4Pelco Manual C505M-B (4/97)
Figure 2. Typical RS-485 Control Applications
3.1.2 The CX9504 4-Wire RS-485 Control System
This system is used in applications where multiple receivers will listen to a single
control channel, but duplex operation is desired. The RS-485 system presents sev-
eral unique installation requirements. These requirements are shown in Figure 2
and are discussed below.
Excessive stub lengths are not acceptable since they produce reflections that cause
message errors. This means that a “star” or “Y” topology is not acceptable.
The extreme ends of the cable must have 120 ohm termination resistors to mini-
mize reflections.