peamodbus

Fault tolerant, TCP modbus implementation in Go that just works. Apt for embedded systems.

This is a WIP.

Protocol sequence diagram:

Mermaid Sequence diagram

sequenceDiagram
    participant Client
    participant Server
    
    critical No requests pending
        note left of Client: peamodbus.Tx.Request*()
        Client ->>+ Server: Request data read or write
        note right of Server: peamodbus.Rx.ReceiveRequest()
        option Handle request on server side
        Server --> Server: Identify func. code and validate request.
        note right of Server: peamodbus.Receive*Response()
        Server --> Server: Access modbus data, read or write.
        note right of Server: peamodbus.Receive*Response()
        option Write response
        note right of Server: peamodbus.Request.PutResponse()
        Server ->>- Client: Write response (nominal or exception)
        note left of Client: peamodbus.Receive*Response()
    end

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Examples

See up to date examples in examples directory. These are just copy pasted from there and may occasionally be out of date.

Modbus Server via TCP

Click to see TCP example

package main

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

	"github.com/soypat/peamodbus"
	"github.com/soypat/peamodbus/modbustcp"
)

// This program creates a modbus server that adds 1 to
// all holding registers on every client request.

func main() {
	dataBank := peamodbus.ConcurrencySafeDataModel(&peamodbus.BlockedModel{})
	sv, err := modbustcp.NewServer(modbustcp.ServerConfig{
		Address:        "localhost:8080",
		ConnectTimeout: 5 * time.Second,
		DataModel:      dataBank,
	})
	if err != nil {
		log.Fatal(err)
	}
	ctx := context.Background()
	for {
		if !sv.IsConnected() {
			fmt.Println("attempting connection")
			err = sv.Accept(ctx)
			if err != nil {
				log.Println("error connecting", err)
			}
			time.Sleep(time.Second)
			continue
		}
		err = sv.HandleNext()
		if err != nil {
			log.Println("error in HandleNext", err)
			time.Sleep(time.Second)
		} else {
			for addr := 0; addr < 125; addr++ {
				value, exc := dataBank.GetHoldingRegister(addr)
				if exc != 0 {
					panic(exc.Error())
				}
				dataBank.SetHoldingRegister(addr, value+1)
			}
		}
		if err := sv.Err(); err != nil {
			log.Println("server error:", err)
		}
	}
}

Modbus Client RTU via USB

Reads from 2 registers in an instrument connected on a port using the go.bug.st/serial library.

Click to see RTU example

package main

import (
	"io"
	"os"
	"time"

	"github.com/soypat/peamodbus/modbusrtu"
	"go.bug.st/serial"
	"golang.org/x/exp/slog"
)

func main() {
	const (
		deviceAddress         = 86
		sensorRegisterAddress = 0
	)

	port, err := serial.Open("/dev/ttyUSB0", &serial.Mode{
		BaudRate: 9600,
		DataBits: 8,
		Parity:   serial.NoParity,
		StopBits: serial.OneStopBit,
	})
	if err != nil {
		panic(err)
	}
	defer port.Close()

	logfp, _ := os.Create("log.txt")
	defer logfp.Close()

	logger := slog.New(slog.NewTextHandler(io.MultiWriter(os.Stdout, logfp), &slog.HandlerOptions{
		Level: slog.LevelDebug,
	}))

	c := modbusrtu.NewClient(modbusrtu.ClientConfig{
		Logger: logger,
	})

	c.SetTransport(port)
	for {
		var tempHumidity [2]uint16
		err = c.ReadHoldingRegisters(deviceAddress, sensorRegisterAddress, tempHumidity[:])
		if err != nil {
			panic(err)
		}
		logger.Info("read", "humidity", float32(tempHumidity[0])/10, "temperature", float32(tempHumidity[1])/10)
		time.Sleep(time.Second)
	}
}