24
Reference Manual
00809-0200-4410, Rev BA
Section 3: Mounting and connection
February 2014
Mounting and connection
Figure 3-6. 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, 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
*Use a sharp non metal tool to switch between resistor options
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.
Tx
Rx
Tx
Rx
(A)
(B)
ON
K40
1
2
3
470
Ω
Pull-down Re
sistor
120
Ω
Te
rminating Re
sistor
470
Ω
Pull-up Re
sistor
DIP Switches*
