C Modbus Slave

Lightweight Modbus slave implementation for embedded C applications.

Features

• Zero dynamic allocations - Can be used without any dynamic allocations
• Transport agnostic - Works with RS485/RS232 serial and Ethernet
• Flexible data access - Direct pointers, callbacks, or constant values
• Compact footprint - Minimal RAM and flash usage for embedded systems
• Fast CRC calculation - Uses lookup table for efficient CRC-16 computation (512 bytes ROM)
• Standards compliant - Implements Modbus specification accurately
• Thread-safe - No global state, multiple instances supported

Prerequisites

• C11 compatible compiler

Quick Start

#include "mbinst.h"

/* 1. Define your data */
static uint16_t s_temperature = 250;

/* 2. Create register map (must be in ascending address order) */
static const struct mbreg_desc_s s_holding_regs[] = {
    {
        .address=0x00,
        .type=MRTYPE_U16,
        .access=MRACC_RW_PTR,
        .read={.pu16=&s_temperature},
        .write={.pu16=&s_temperature}
    }
};

/* 3. Create instance */
static struct mbinst_s s_inst = {
    .hold_regs=s_holding_regs,
    .n_hold_regs=sizeof s_holding_regs / sizeof s_holding_regs[0]
};

/* 4. Initialize internal instance state */
mbinst_init(&inst);

/* 5. Handle incoming requests */
uint8_t req[MBADU_SIZE_MAX];
size_t req_len = fetch_request(req); /* Fetch data from your transport layer (see examples/**) */

uint8_t resp[MBADU_SIZE_MAX];
size_t resp_len = mbadu_handle_req(&s_inst, req, req_len, resp);
if (resp_len) {
    send_response(resp, resp_len); /* Send response via your transport layer (see examples/**) */
}

Documentation

• Building & Setup - Build instructions and file dependencies
• API Reference - API documentation and function signatures
• Examples - Usage examples for different scenarios
• Testing - Unit testing and validation tools
• Glossary - Abbreviations and technical terms

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.