File syn_modbus.c

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Modbus RTU slave implementation.

• #include "../util/syn_assert.h"
• #include "syn_modbus.h"
• #include <string.h>
• #include "../util/syn_pack.h"

Public Functions

Type	Name
void	syn_modbus_feed (SYN_Modbus * mb, uint8_t byte) Feed a byte to the Modbus receiver.
void	syn_modbus_init (SYN_Modbus * mb, const SYN_Modbus_Config * cfg, uint8_t * buf, uint16_t buf_size) Initialize the Modbus RTU slave.
bool	syn_modbus_poll (SYN_Modbus * mb) Poll for incoming Modbus requests and process them.
bool	syn_modbus_process (SYN_Modbus * mb) Process a complete frame (call after feed + silence detection).
void	syn_modbus_reset (SYN_Modbus * mb) Reset the receiver state.

Public Static Functions

Type	Name
void	append_crc (uint8_t * buf, uint16_t len) Append CRC-16 Modbus to a frame buffer.
bool	check_crc (const uint8_t * buf, uint16_t len) Verify the CRC-16 Modbus trailer.
bool	get_bit (const uint8_t * bits, uint16_t idx) Helper to get bit from array.
void	handle_diagnostics (SYN_Modbus * mb) Handle Diagnostics (FC 0x08).
void	handle_get_comm_event_cnt (SYN_Modbus * mb) Handle Get Comm Event Counter (FC 0x0B).
void	handle_get_comm_event_log (SYN_Modbus * mb) Handle Get Comm Event Log (FC 0x0C).
void	handle_mask_write_register (SYN_Modbus * mb) Handle Mask Write Register (FC 0x16).
void	handle_read_bits (SYN_Modbus * mb, uint16_t frame_len, const uint8_t * bits, uint16_t total_bits) Handle Read Coils (FC 0x01) and Read Discrete Inputs (FC 0x02).
void	handle_read_device_info (SYN_Modbus * mb) Handle Modbus read device identification (FC 0x2B / MEI 0x0E).
void	handle_read_exception_status (SYN_Modbus * mb) Handle Modbus read exception status (FC 0x07).
void	handle_read_fifo_queue (SYN_Modbus * mb) Handle Read FIFO Queue (FC 0x18).
void	handle_read_file_record (SYN_Modbus * mb) Handle Modbus read file record (FC 0x14).
void	handle_read_regs (SYN_Modbus * mb, uint16_t frame_len, const uint16_t * regs, uint16_t reg_count) Handle Modbus read holding/input register request.
void	handle_read_write_multiple (SYN_Modbus * mb) Handle Modbus read/write multiple registers (FC 0x17).
void	handle_report_server_id (SYN_Modbus * mb) Handle Report Server ID (FC 0x11).
void	handle_write_file_record (SYN_Modbus * mb) Handle Modbus write file record (FC 0x15).
void	handle_write_multiple (SYN_Modbus * mb, uint16_t frame_len) Handle Modbus write multiple registers request.
void	handle_write_multiple_coils (SYN_Modbus * mb, uint16_t frame_len) Handle Write Multiple Coils (FC 0x0F).
void	handle_write_single (SYN_Modbus * mb, uint16_t frame_len) Handle Modbus write single register request.
void	handle_write_single_coil (SYN_Modbus * mb, uint16_t frame_len) Handle Write Single Coil (FC 0x05).
uint16_t	read_u16 (const uint8_t * buf)
void	send_exception (SYN_Modbus * mb, uint8_t func, uint8_t ex_code) Send a Modbus exception response.
void	send_response (SYN_Modbus * mb, uint16_t len) Append CRC and transmit a response frame.
void	set_bit (uint8_t * bits, uint16_t idx, bool val) Helper to set bit in array.
void	write_u16 (uint8_t * buf, uint16_t val)

Macros

Type	Name
define	MB_MAX_PDU_DATA 252
define	MB_MIN_FRAME_LEN 4
define	MB_SILENCE_MS 5

Public Functions Documentation

function syn_modbus_feed

Feed a byte to the Modbus receiver.

void syn_modbus_feed (
    SYN_Modbus * mb,
    uint8_t byte
) 

Alternative to poll() — call from UART ISR if you want interrupt-driven reception.

Parameters:

• mb Modbus instance.
• byte Received byte.

---

function syn_modbus_init

Initialize the Modbus RTU slave.

void syn_modbus_init (
    SYN_Modbus * mb,
    const SYN_Modbus_Config * cfg,
    uint8_t * buf,
    uint16_t buf_size
) 

Parameters:

• mb Modbus instance.
• cfg Configuration.
• buf Frame buffer (must be at least 256 bytes).
• buf_size Buffer capacity.

---

function syn_modbus_poll

Poll for incoming Modbus requests and process them.

bool syn_modbus_poll (
    SYN_Modbus * mb
) 

Call from your main loop. This reads bytes from UART, detects frame boundaries (3.5 character times silence), validates CRC, and processes the request.

Parameters:

• mb Modbus instance.

Returns:

true if a request was processed.

---

function syn_modbus_process

Process a complete frame (call after feed + silence detection).

bool syn_modbus_process (
    SYN_Modbus * mb
) 

Parameters:

• mb Modbus instance.

Returns:

true if a response was sent.

---

function syn_modbus_reset

Reset the receiver state.

void syn_modbus_reset (
    SYN_Modbus * mb
) 

Parameters:

• mb Modbus instance.

---

Public Static Functions Documentation

function append_crc

Append CRC-16 Modbus to a frame buffer.

static void append_crc (
    uint8_t * buf,
    uint16_t len
) 

Parameters:

• buf Frame buffer.
• len Frame length (excluding CRC).

---

function check_crc

Verify the CRC-16 Modbus trailer.

static bool check_crc (
    const uint8_t * buf,
    uint16_t len
) 

Parameters:

• buf Frame buffer.
• len Total frame length (including CRC).

Returns:

true if CRC matches.

---

function get_bit

Helper to get bit from array.

static bool get_bit (
    const uint8_t * bits,
    uint16_t idx
) 

---

function handle_diagnostics

Handle Diagnostics (FC 0x08).

static void handle_diagnostics (
    SYN_Modbus * mb
) 

---

function handle_get_comm_event_cnt

Handle Get Comm Event Counter (FC 0x0B).

static void handle_get_comm_event_cnt (
    SYN_Modbus * mb
) 

---

function handle_get_comm_event_log

Handle Get Comm Event Log (FC 0x0C).

static void handle_get_comm_event_log (
    SYN_Modbus * mb
) 

---

function handle_mask_write_register

Handle Mask Write Register (FC 0x16).

static void handle_mask_write_register (
    SYN_Modbus * mb
) 

---

function handle_read_bits

Handle Read Coils (FC 0x01) and Read Discrete Inputs (FC 0x02).

static void handle_read_bits (
    SYN_Modbus * mb,
    uint16_t frame_len,
    const uint8_t * bits,
    uint16_t total_bits
) 

---

function handle_read_device_info

Handle Modbus read device identification (FC 0x2B / MEI 0x0E).

static void handle_read_device_info (
    SYN_Modbus * mb
) 

Parameters:

• mb Modbus instance.

---

function handle_read_exception_status

Handle Modbus read exception status (FC 0x07).

static void handle_read_exception_status (
    SYN_Modbus * mb
) 

Parameters:

• mb Modbus instance.

---

function handle_read_fifo_queue

Handle Read FIFO Queue (FC 0x18).

static void handle_read_fifo_queue (
    SYN_Modbus * mb
) 

---

function handle_read_file_record

Handle Modbus read file record (FC 0x14).

static void handle_read_file_record (
    SYN_Modbus * mb
) 

Parameters:

• mb Modbus instance.

---

function handle_read_regs

Handle Modbus read holding/input register request.

static void handle_read_regs (
    SYN_Modbus * mb,
    uint16_t frame_len,
    const uint16_t * regs,
    uint16_t reg_count
) 

Parameters:

• mb Modbus instance.
• regs Register array.
• reg_count Total register count.

---

function handle_read_write_multiple

Handle Modbus read/write multiple registers (FC 0x17).

static void handle_read_write_multiple (
    SYN_Modbus * mb
) 

Parameters:

• mb Modbus instance.

---

function handle_report_server_id

Handle Report Server ID (FC 0x11).

static void handle_report_server_id (
    SYN_Modbus * mb
) 

---

function handle_write_file_record

Handle Modbus write file record (FC 0x15).

static void handle_write_file_record (
    SYN_Modbus * mb
) 

Parameters:

• mb Modbus instance.

---

function handle_write_multiple

Handle Modbus write multiple registers request.

static void handle_write_multiple (
    SYN_Modbus * mb,
    uint16_t frame_len
) 

Parameters:

• mb Modbus instance.

---

function handle_write_multiple_coils

Handle Write Multiple Coils (FC 0x0F).

static void handle_write_multiple_coils (
    SYN_Modbus * mb,
    uint16_t frame_len
) 

---

function handle_write_single

Handle Modbus write single register request.

static void handle_write_single (
    SYN_Modbus * mb,
    uint16_t frame_len
) 

Parameters:

• mb Modbus instance.

---

function handle_write_single_coil

Handle Write Single Coil (FC 0x05).

static void handle_write_single_coil (
    SYN_Modbus * mb,
    uint16_t frame_len
) 

---

function read_u16

static inline uint16_t read_u16 (
    const uint8_t * buf
) 

---

function send_exception

Send a Modbus exception response.

static void send_exception (
    SYN_Modbus * mb,
    uint8_t func,
    uint8_t ex_code
) 

Parameters:

• mb Modbus instance.
• func Function code that caused the exception.
• ex_code Exception code.

---

function send_response

Append CRC and transmit a response frame.

static void send_response (
    SYN_Modbus * mb,
    uint16_t len
) 

Parameters:

• mb Modbus instance.
• len PDU length (excluding CRC).

---

function set_bit

Helper to set bit in array.

static void set_bit (
    uint8_t * bits,
    uint16_t idx,
    bool val
) 

---

function write_u16

static inline void write_u16 (
    uint8_t * buf,
    uint16_t val
) 

---

Macro Definition Documentation

define MB_MAX_PDU_DATA

#define MB_MAX_PDU_DATA `252`

Max PDU data bytes.

---

define MB_MIN_FRAME_LEN

#define MB_MIN_FRAME_LEN `4`

Minimum: addr + func + CRC16.

---

define MB_SILENCE_MS

#define MB_SILENCE_MS `5`

Inter-frame silence (ms).

---

---

The documentation for this class was generated from the following file src/syntropic/proto/syn_modbus.c