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© Sealevel Systems, Inc. 7205e Manual | SL9239 12/2022
Electrical Interface Modes
Each port on the 7205e can be individually configured for RS-232, RS-422, or RS-485 mode. The first two
switches labeled ‘M1’ and ‘M0’ are used to select the electrical interface. To simplify installation and testing
the board is shipped in RS-422 mode. Use the settings in the table below to assist you in configuring the
electrical interface.
M1
M0
Interface
OFF
OFF
RS-232
OFF
ON
RS-422 [Default]
ON
OFF
RS-485 ECHO Enabled
ON
ON
RS-485 No ECHO
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 1K
ohm 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 120Ω termination, OFF to remove
PU
ON
Adds 1K Ω pull
-up resistor in RS-422/485
Off to remove
PD
ON
Adds 1K 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
