Designing RS-485 Links and NetworksRS-485 transceiver, or other component. The primary node’s serial port con-
nects to a series of RS-485 transceivers. Each transceiver in turn connects via
RS-485 to one or more nodes in the network.
When the primary node transmits, the data passes through all of the transceiv-
ers to the secondary nodes. When a secondary node transmits, a transceiver
passes the data to the primary node but not to the other secondary nodes.
When all of the transceivers’ R outputs are logic highs, a 4.7k resistor holds the
primary node’s input high. When a transceiver’s R output goes low, the primary
node’s input goes low.
All of the transceivers derive their driver-enable signals from the data as
described in Chapter 6. Using this method to control the driver-enable signals
greatly simplifies the network programming for the circuit.Figure 7-17: With this configuration, the primary node sees data from all of the
secondary nodes and the secondary nodes see traffic from the primary node only.
Circuit design courtesy of R.E. Smith (www.rs485.com).