Specifications

Chapter 5 - Software RS-422 And RS-485 Applications Ebook
Page 70 of 137 Manual Documentation Number: <DocNumber>
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Two Wire Master-Slave Systems
Two wire configurations add a small amount of complexity to the
system. The RS-485 driver must be tri-stated 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 tri-state condition becomes a very
important parameter in this type system. If a slave attempts to reply
before the master has tri-stated (released) 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 tri-state its driver within that amount of
time. B&B Electronics' Automatic Send Data control circuits tri-state the
driver within one character length of the end of a transmission.
Figure 53: Two-wire Master-Slave System
Multi-Master RS-485 Systems
Each node in a multi-master type RS-485 system can initiate its own
transmission creating the potential for data collisions. This type system
requires the designer to implement a more sophisticated method of
error detection, including methods such as line contention detection,
acknowledgment of transmissions and a system for resending
corrupted data.