21
Reference Manual
00809-0200-4420, Rev HC
Section 3: Installation
July 2015
Installation
If the existing data bus uses a 4 wire Full Duplex configuration, see Figure 3-8 to convert to a
2-wire Half Duplex configuration.
Figure 3-8. Convert from Full to Half Duplex
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
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 470Ω 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 120Ω 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.
ON
K40
1
2
3
470Ω Pull-down Resistor
120Ω Terminating Resistor
470Ω Pull-up Resistor
Electronics
Radio
DIP Switches
Main Circuit Board
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