Wiring 9 — RS-485 / Modbus RTU Daisy Chain
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Briefing
Modbus RTU uses RS-485 differential signalling — two wires (A and B) that carry complementary signals for noise immunity over long runs. **Rules for RS-485:** 1. **Daisy chain** topology: one cable running device-to-device (not star wiring). 2. **Termination resistors** (120 Ω) at **both ends** of the bus to prevent reflections. 3. **Polarity**: A (−) and B (+) must be consistent across all devices. 4. Maximum 32 unit loads (nodes) on one segment. Wire the PLC, Inverter 1, and Inverter 2 in a daisy chain with termination.
Objectives
- Wire PLC RS-485 A (−) to daisy chain
- Wire PLC RS-485 B (+) to daisy chain
- Add 120 Ω termination at the PLC end
- Add 120 Ω termination at the last device (Inverter 2)
Hints
- Only the two ENDS of the daisy chain get termination resistors — not every device.
- A/B polarity must be consistent; swapping A and B inverts the signal and communication fails.
I/O Table
Inputs
RS485_ARS-485 A (−) differential line
BOOL · %I0.0
RS485_BRS-485 B (+) differential line
BOOL · %I0.1
TERM_END1120 Ω termination at PLC (first node)
BOOL · %I0.2
TERM_END2120 Ω termination at last device (Inverter 2)
BOOL · %I0.3
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