Connect Tech NT960/PCI - Manual (Page 69)

Chapter 4: Appendix D - RS-485/422 Option 4-17
Intellicon-NT960/PCI User's Manual, ver. 0.02
Line bias feature
During the operation of a half duplex, or multi-drop RS-485/422
connection, there are situations (time intervals) when there is NO
"generator station" driving the wires that interconnect the various
stations. During these intervals the connection is susceptible to
electrical interference from outside sources. The RS-485/422
SLIM has fixed bias circuits to reduce the sensitivity to electrical
interference when no "generator station" (TxD and RTS) is
driving the cable. These bias circuits force the "receiver stations"
(RxD and CTS) into a known state.
Line load options
RS-485/422 lines normally need some type of load to prevent an
impedance mismatch that causes signal interference. The RS-
485/422 specification defines line termination loads placed on
either end of a twisted pair cable segment. Please refer to Figure
17 as an example of a typical circuit for a RS-485 application.
Figure 17: Typical RS-485 circuit
G R
G
R
Station A
Station B Station C
Station D
G = generationg station
R = receiving station
Typical RS-485 Circuit
where: