3.4.4 Terminating resistors
Three DIP switches are provided to enable various terminating resistors to the RS-485 data
bus. The switches are found inside the electronics housing near the top center of the main
circuit board (Figure 3-9).
Figure 3-9: RS-485 Resistor DIP Switches
A. Main circuit board
B. Radio
C. Electronics
D. DIP switches
E. 470Ω pull-down resistor
F. 120Ω terminating resistor
G. 470Ω pull-up resistor
Switches 1 and 3 are connected to pull-up and pull down resistors. Switch 1 is for the Tx +
(A) line and Switch 3 is for the Rx – (B) line. These 470 Ω resistors are used to prevent noise
from being interpreted as valid communications during periods when no actual
communications are occurring. Only one set of pull-up and pull-down resistors should be
active on the RS-485 data bus at time.
Switch 2 is connected to a 120 Ω terminating resistor. This resistor is used to dampen
signal reflections on long cable runs. RS-485 specifications indicate that the data bus
should be terminated at both ends (Figure 3-10). However termination should only be
used with high data rates (above 115 kbps) and long cable runs.
Installation Reference Manual
September 2020 00809-0200-4420
26 Emerson.com/Rosemount
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