Aaeon Raptor MicroATX Installation Manual page 72

Raptor MicroATX – Installation Guide
RS-485 is Differential and Multi-Point Connected
Differential Transmission
Please, read the Differential Transmission explanation in the previous
RS-422 section.
Termination Resistors
Follow instructions in the previous RS-422 and RS-485 sections. The
termination resistors available through SW1 are rated to 120Ω.
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.
62