Wiring 9 — RS-485 Modbus RTU Daisy Chain with Termination

RS-485 Modbus RTU Daisy Chain with Termination

Why RS-485?

Modbus RTU communicates over RS-485 — a differential signalling standard designed for long cable runs in electrically noisy industrial environments. Instead of sending a signal relative to a common ground, RS-485 uses two wires called A and B that carry the same data with opposite polarity at all times. The receiver measures the voltage difference between A and B. Any noise picked up equally on both conductors (common-mode noise — inductive and capacitive interference from nearby motor drives, relay coils, and power cables) cancels out in the subtraction. RS-485 cables routinely run 1 200 metres in switchgear cabinets that would destroy an RS-232 link.

The Four Critical Wires per Node

Every Modbus node on the segment needs four connections:

1. A — non-inverting differential line.
2. B — inverting differential line.
3. COM / 0V — a common reference wire that runs alongside A and B. Without it the differential signal can drift outside the receiver's common-mode input range (typically ±7 V), and communication becomes intermittent or fails entirely. This is the most-skipped wire in field installations and the most common cause of "works in the lab, fails in the cabinet" Modbus problems.
4. Cable shield — bonded to PE at one end only (typically the master end). Bonding both ends creates a ground loop: any potential difference between the two earth points drives a circulating current down the shield, inducing noise on the very conductors you are trying to protect.

Daisy-Chain Topology — No Stars, No Spurs

RS-485 must be wired as a single linear chain: master → slave 1 → slave 2 → ... → last slave. The cable passes through each device in turn. Star wiring (one cable out from a central hub to each device) and spurs (short branches off the main cable) cause signal reflections that corrupt data, especially at higher baud rates (19 200 bps and above). Short spurs under approximately 30 cm are tolerated in practice because their reflection returns before the driver has finished the bit transition; longer spurs are not.

Termination at the Extremes Only

When a voltage step travels down a cable and reaches the far end, it reflects back unless the cable's characteristic impedance is matched at that end. Belden 9841 and similar twisted-pair cable have a characteristic impedance of approximately 120 Ω. Place a 120 Ω resistor across A-B at each extreme end of the chain (master end and last-slave end) — not at any intermediate node. Termination in the middle halves the load impedance seen by the drivers and reduces the signal amplitude. Termination at one end only leaves the other end unterminated, and that reflection arrives back exactly when the driver is trying to resolve the next bit.

Biasing

When no node is transmitting, the A and B lines float to an undefined voltage. Some receivers interpret a floating line as a mark (logic 1) because a mark is the idle state; others interpret it as a space (logic 0), causing a framing error the moment the first byte arrives. Most modern Modbus master ports include built-in bias resistors — a pull-up on B and a pull-down on A that force the lines to a known idle state (typically 4-5 V differential) between transmissions. In this lesson the master node is assumed to have biasing built in; no separate bias resistor pair is modelled.

Common Modbus Wiring Mistakes

• Missing COM wire. All three conductors (A, B, COM) must run between every adjacent pair of nodes. Omitting COM causes intermittent failures that worsen over cable length.
• Terminating in the middle. A terminator across the A-B of a middle node loads the bus permanently. Symptoms: weak signal, high error rate, or complete failure at higher baud rates.
• Spurs longer than ~30 cm. A device tapped off the main cable with a spur longer than 30 cm acts as a stub antenna. Remove the spur; splice the device into the main cable run.
• Ground loop on the shield. Bonding the cable shield to PE at more than one point creates a low-resistance loop. The circulating current induced by earth-potential differences is a significant noise source on the signal conductors.