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FAQ: Connections for RS-485 or RS-422 to the 4WSD9OTB or 4WSD25OTB Converters?

Q: Connections for RS-485 or RS-422 to the 4WSD9OTB or 4WSD25OTB Converters?

Figure 1: 2-Wire RS_485

Mode/Switches

Switch 1 (Tx)

Switch 2 (Rx)

Switch 3 (bridge)

Switch 4 (bridge)

RS-485 2-Wire Mode (half duplex)

RS-485

Echo Off

2-Wire

2-Wire

RS-485 4-Wire Mode
(full duplex)

RS-485

Echo On

4-Wire

4-Wire

 

Figure 2: 4-Wire RS-485

 

Figure 3: 4-Wire RS-422

Mode/Switches

Switch 1 (Tx)

Switch 2 (Rx)

Switch 3 (bridge)

Switch 4 (bridge)

RS-422 4-Wire Mode
(full duplex)

RS-422

Echo On

4-Wire

4-Wire

 

Figure 4: 2-Wire RS-422 - No Receive

Table 1 – Switch Settings for Operating Mode


Mode/Switches

Switch 1 (Tx)

Switch 2 (Rx)

Switch 3 (bridge)

Switch 4 (bridge)

RS-485 2-Wire Mode (half duplex)

RS-485

Echo Off

2-Wire

2-Wire

RS-485 4-Wire Mode
(full duplex)

RS-485

Echo On

4-Wire

4-Wire

RS-422 4-Wire Mode
(full duplex)

RS-422

Echo On

4-Wire

4-Wire

Loopback Test Mode*

As Desired

Echo On

2-Wire

2-Wire

*Used with HyperTerminal or another terminal program to confirm operation of data through converter.

The ComTest program is available on our website in the Technical Library (Tech Notes) under Software Tools.

Figure 5: Extending RS-232 with RS-422

Null Modem Cable Connections

2-wire
Full Duplex 4-wire operation usually works best for RS-232 signals. Half Duplex operation using “2-wire” connections is usually NOT satisfactory because the RS-485 bus takes time to “turn-around” between transmit and receive, some of the first byte sent may be lost. RS-232 devices don’t wait before responding to a data request, but RS-485 devices designed for 2-wire operation wait.

This information should help you make the needed connections.