Specifications

Chapter 5 - Software RS-422 And RS-485 Applications Ebook
Page 68 of 137 Manual Documentation Number: <DocNumber>
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RS-485 Receiver Control
The RS-485 receiver also has an enable input. Since RS-485 systems
using a two-wire configuration connect the driver to receiver in a
loopback fashion, this feature is often used to disable the receiver
during transmission to prevent the echo of local data. Another
approach is to leave the RS-485 receiver enabled and monitor the
loopback data for errors which would indicate that line contention has
occurred. Although a good loopback signal does not guarantee data
integrity, it does offer a degree of error detection.
Master-Slave Systems
A master-slave system has one master node that issues commands to
each of the slave nodes and processes responses. Slave nodes do not
typically transmit data without a request from the master node, and do
not communicate with each other. Each slave must have a unique
address so that it can be addressed independently of other nodes.
These systems can be configured as two-wire or four-wire. Four-wire
systems may use an RS-422 master (the driver is always enabled) and
RS-485 slaves to reduce system complexity.
Figure 51: A Master/Slave System
Four Wire Master-Slave Systems
This configuration reduces software complexity at the host since the
driver and receiver are always enabled, at the expense of installing two
extra conductors in the system. The master node simply prefixes