Impulse 1105 - Manual (Page 6)

Card Setup
Sealevel Systems DIN485+I Page 4
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 DIN485+I 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 receiver 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)
ON
1
2
OFF
3
4
5
6
Figure 5 – DIP-switch SW2, Line Termination and Echo Off/On