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© Sealevel Systems, Inc.
H1400811-8.4 Manual | SL9363 5/2022
RS-485 Termination and Pull-up/Pull-down Resistors
Each RS-485 Isolated port has selectable line conditioning via the three DIP switches to the side of the RS-
485 terminal block. These switches control whether termination and pull-up/pull-down resistors are active
for the port.
DIP Switch
Position
Result
TERM
OFF (Down or away from screen)
ON (Up or toward screen)
DISABLED - No termination
ENABLED - Terminate between D+ and D-
lines with 120 ohm resistor
PU
OFF (Down or away from screen)
ON (Up or toward screen)
DISABLED - No pull-up
ENABLED- Pull-up D+ line to isolated 3.3V
ISO1 plane via 510 ohm resistor
PD
OFF (Down or away from screen)
ON (Up or toward screen)
DISABLED - No pull-down
ENABLED - Pull-down D- line to isolated
GND ISO1 plane via 510 ohm resistor
The pull-up and pull-down resistors ensure that the D+ and D- lines are at a known state when not being
driven by the RS-485 bus. The first and last devices in a RS-485 bus should enable line termination, as well
as the pull-up and pull-down resistors.
Pull-up/Pull-down Resistors (DIP Switches)
