Specifications

34 RS-422/485 Application Note
 Copyright B&B Electronics -- Revised October 1997
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RS-485 Receiver Control
The RS-485 receiver also has an enable signal. 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 guaranty
data integrity it does offer a degree of error detection.
Master-Slave Systems
A master-slave type system has one node that issues commands to each of
the “slave” nodes and processes responses. Slave nodes will 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 independent of other nodes. These type systems can be configured as
two wire or four wire. Four wire systems often use an RS-422 master (the driver
is always enabled) and RS-485 slaves to reduce system complexity.
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 commands with the
appropriate address of the slave. There are no data echo or turn around delays to
consider. Since each of the slave transmitters share the same pair of wires, care
must be taken that the master never requests data from multiple nodes
simultaneously or data collisions will result.
Two Wire Master-Slave Systems
Two wire configurations add a small amount of complexity to the system.
The RS-485 driver must be tristated when not in use to allow other nodes to use
the shared pair of wires. The time delay between the end of a transmission and
the tristate condition becomes a very important parameter in this type system. If
a slave attempts to reply before the master has tristated the line, a collision will
occur and data will be lost. The system designer must know the response time or
turn around delay of each of the slave nodes and assure that the master will
tristate its driver within that amount of time. B&B Electronics’ Automatic Send
Data control circuits tristate the driver within one character length of the end of a
transmission.