLC via RS-232 using only the Tx, Rx, and GND at 9600/8/None/1
Basically what I saw was that whenever any of the VFDs for the motors where running, my scanner read rates went to hell. But the RS-232 connection continued to work fine. The PLC read the bad-scan data with no problem.
I finally resolved the problem by connecting the shield for the RS-232 line to earth ground on both ends. I was grasping at straws by the point I tried this and just happened to get lucky.
This makes no sense to me. Doesn't this create a ground loop? I've always heard to only connect the shield to earth ground on one side to properly eliminate noise and never on both ends.
I almost have the feeling that the scanner which is on a seperate 120vac circuit isn't grounded properly and by hooking up the ground on both ends I was providing the scanner a proper ground but I really have no idea. I have a scope meter, but am pretty clueless with what I'm looking for as far as detecting line noise.
Any info you kind fellers have on the subject would be greatly appreciated.