Modbus RTU Survival Guide | RS485 & Troubleshooting

On paper, Modbus RTU over RS485 is simple. In practice, it is one of the biggest sources of headaches on the plant floor. Why? Because RS485 is a physical transmission line subject to the laws of electromagnetism, not just bits and bytes.

If your network has random “timeouts” or constant CRC errors, it’s not a bug in your code; it’s a physics problem. In this guide, we are going to “dissect” the physical layer.

1. RS485 Physics: Termination and Reflections

An RS485 cable must be treated as a transmission line. When a signal travels down a cable and reaches an open end, the energy “bounces” and returns, colliding with new data. This is called signal reflection.

The Remedy: 120Ω Resistor

You must place a 120 Ohm resistor at the absolute physical ends of the bus (and only at the ends).

Common Error: Putting resistors on every slave. This overloads the driver and burns the port.
Effect: The resistor absorbs the energy at the end of the cable, eliminating the bounce.

2. Fail-Safe Biasing

When no device is transmitting, the bus enters a “high impedance” state. At this time, the differential voltage between A and B can be 0V. For an RS485 receiver, 0V is an indeterminate zone: ambient noise can be interpreted as false bits, generating “Frame Error” or garbage bytes.

Technical Implementation

You need to force a minimum differential voltage (typically >200mV) using pull-up and pull-down resistors:

Pull-up (560Ω to 1kΩ) on the D+ (A) line to VCC (5V).
Pull-down (560Ω to 1kΩ) on the D- (B) line to GND.

[!TIP] Many industrial converters (Moxa, Advantech) have internal DIP switches to enable this. Use them!

3. The Third Wire: Common Ground

“RS485 only needs two wires.” Lie. Although it is a differential protocol, transceivers have a common-mode voltage limit (usually -7V to +12V). If the ground of a PLC in Panel A is 50V apart from the ground of a sensor in Panel B, the RS485 chip will burn out or stop reading.

Solution: Use shielded twisted-pair cable. Use the shield or a third conductor wire to join the G or SG terminals of all devices.

4. Troubleshooting with Code

To diagnose, you need tools that don’t assume everything is fine. I’ve created a repository with utilities for this:

👉 GitHub: modbus-troubleshooting-toolkit

Slave ID Scanning (Practical Snippet)

If you don’t know the ID of a slave, you can use this Python snippet (requires pyserial and pymodbus):

import serial
from pymodbus.client import ModbusSerialClient

client = ModbusSerialClient(
    port='/dev/ttyUSB0',
    baudrate=9600,
    stopbits=1,
    bytesize=8,
    parity='N'
)

for slave_id in range(1, 248):
    result = client.read_holding_registers(0, 1, slave=slave_id)
    if not result.isError():
        print(f"Slave found at ID: {slave_id}!")