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).
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Communicating without wires has great appeal. Wireless data can pass through
walls and travel for miles. Portable devices are easier to use when there are no
data cables to attach and route. Plus, wireless communicating is fun. Even in an
age when wireless phones and networks are routine, there??™s something entertaining
about designing a system that sends and receives information with no
visible connection between the sender and receiver.
A data-communication system must carry information to the receiver at the
desired speed, without errors, and without causing interference to other electronic
devices.
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