Terminating Resistors - Emerson Smart Wireless Gateway Reference Manual

Reference Manual
00809-0200-4420, Rev GA
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. 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  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
rates (above 115 kbps) and long cable runs.
Figure 3-9. Typical Half Duplex (2-wire) Network
Device 1
Mounting and Connection
(Figure 3-8).
Main Circuit Board
Radio
Electronics
(Figure
Up to 4000 ft. (1220 m)
A
B
Device N
(up to 32
possible)
Section 3: Mounting and Connection
DIP Switches
3-9). However termination should only be used with high data
Terminators required
only for high data rates
and long cable runs
470Ω Pull-down Resistor
120Ω Terminating Resistor
470Ω Pull-up Resistor
Device 2
April 2013
23