Modbus

Modbus is the most widely deployed protocol in industrial automation. It dates from 1979, it is simple, and almost every energy meter, solar inverter, battery system, drive and PLC speaks it. A client (master) reads and writes four data tables on a server (slave): coils and discrete inputs (bits), and input registers and holding registers (16-bit words).

Where you meet it

• Energy meters, solar inverters and battery systems, often with a SunSpec register map on top
• PLCs, drives and remote I/O on the plant floor
• RS-485 field buses (Modbus RTU) and serial-to-Ethernet gateways

Quickstart

From the command line

Download modbusctl from GitHub Releases, then:

# What is this device? (FC43 Device Identification + FC17 Report Server ID)
modbusctl client identify --ip 192.168.1.10 --server-id

# Which register blocks are readable? (adaptive scan, saved to MCAP)
modbusctl client scan --ip 192.168.1.10 --algo smart --start 0 --end 5000 --output scan.mcap

# Poll four holding registers every second until Ctrl-C
modbusctl client poll --ip 192.168.1.10 --start 100 --count 4

# Serve the scan results as a simulated device on port 1502
modbusctl server static --input scan.mcap --port 1502

Addresses are raw Modbus PDU addresses (0–65535), not “4xxxx” register labels.

From Go

go get github.com/otfabric/go-modbus

Read four holding registers from unit 1 over Modbus TCP:

package main

import (
  "context"
  "fmt"
  "log"
  "time"

  "github.com/otfabric/go-modbus"
)

func main() {
  client, err := modbus.NewClient(modbus.NewConfig(
    modbus.TransportConfig{URL: "tcp://192.168.1.10:502", DialTimeout: 5 * time.Second},
    modbus.ExecutionConfig{Timeout: 3 * time.Second},
    modbus.ObservabilityConfig{},
  ))
  if err != nil {
    log.Fatal(err)
  }
  if err := client.Open(); err != nil {
    log.Fatal(err)
  }
  defer client.Close()

  ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
  defer cancel()

  regs, err := client.ReadRegisters(ctx, 1, 100, 4, modbus.HoldingRegister)
  if err != nil {
    log.Fatal(err)
  }
  fmt.Println(regs)
}

What go-modbus supports

Interoperability

modbus-interop runs go-modbus against libmodbus (C), PyModbus (Python), digitalpetri/modbus (Java), NModbus (C#) and tokio-modbus (Rust), as both client and server.