nanoMODBUS - A compact MODBUS RTU/TCP C library for microcontrollers

nanoMODBUS is a small C library that implements the Modbus protocol. It is especially useful in resource-constrained systems like microcontrollers.
Its main features are:

• Compact size
  • Only ~1000 lines of code
  • Client and server code can be disabled, if not needed
• No dynamic memory allocations
• Transports:
  • RTU
  • TCP
• Roles:
  • Client
  • Server
• Function codes:
  • 01 (0x01) Read Coils
  • 02 (0x02) Read Discrete Inputs
  • 03 (0x03) Read Holding Registers
  • 04 (0x04) Read Input Registers
  • 05 (0x05) Write Single Coil
  • 06 (0x06) Write Single Register
  • 15 (0x0F) Write Multiple Coils
  • 16 (0x10) Write Multiple registers
• Platform-agnostic
  • Requires only C99 and its standard library
  • Data transport read/write function are implemented by the user
• Broadcast requests and responses

At a glance

#include <stdio.h>

#include "nanomodbus.h"
#include "my_platform_stuff.h"

int main(int argc, char* argv[]) {
    // Set up the TCP connection
    void* conn = my_connect_tcp(argv[1], argv[2]);
    if (!conn) {
        fprintf(stderr, "Error connecting to server\n");
        return 1;
    }

    // my_transport_read() and my_transport_write() are implemented by the user 
    nmbs_platform_conf platform_conf;
    platform_conf.transport = NMBS_TRANSPORT_TCP;
    platform_conf.read = my_transport_read;
    platform_conf.write = my_transport_write;
    platform_conf.arg = conn;    // Passing our TCP connection handle to the read/write functions

    // Create the modbus client
    nmbs_t nmbs;
    nmbs_error err = nmbs_client_create(&nmbs, &platform_conf);
    if (err != NMBS_ERROR_NONE) {
        fprintf(stderr, "Error creating modbus client\n");
        return 1;
    }

    // Set only the response timeout. Byte timeout will be handled by the TCP connection
    nmbs_set_read_timeout(&nmbs, 1000);

    // Write 2 holding registers at address 26
    uint16_t w_regs[2] = {123, 124};
    err = nmbs_write_multiple_registers(&nmbs, 26, 2, w_regs);
    if (err != NMBS_ERROR_NONE) {
        fprintf(stderr, "Error writing register at address 26 - %s", nmbs_strerror(err));
        return 1;
    }

    // Read 2 holding registers from address 26
    uint16_t r_regs[2];
    err = nmbs_read_holding_registers(&nmbs, 26, 2, r_regs);
    if (err != NMBS_ERROR_NONE) {
        fprintf(stderr, "Error reading 2 holding registers at address 26 - %s\n", nmbs_strerror(err));
        return 1;
    }
    
    // Close the TCP connection
    my_disconnect(conn);
    
    return 0;
}

Installation

Just copy nanomodbus.c and nanomodbus.h inside your application codebase.

API reference

API reference is available in the repository's GitHub Pages.

Platform functions

nanoMODBUS requires the implementation of 2 platform-specific functions, defined as function pointers when creating a client/server instance.

Transport read/write

int32_t read(uint8_t* buf, uint16_t count, int32_t byte_timeout_ms, void* arg);
int32_t write(const uint8_t* buf, uint16_t count, int32_t byte_timeout_ms, void* arg);

These are your platform-specific functions that read/write data to/from a serial port or a TCP connection.
Both methods should block until either:

• count bytes of data are read/written
• the byte timeout, with byte_timeout_ms >= 0, expires

A value < 0 for byte_timeout_ms means no timeout.

Their return value should be the number of bytes actually read/written, or < 0 in case of error.
A return value between 0 and count - 1 will be treated as if a timeout occurred on the transport side. All other values will be treated as transport errors.

Platform functions argument

Platform functions and server callbacks can access arbitrary user data through their void* arg argument. The argument is useful, for example, to pass the connection a function should operate on.
Its initial value can be set inside the nmbs_platform_conf struct when creating the nmbs_t instance, and changed at any time via the nmbs_set_platform_arg API method.

Platform endianness

nanoMODBUS will attempt to detect the endianness of the platform at build time. If the automatic detection fails, you can manually set the endianness of the platform by defining either NMBS_BIG_ENDIAN or NMBS_LITTLE_ENDIAN in your build flags.

Tests and examples

Tests and examples can be built and run on Linux with CMake:

mkdir build && cd build
cmake ..
make

Please refer to examples/arduino/README.md for more info about building and running Arduino examples.

Misc

• To reduce code size, you can define the following #defines:
  • NMBS_CLIENT_DISABLED to disable all client code
  • NMBS_SERVER_DISABLED to disable all server code
  • To disable individual server callbacks, define the following:
    • NMBS_SERVER_READ_COILS_DISABLED
    • NMBS_SERVER_READ_DISCRETE_INPUTS_DISABLED
    • NMBS_SERVER_READ_HOLDING_REGISTERS_DISABLED
    • NMBS_SERVER_READ_INPUT_REGISTERS_DISABLED
    • NMBS_SERVER_WRITE_SINGLE_COIL_DISABLED
    • NMBS_SERVER_WRITE_SINGLE_REGISTER_DISABLED
    • NMBS_SERVER_WRITE_MULTIPLE_COILS_DISABLED
    • NMBS_SERVER_WRITE_MULTIPLE_REGISTERS_DISABLED
  • NMBS_STRERROR_DISABLED to disable the code that converts nmbs_errors to strings
• Debug prints about received and sent messages can be enabled by defining NMBS_DEBUG