RS-485 wiring, termination and bias

An RS-485 bus for Modbus RTU is one twisted-pair trunk, daisy-chained from device to device, with a terminator at each physical end and bias at one point. Most RS-485 faults come from breaking one of those rules: a star branch, a third terminator, bias at two devices or swapped conductors. This guide explains each rule and how to check the installed state.

This guide is part of the Modbus and RS-485 series. The RS-485 vs Modbus guide explains what each of the two names defines. This guide uses the Modbus serial line guide for the protocol-specific values. Always check them against the instructions for the exact devices.

Wire one daisy-chained trunk

Run one cable from the first device to the last, and connect each device on the way. Do not branch the bus into a star.

Swipe to see the whole drawing

Each connection from the trunk to a device is a stub (the Modbus guide calls it a derivation). Keep stubs as short as the terminals allow. The Modbus serial guide sets a limit of 20 m for one stub, and 40 m divided by the number of stubs on a multi-port tap. A long branch to a distant meter is a second trunk, not a stub, and it needs a repeater.

Draw the bus as installed: the cable route, the length of each section, every device, every joint and every repeater. The master does not have to be at one end. A controller in a middle cabinet is in the middle of the trunk.

Terminate the two ends, and only the ends

A terminator stops the signal reflecting back from the end of the cable. It goes across the two signal conductors at each physical end of the trunk. The Modbus serial guide requires a termination near each end and allows no more than two on one pair. It also forbids a terminator on a stub.

The value should match the characteristic impedance of the cable, typically 120 Ω for RS-485 twisted pair. The Modbus serial guide gives two choices:

• a 150 Ω, 0.5 W resistor; or
• a 120 Ω, 0.25 W resistor in series with a 1 nF capacitor, which is the better choice when the line is biased.

Many meters and interfaces have a built-in terminator on a switch or a jumper. Count every enabled one. The terminators are in parallel, so each extra one lowers the load that the drivers see:

The RS-485 termination and bias checker finds contradictions such as three terminators or bias at two points:

Bias at one point, if a device needs it

When no device transmits, nothing drives the bus. Noise can then look like data to a receiver. Bias (the Modbus guide calls it polarisation) holds the idle bus in a defined state: a pull-up resistor from D1 to 5 V and a pull-down from D0 to common. The Modbus serial guide sets these rules:

• each device's documentation must say whether it needs bias and whether it provides it;
• if any device needs bias, fit one pair of resistors of 450 to 650 Ω, at one point for the whole bus, usually at the master;
• no other device may apply bias;
• with bias fitted, the number of devices allowed on the bus is 4 fewer.

Many modern transceivers are "failsafe": they read an idle bus correctly without bias. Check the device documentation rather than adding bias by habit.

Why termination and bias interact

A receiver needs at least 200 mV between the two conductors to read a defined state. Bias makes that voltage through a divider with the terminators. With the idle bus, 5 V bias supply and 650 Ω resistors:

With plain 120 Ω terminators there is almost no margin above 200 mV, before any tolerance or noise. That is why the Modbus guide prefers the capacitor version on a biased bus: the capacitor blocks the DC bias current, and the resistor still terminates the fast signal edges. These figures ignore the receivers, the cable and component tolerances. Use them to understand the design, not to accept a bus.

Polarity, common and shield

A and B are not consistent. The Modbus serial guide names the conductors D0 and D1, and maps them to the RS-485 terminal names A/A' and B/B'. Manufacturers do not all follow this. Some label their terminals A/B the other way round, and others use D+/D−, 485+/485− or TX+/TX−. Use the signal definition in each device's manual, not the letter printed on the terminal. The Modbus guide's recommended colours are yellow for D1, brown for D0 and grey for common.

Connect the common. RS-485 is differential, but the receivers still need the voltages of the two conductors to stay inside their common-mode range. The Modbus guide includes a common conductor, connected to protective earth at one point only, usually at the master. Isolated interfaces change this. Follow the device documentation and the site earthing design.

Treat the shield separately. The shield, the signal common and protective earth have different jobs. Do not join them by habit.

Cable length, devices and baud rate

Higher baud rates, more devices and poorer cable shorten the length. The TI design guide explains how cable capacitance and signal rise time set the practical limit.

An electrically correct bus can still be too slow. The Modbus RTU timing calculator shows how long one polling cycle takes. A higher baud rate shortens the time on the wire, not the response time of each device, and one offline device can add a full timeout to every cycle.

Symptoms and their likely causes

Change one thing at a time, and record what you changed. The Modbus troubleshooting guide continues with protocol faults.

Keep a short bus with a ZMB

A ZMB Modbus interface sits next to the equipment that it reads and sends the registers over the Zigbee mesh to the Gateway. The RS-485 cable then runs a few metres inside one panel, not across the building, and the ZMB is the master at one end of that short trunk. The Modbus RS-485 commissioning guide covers the full commissioning sequence.