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

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

• Compact size, only ~1000 lines of code
• 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 "modbusino.h"
#include "my_platform_stuff.h"
#include <stdio.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_byte, my_transport_write_byte and my_sleep are implemented by the user 
    mbsn_platform_conf platform_conf;
    platform_conf.transport = MBSN_TRANSPORT_TCP;
    platform_conf.read_byte = my_transport_read_byte;
    platform_conf.write_byte = my_transport_write_byte;
    platform_conf.sleep = my_sleep_linux;
    platform_conf.arg = conn;    // Passing our TCP connection handle to the read/write functions

    // Create the modbus client
    mbsn_t mbsn;
    mbsn_error err = mbsn_client_create(&mbsn, &platform_conf);
    if (err != MBSN_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
    mbsn_set_read_timeout(&mbsn, 1000);

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

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

Installation

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

API reference

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

Platform functions

MODBUSino requires the implementation of 3 platform-specific functions, which are passed as function pointers when creating a client/server instance.

Transport read/write

int read_byte(uint8_t* b, int32_t, void* arg);
int write_byte(uint8_t b, int32_t, void* arg);

These are your platform-specific functions that read/write data to/from a serial port or a TCP connection.
Both functions should block until the requested byte is read/written.
If your implementation uses a read/write timeout, and the timeout expires, the function should return 0.
Their return values should be:

• 1 in case of success
• 0 if no data is available immediately or after an internal timeout expiration
• -1 in case of error

Sleep

void sleep(uint32_t milliseconds, void* arg);

This function should sleep for the specified amount of milliseconds.

Platform functions argument

Platform functions can access arbitrary user data through their void* arg argument. The argument is useful, for example, to pass to read/write function the connection they should operate on.
Its initial value can be set inside the mbsn_platform_conf struct when creating the client/server instance, and changed at any time via the mbsn_set_platform_arg API method.

Platform endianness

MODBUSino 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 MBSN_BIG_ENDIAN or MBSN_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

Misc

• To reduce code size, you can define MBSN_STRERROR_DISABLED to disable the code that converts mbsn_errors to strings
• Debug prints about received and sent messages can be enabled by defining MBSN_DEBUG