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© Sealevel Systems, Inc. 1102 Manual | SL9135 6/2021
Line Termination and Receiver Echo Off/On
Typically, each end of the RS-485 bus must have a line-terminating resistor (RS-422 terminates at the
receive end only). A 120-ohm resistor is across each RS-422/485 input in addition to a 1K-ohm pull-up/pull-
down combination that biases the receiver inputs. DIP-switch SW2 allows customization of this interface
to specific requirements. Each DIP-switch position corresponds to a specific portion of the interface. If
multiple IC485+
adapters are configured in an RS-485 network, only the adapters on each end should have
DIP-switch SW2 position 1 on. One adapter should have DIP-switch SW2 positions 2 and 3 on.
Refer to the
table below for each position’s operation.
DIP-switch SW2 position 6 is used to control the RS-485 enable/disable functions for the receiver circuit
and determines the state of the RS-422/485 driver. The RS-
485 ‘Echo’ is the result of connecting the rece
iver
inputs to the transmitter outputs. Every time a character is transmitted, it is also received. This can be
beneficial if the software can handle echoing (i.e., using received characters to throttle the transmitter) or
it can confuse the system if the software
does not. To enable the ‘No Echo’ mode DIP
-switch SW2 position
6 should be on.
SW2
Function
1
Adds or removes the 120-ohm termination.
2
Adds or removes the 1K-ohm pull-up resistor in the RS-422/RS-485 receiver circuit (Receive data
only)
3
Adds or removes the 1K-ohm pull-down resistor in the RS-422/RS-485 receiver circuit (Receive
data only)
4
Connects TX+ to RX+ for RS-485 two-wire operation
5
Connects TX- to RX- for RS-485 two-wire operation
6
Receiver Echo Off/On (On = Echo Off)
Figure 5
–
Dip switch SW2, Line Termination and Echo Off/On
