In 4-wire transmission (full duplex), 4 wires run from Transmit Data (TXD) or Receive Data (RXD) on the master to the corresponding TXD or RXD on the slave(s). This allows simultaneous bi-directional communication.
1. Master to Slave(s):
- TXD+ ↔ RXD+
- TXD- ↔ RXD-
2. Slave(s) to Master
- RXD+ ↔ TXD+
- RXD- ↔ TXD-
In 2-wire transmission (half-duplex), Transmit Data (TXD+) and Receive Data (RXD+) on the master are wired together to TXD+ and RXD+ on the slave(s). TXD- and RXD- on the master are wired together to TXD- and RXD- on the slave(s). Only one pair of cables connects the master and the slave(s). Since the RS-485 protocol requires communication to flow in one direction at a time, 2-wire half-duplex mode results in non-simultaneous bi-directional communication.
In all cases terminating resistors must be placed at the extreme end or ends of a cable. As signals travel through cabling it creates reflections, terminating resistors aim to reduce and minimise these reflections. In the case of circuits with only 1 or 2 slaves terminating resistors must still be used.