Card Setup
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 OMG-CONV-485-ISO 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.
1Adds or removes the 120-ohm termination
2Adds or removes the 1K-ohm pull-up resistor in the RS-422/RS-485 receiver circuit (Receive data only)
3Adds or removes the 1K-ohm pull-down resistor in the RS-422/RS-485 receiver circuit (Receive data only)
4Connects TX+ to RX+ for RS-485 two-wire operation
5Connects TX- to RX- for RS-485 two-wire operation
6Receiver Echo Off/On (On = Echo Off)
OFF ON
1 
2 
3 
4 
5 
6 
Figure 5 – Dip switch SW2, Line Termination and Echo Off/On
