Wiring 15 — RS-485 Modbus RTU Drop

RS-485 Modbus RTU Drop

RS-485 is the physical-layer standard used by Modbus RTU — the most widely deployed fieldbus protocol in industrial automation. You will encounter it on flow meters, temperature transmitters, variable speed drives, and PLCs from virtually every vendor.

Three rules you must know

1. Polarity matters RS-485 uses a differential pair: A (non-inverting, DATA+) and B (inverting, DATA−). Every device on the bus must connect A to A and B to B. Swapping A and B on even one device reverses the idle polarity for all devices — communication stops for the whole bus, not just the swapped device.

2. Termination prevents reflections At each physical end of the bus a 120 Ω termination resistor must be placed across A and B. Without it, high-frequency signal edges bounce back along the cable and corrupt waveforms. At 9 600 baud the reflections are usually tolerable; above 19 200 baud they often cause frame errors. In this lab an RS485Terminator module is placed at the far end of the drop (the PLC's built-in termination covers the near end).

3. Bus topology, not star RS-485 is a daisy-chain bus. Each device connects in line along the cable: PLC → device 1 → device 2 → terminator. A star topology (one master connected to multiple separate spurs) creates impedance mismatches that corrupt fast Modbus polling and is prohibited by the RS-485 standard.

What to wire

1. Seed +24 V and 0 V buses from the PSU. Power the PLC from the bus.
2. PLC comms: plc-1.X0 (DATA+ proxy) → tb-rs485-a.t0 and plc-1.X1 (DATA− proxy) → tb-rs485-b.t0.
3. Distribute the bus: tb-rs485-a.t1 → slave-1.A and tb-rs485-b.t1 → slave-1.B (A to A, B to B — polarity!).
4. Terminate the bus: tb-rs485-a.t2 → term-1.A and tb-rs485-b.t2 → term-1.B (terminator at the far end).
5. Power and earth the Modbus slave: tb-24v.t2 → slave-1.+24V; tb-0v.t2 → slave-1.0V; slave-1.PE → gnd-bar.pe2.
6. PE bonds: psu-1.PE → gnd-bar.pe0; psu-1.0V → gnd-bar.pe1.