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Modbus

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Modbus is a simple, royalty-free, master/server (now client/server) protocol introduced by Modicon in 1979. It has outlived nearly every “modern” competitor because it is easy to implement, well-documented, and universally supported.

Overview

  • One client (master) talks to many servers (slaves).
  • Server addresses 1–247; 0 is broadcast (RTU/ASCII only).
  • No native authentication or encryption — segment the network.
  • Maintained by the Modbus Organization.

Variants

  • Modbus RTU — binary over RS-485 / RS-232; most common in the field.
  • Modbus ASCII — readable text over serial; rarely used.
  • Modbus TCP — over Ethernet, TCP port 502; no CRC (TCP handles it).
  • Modbus over UDP, Modbus over TLS — newer additions.

Data Model

  • Discrete Inputs — 1-bit, read-only (10001–19999).
  • Coils — 1-bit, read/write (00001–09999).
  • Input Registers — 16-bit, read-only (30001–39999).
  • Holding Registers — 16-bit, read/write (40001–49999).
  • 32-bit values use 2 consecutive registers; watch endianness (big / little / mixed).

Function Codes

  • 0x01 Read Coils.
  • 0x02 Read Discrete Inputs.
  • 0x03 Read Holding Registers.
  • 0x04 Read Input Registers.
  • 0x05 Write Single Coil.
  • 0x06 Write Single Register.
  • 0x0F Write Multiple Coils.
  • 0x10 Write Multiple Registers.
  • 0x17 Read/Write Multiple Registers.

Wiring & Timing

  • RS-485: twisted pair, 2-wire (A/B) or 4-wire; 120 Ω termination at both ends.
  • Up to 32 nodes per segment (more with low-load transceivers); 1200 m max at 9600 baud.
  • Common baud rates: 9600, 19200, 38400, 57600, 115200.
  • RTU inter-character silence: 1.5× char time; inter-message silence: 3.5× char time.

Troubleshooting

  • Check baud, parity, stop bits match on every node.
  • Verify A/B polarity (a swapped pair is the #1 symptom).
  • Termination resistor present at both ends (not the middle).
  • Use a Modbus poller (Modbus Poll, ModScan, mbpoll) to isolate device vs master.
  • For TCP: check port 502 firewall, server unit ID, slave timeout.
  • Exception codes 01–04 are the most common: bad function / address / value / device failure.
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