Specifications

Chapter 1 - Overview RS-422 And RS-485 Applications Ebook
Page 20 of 137 Manual Documentation Number: <DocNumber>
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Care must be taken when setting the period of the timing circuit. If the
time is too short, it could time out before the entire character has been
transmitted, which would result in retries (the system would repeatedly
try to re-send the message). If the time is too long, the start of the
data from the last node polled could be lost. (In a master/slave system
the master requests data, so no other node should be sending or
replying other than the one addressed by the master.)
For additional details refer to Send Data Control Timing Diagrams (on
page 96).
Bit-wise Enable Timing Control
Some RS-485 devices use bit-wise enable timing for tri-state control.
In this method, the RS-485 driver is enabled each time the RS-232 TD
line is in the space state. When the RS-232 TD data goes from space to
mark, the RS-485 driver is driven back to the mark state and then
released. The biasing resistors ensure that the output lines remain in
the mark state. On each change from a space to a mark this occurs. (It
is important that clear leading and trailing edges are created on the
output of the driver because these are used to determine the center of
each bit.)