Specifications

Chapter 5 - Software RS-422 And RS-485 Applications Ebook
Page 66 of 137 Manual Documentation Number: <DocNumber>
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RS-422 Systems
RS-422 system software differs little from that used with familiar point-
to-point RS-232 communication systems. RS-422 is often used to
simply extend the distance between nodes over the capabilities of RS-
232. RS-422 can also be used as the master node in a four-wire
master-slave network.
Figure 48: RS-422 Point-to-Point Link
When selecting or writing software for RS-422 systems the designer
should be aware of the signals being used by the hardware in the
system. Many RS-422 systems do not implement the hardware
handshake lines often found in RS-232 systems due to the cost of
running additional conductors over long distances.
RS-485 Driver Control
The principle difference between RS-422 and RS-485 is that the RS-
485 driver can be put into a high impedance, tri-state mode, which
allows other drivers to transmit over the same pair of wires. There are
several methods of accomplishing this task.
One common method is to use a control line (often the RTS handshake
line) to enable and disable the driver. The host software must assert
the RTS line and enable the driver before beginning a transmission,
then release the RTS line after the completion of the transmission.
Since only a single RS-485 driver can be enabled on a network at one
time it is important that the driver is disabled as quickly as possible
after transmission to avoid two drivers trying to control the lines
simultaneously, a condition called line contention.