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© Sealevel Systems, Inc. 7204e Manual | SL9244 11/2022
Termination & Biasing Resistors
Typically, each end of the RS-485 bus must have line terminating resistors enabled (RS-422 should
terminate the receive end only). A 120 ohm resistor is across each RS-422/485 input in addition to a 1 K
Ω
pull-up and pull-down combination that biases the receiver inputs. Use the switches detailed in the table
below to customize the interface to your specific requirements.
If using the board in an RS-485 two-wire network, enable the two switches labeled ‘L’, which tie the
positive/negative transmit/receive pairs together.
Switch
Default
Description
T
ON
Adds 120
Ω
termination, OFF to remove
PU
ON
Adds 1 K
Ω pull
-up resistor in RS-422/485
Off to remove
PD
ON
Adds 1 K
Ω pull
-down resistor in RS-422/485
OFF to remove
L
OFF
ON to connect TX– to RX– for RS-485 two-wire
L
OFF
ON to connect TX+ to RX+ for RS-485 two-wire