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

Getting Started

Prerequisites

• C11 compatible compiler
• Basic knowledge of Modbus protocol (Wikipedia)

Quick Example

Here’s a minimal example to get you started:

#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/**) */
}

Tip: Check the examples/ directory for complete working examples with different transport layers (Serial, Ethernet).

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

Code Quality

This project follows MISRA C:2023 coding standards with a few documented exceptions where necessary for functionality or compatibility. The codebase emphasizes:

• Static analysis compliance for safety-critical applications
• Predictable behavior and reduced complexity
• Enhanced portability across embedded platforms
• Clear documentation of any deviations from the standard

Contributing

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