modbus-rtu v0.1.0

Modbus RTU frame builder and parser.

Application scenarios

• A UART sensor or drive on an RS-485 bus.
• A gateway between Modbus and a local protocol.
• A bench tool that checks slave responses.

Contract

uint16_t modbus_crc16(const uint8_t *data, uint16_t length);
uint8_t modbus_build_read(uint8_t slave, uint8_t function,
                          uint16_t address, uint16_t count, uint8_t *frame);
uint8_t modbus_parse_read(const uint8_t *frame, uint16_t length,
                          uint8_t slave, uint8_t function,
                          uint16_t *values, uint8_t capacity, uint8_t *count);
uint8_t modbus_parse_exception(const uint8_t *frame, uint16_t length,
                               uint8_t slave, uint8_t function, uint8_t *code);

• modbus_build_read builds an 8-byte request with the CRC appended low

byte first. It supports function 0x03 and counts 1..125.

• modbus_parse_read validates slave, function, byte count, and CRC, then

decodes big-endian 16-bit registers.

• modbus_parse_exception accepts the 5-byte exception response.
• Returns 1 on success, 0 on a malformed frame.

Parameters

• Slave address 1..247; broadcast 0 is not a read request.
• Function 0x03 (holding registers); other functions use the same frame shape.
• UART settings are project-specific: 8N1, 9600/19200/115200, and a timeout.

Memory and timing

• No allocation; the caller owns the frame.
• A request is 8 bytes; a response is 5 + 2 * count bytes.
• A 3.5-character silent interval delimits frames; the UART adapter owns it.

Integration

uint8_t frame[8];
uint16_t registers[4];
uint8_t count;
modbus_build_read(1, 0x03, 0x0000, 4, frame);
/* send frame, receive response into frame */
if (modbus_parse_read(frame, response_length, 1, 0x03, registers, 4, &count)) {
    use(registers, count);
}

Adapters

• The adapter owns UART framing, DE/RE for RS-485, and the inter-frame timeout.
• Reference: Modbus over Serial Line Specification V1.02 and the

emelianov/modbus-esp8266 frame behavior (MIT).

• This implementation is original.

When not to use

• Modbus TCP; use an MBAP header layer.
• A bus without a defined turnaround or termination.

Verification

make test covers the CRC vector, request bytes, a valid response, an

exception, and malformed frames.