Product specifications

21
Reference Manual
00809-0200-4420, Rev HB
Section 3: Mounting and Connection
January 2015
Mounting and Connection
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 3-8).
Figure 3-8. 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 Ù 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 3-9). However termination should only be used with high data
rates (above 115 kbps) and long cable runs.
Figure 3-9. Typical Half Duplex (2-wire) Network
ON
K40
1
2
3
470Ω Pull-down Resistor
120Ω Terminating Resisto
r
470Ω Pull-up Resistor
Electronics
Radio
DIP Switches
Main Circuit Board
Up to 4000 ft. (1220 m)
Device 1
Device 2
A
B
Terminators required
only for high data rates
and long cable runs
Device N
(up to 32
possible)