Rockwell Automation T8xxx Trusted Communications Interface - Manual (Page 52)

Trusted
TM
Communication Interface T8151B
Issue 21 Apr 10 PD-T8151B 52
6.2. RS485 Communications Standard
RS485 is a differential twisted pair interface. A key feature of RS485 over RS422 is its ability to support
configurations where multiple line-drivers share a common twisted pair, as distinct from multi-drop (i.e.
multiple receivers only) which is supported by both RS422 and RS485. There are several modes of
operation for the RS485/RS422 ports of the Communication Interface.
6.2.1. RS485 Full Duplex
This mode is a 4-wire point to point connection. The System Configuration setting for this mode is
‘rs485fd’.
The receive end of the link should always be terminated. The Communications Interface shall be AC
terminated with a 120R resistor and 68nF capacitor in series across the receive pair. The Interface
Adapter (8153 for the CI and 8130 for the MP) provides link selectable terminations for the receive
circuit.
A diagram of topology is shown in Figure 20 below.
A
B
TX
A
B
RX
A
B
TX
A
B
RX
Term
Term
Figure 20 RS485 Full Duplex Topology