Contemporary Control Systems EXP Expansion Modules User Manual - Manual (Page 17)

Figure 2-1. EIA-485 Daughterboards
For EIA-485 DC operation, it is very important that all devices
on the wiring segment be referenced to the same ground
potential in order that the common mode voltage requirement
(+/–7 Vdc) of the EIA-485 specification is achieved. This can be
accomplished by running a separate ground wire between all
hubs and computers or by relying upon the third wire ground of
the power connector assuming that the DC power return is
connected to chassis ground on all computers (this is the case
with the MOD HUB). Another approach would be to connect
the DC common of each computer to a cold water pipe.
Connected systems, each with different elevated grounds, can
cause unreliable communications or damage to the EIA-485
differential drivers. Therefore, it is important that an adequate
grounding method be implemented.
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