Chassis Plans ATXP-875P, Technical Reference

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Appendix C: Communications Devices ATXP-875P – Technical Reference
76 Chassis Plans
Termination Resistors
Follow instructions in the previous RS-422 and RS-485 sections. The termination resistors available are rated to
120
Ω
.
Ground Connections
All 422- and 485-compliant system configurations shown up to this point do not have incorporated signal-return
paths to ground. Obviously, having a solid ground connection so that both receivers and drivers can talk error free is
imperative. The figure below shows how to make this connection and recommends adding some resistance between
logic and chassis ground to avoid excess ground-loop currents. Logic ground does not have any resistance in its path
from the driver or receiver. A potential problem might exist, especially during transients, when a high-voltage
potential between the remote grounds could develop. Therefore, some resistance between them is recommended.