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RS422/485 and ground wire.

HellasTechn

Apr 14, 2013
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Apr 14, 2013
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In my DIY topology i have 4 rs422 devices where 1 is sender and 3 are receivers.
Yesterday i took my DMM and tested the power supplies of my devices.All of them use switching power supplies. Surprisingly i dscovered that one of them had the negative lead connected directly to chasis ground with out any resistance. the 2nd one had the negative lead connected to chasis GND through 60 ohm resistance. the last two devices share the same switching PSU where i noticed something strange. when the PSU was off it's ground lead would show no connectio to chasis ground but when turned on it would show about 30 ohm from the negative lead to the chasis gnd. What am i doing wrong ?

Bottomline is that i decided not to connect the signal grounds (negative leads of all psu's) together in fear of ground loops. So for now at least my bus works just fine with only A and B lines connected and chasis ground serving as common (at least in the first two cases).
 
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