4WSD9R1203-2/4
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Table 3: Switch settings for
RS-485 2-wire half-duplex
Switch # Position
1
RS-485
2
Echo Off
3
2-wire
4
2-wire
Table 4: Switch settings for
RS-485 4-wire full-duplex
Switch # Position
1
RS-485
2
Echo ON
3
4-wire
4
4-wire
RS-485 2-Wire Half-Duplex
This communications mode is used to connect several RS-485 devices to the same
network with a minimum number of wires when only one unit talks at a time. The RS-485
driver is automatically enabled during each spacing state on the RS-232 side. During the
marking or idle state, the RS-485 driver is disabled and the data lines are held in the
marking state by the 4.7K Ohm pull-up and pull-down resistors. The value of these
resistors may need to be changed to a different value when termination is used in order
to maintain the proper DC bias during the idle state.
See B&B Electronics’ free RS-422/RS-485 Application Note for more information on
termination and DC biasing of an RS-485 network. To set up for this mode on the
4WSD9R, place switch 1 in the RS-485 position, switch 2 in the echo off position, and
switches 3 and 4 in the 2-wire position. Follow the wiring diagram in Figure1 to connect
to a 2-wire half-duplex network.
Figure 1: RS-485 2-wire half-duplex diagram
RS-485 4-Wire Full-Duplex
This communications mode is typically one of the slaves in a master/slave RS-485
setup. The RS-485 driver is automatically enabled during each spacing state on the
RS-232 side. During the marking or idle state, the RS-485 driver is disabled and the
data lines are held in the marking state by the 4.7K Ohm pull-up and pull-down
resistors. The value of these resistors may need to be changed to a different value
when termination is used in order to maintain the proper DC bias during the idle state.
See B&B Electronics’ free RS-422/RS-485 Application Note for more information on
termination and DC biasing of an RS-485 network.
The receiver is enabled at all times, allowing devices to receive information even while they are responding
to a request. To set up for this mode on the 4WSD9R, place switch 1 in the RS-485 position, switch 2 in the
echo on position, and switches 3 and 4 in the 4-wire position. The wiring between the slave and the master
is identical to the RS-422 4-wire full-duplex diagram shown in Figure 3, and in many cases the master in a
master/slave RS-485 network is set up as an RS-422 device. When multiple slaves are on the same
network, the connection between the master and each slave must be the same as shown in Figure 2. This
will cause all slaves to have their transmitters connected and their receivers connected, and not allow
communications between the slaves themselves.
9-pin M/F cable
