Chapter 6
An RS-485 port can use any of three configurations (Figure 6-13):- Use two output bits and control the driver’s and receiver’s enable inputs sep-
arately. This arrangement provides the most flexible control but requires two
port bits. - Use one output to control both enable lines. The driver’s enable input is
active-high and the receiver’s enable input is active-low so either the driver
or receiver is always enabled and both are never enabled at the same time.
This configuration is useful if a node doesn’t need to receive its own trans-
missions. - Control the driver-enable input only. Tie the receiver’s enable input low to
keep the receiver enabled at all times. With this configuration, a node
receives the data it sends and thus can verify that the data transmitted.
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When using software control, the transmitting node must disable the
driver-enable line as soon as possible after transmitting all of the data. To deter-
mine when to disable the driver, the software can wait for the transmit buffer to
be empty, read back all transmitted data, or calculate a delay time.
An indication that the transmit buffer is empty can be useful in detecting when
it’s safe to disable the driver.Figure 6-13: Three options for controlling the driver-enable and receiver-enable inputs
on an RS-485 transceiver.