19
Reference Manual
00809-0200-4410, Rev BC
Mounting and Connection
July 2017
Installation
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, located behind an access slot on the upper right side.
The switches number bottom to top 1 through 3 and the upward position is ON.
Figure 3-7. RS-485 Resistor DIP Switches
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 ohm 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 ohm 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 3-8). However termination should only be used with high data rates (above 115 kbps)
and long cable runs.
Figure 3-8. Typical Half Duplex (2-wire) Network
A. Tx line
B. Rx line
DIP Switches
(1)
1. Use a sharp non metal tool to switch between resistor options.
A. 470 Ω pull-up resistor
B. 120 Ω terminating resistor
C. 470 Ω pull-down resistor
ON
K40
1
2
3
ABC
Up to 4000 ft. (1220 m)
Device 1
A
B
Terminators required
only for high data
rates and long cable
runs
Device N
(up to 32
possible)
Device 2
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