5
Technical Background on RS485
5.5 4-Wire Scheme
RS422 requires dedicated wire pairs for transmit and receive. The transmit wires are used to send
data to as many as 10 receivers, as stated in the specifications of RS422. Since the VScom products
use RS485 line driver technology, up to 32 receivers are possible.
While one pair is used to transmit, a second pair is available to receive data at the same time.
When only two devices are connected, this is a possible replacement of classic RS232 connections
(PointtoPoint).
In RS485 4-wire mode the transmit wires may be shared between dedicated stations. As an example
a second station can be a backup master for the network. Masters can send data and commands
to one station, while they receive information from another device.
Figure 25: 4-wire cabling scheme
Figure
25
displays the wire pairs Tx
±
and Rx
±
as named for Devices 1 and 2. There are more
slave devices, only two of them are shown. The slaves transmit on the Rx-lines, and receive on the
Tx-lines. To implement the Common Voltage Range as specified the GND line is also connected.
Masters on the network are identified because they transmit on the Tx-lines. The two Masters in
figure
25
have to synchronize their use of the Tx-lines be extra means. E.g. Dev 2 can be a backup
master, which is manually activated.
Also indicated in figure
25
are polarization resistors P1/P2 for Tx
±
and P3/P4 for Rx
±
, only once
per wire pair. Further Termination resistors T1/T2 for Tx
±
and T3/T4 for Rx
±
are added, one at
each end of the cable. To ensure the Common Voltage Range the GND is also connected at each
device, making this a 5-wire connection.
June 2009
USB-COM PRO User Manual
29
