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Modbus RTU slave implementation. More...

  • #include "../common/syn_defs.h"
  • #include "../drivers/syn_uart.h"
  • #include "../port/syn_port_system.h"
  • #include "../util/syn_crc.h"
  • #include <stdbool.h>
  • #include <stddef.h>
  • #include <stdint.h>

Classes

Type Name
struct SYN_Modbus
Modbus RTU slave instance — frame buffer, config, and statistics.
struct SYN_Modbus_Config
Modbus RTU slave configuration.
struct SYN_Modbus_DeviceInfo
Device Information structure for FC 0x2B / 0x0E.

Public Types

Type Name
typedef bool(* SYN_Modbus_ReadFileRecordCallback
Callback for reading file records (FC 0x14).
typedef bool(* SYN_Modbus_WriteCallback
Called before a write to holding registers is applied.
typedef bool(* SYN_Modbus_WriteFileRecordCallback
Callback for writing file records (FC 0x15).

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.

Macros

Type Name
define SYN_MB_EX_DEVICE_FAILURE 0x04
define SYN_MB_EX_ILLEGAL_ADDR 0x02
define SYN_MB_EX_ILLEGAL_FUNC 0x01
define SYN_MB_EX_ILLEGAL_VALUE 0x03
define SYN_MB_FC_DIAGNOSTICS 0x08
define SYN_MB_FC_GET_COMM_EVENT_CNT 0x0B
define SYN_MB_FC_GET_COMM_EVENT_LOG 0x0C
define SYN_MB_FC_MASK_WRITE_REGISTER 0x16
define SYN_MB_FC_READ_COILS 0x01
define SYN_MB_FC_READ_DEVICE_INFO 0x2B
define SYN_MB_FC_READ_DISCRETE_INPUTS 0x02
define SYN_MB_FC_READ_EXCEPTION_STATUS 0x07
define SYN_MB_FC_READ_FIFO_QUEUE 0x18
define SYN_MB_FC_READ_FILE_RECORD 0x14
define SYN_MB_FC_READ_HOLDING 0x03
define SYN_MB_FC_READ_INPUT 0x04
define SYN_MB_FC_READ_WRITE_MULTIPLE 0x17
define SYN_MB_FC_REPORT_SERVER_ID 0x11
define SYN_MB_FC_WRITE_FILE_RECORD 0x15
define SYN_MB_FC_WRITE_MULTIPLE 0x10
define SYN_MB_FC_WRITE_MULTIPLE_COILS 0x0F
define SYN_MB_FC_WRITE_SINGLE 0x06
define SYN_MB_FC_WRITE_SINGLE_COIL 0x05
define SYN_MB_MEI_TYPE_READ_DEVICE_ID 0x0E

Detailed Description

Implements a Modbus RTU slave using UART, CRC-16/Modbus (in syn_crc), and register/bit maps. Supports all 19 function codes:

  • 0x01: Read Coils - 0x0F: Write Multiple Coils
  • 0x02: Read Discrete Inputs - 0x10: Write Multiple Registers
  • 0x03: Read Holding Registers - 0x11: Report Server ID
  • 0x04: Read Input Registers - 0x14: Read File Record
  • 0x05: Write Single Coil - 0x15: Write File Record
  • 0x06: Write Single Register - 0x16: Mask Write Register
  • 0x07: Read Exception Status - 0x17: Read/Write Multiple Registers
  • 0x08: Serial Line Diagnostics - 0x18: Read FIFO Queue
  • 0x0B: Get Comm Event Counter - 0x2B: Read Device Identification
  • 0x0C: Get Comm Event Log

The register/coil maps are application-owned arrays. The Modbus module handles framing, CRC validation, and exception responses.

** **

static uint16_t holding_regs[32];
static uint16_t input_regs[16];
static SYN_Modbus mb;
static uint8_t mb_buf[256];

SYN_Modbus_Config cfg = {
    .slave_addr = 1,
    .uart = SYN_UART0,
    .holding_regs = holding_regs,
    .holding_count = 32,
    .input_regs = input_regs,
    .input_count = 16,
};
syn_modbus_init(&mb, &cfg, mb_buf, sizeof(mb_buf));

// In main loop:
syn_modbus_poll(&mb);  // checks UART, processes requests

// Your app writes input registers:
input_regs[0] = adc_value;

// Your app reads holding registers (written by master):
uint16_t setpoint = holding_regs[0];

Public Types Documentation

typedef SYN_Modbus_ReadFileRecordCallback

Callback for reading file records (FC 0x14). 

typedef bool(* SYN_Modbus_ReadFileRecordCallback) (struct SYN_Modbus *mb, uint16_t file_num, uint16_t record_num, uint16_t record_len, uint16_t *record_data, void *ctx);

typedef SYN_Modbus_WriteCallback

Called before a write to holding registers is applied. 

typedef bool(* SYN_Modbus_WriteCallback) (struct SYN_Modbus *mb, uint16_t addr, uint16_t count, void *ctx);

Parameters:

  • mb Modbus instance.
  • addr Register start address.
  • count Number of registers.
  • ctx User context.

Returns:

true to allow the write, false to reject (exception 0x03).

typedef SYN_Modbus_WriteFileRecordCallback

Callback for writing file records (FC 0x15). 

typedef bool(* SYN_Modbus_WriteFileRecordCallback) (struct SYN_Modbus *mb, uint16_t file_num, uint16_t record_num, uint16_t record_len, const uint16_t *record_data, void *ctx);

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.

Macro Definition Documentation

define SYN_MB_EX_DEVICE_FAILURE

#define SYN_MB_EX_DEVICE_FAILURE `0x04`

Device failure

define SYN_MB_EX_ILLEGAL_ADDR

#define SYN_MB_EX_ILLEGAL_ADDR `0x02`

Illegal data address

define SYN_MB_EX_ILLEGAL_FUNC

#define SYN_MB_EX_ILLEGAL_FUNC `0x01`

Illegal function code

define SYN_MB_EX_ILLEGAL_VALUE

#define SYN_MB_EX_ILLEGAL_VALUE `0x03`

Illegal data value

define SYN_MB_FC_DIAGNOSTICS

#define SYN_MB_FC_DIAGNOSTICS `0x08`

Serial line diagnostics

define SYN_MB_FC_GET_COMM_EVENT_CNT

#define SYN_MB_FC_GET_COMM_EVENT_CNT `0x0B`

Get comm event counter

define SYN_MB_FC_GET_COMM_EVENT_LOG

#define SYN_MB_FC_GET_COMM_EVENT_LOG `0x0C`

Get comm event log

define SYN_MB_FC_MASK_WRITE_REGISTER

#define SYN_MB_FC_MASK_WRITE_REGISTER `0x16`

Mask write register

define SYN_MB_FC_READ_COILS

#define SYN_MB_FC_READ_COILS `0x01`

Read coils (bits)

define SYN_MB_FC_READ_DEVICE_INFO

#define SYN_MB_FC_READ_DEVICE_INFO `0x2B`

Encapsulated interface trans

define SYN_MB_FC_READ_DISCRETE_INPUTS

#define SYN_MB_FC_READ_DISCRETE_INPUTS `0x02`

Read discrete inputs (bits)

define SYN_MB_FC_READ_EXCEPTION_STATUS

#define SYN_MB_FC_READ_EXCEPTION_STATUS `0x07`

Read exception status

define SYN_MB_FC_READ_FIFO_QUEUE

#define SYN_MB_FC_READ_FIFO_QUEUE `0x18`

Read FIFO queue

define SYN_MB_FC_READ_FILE_RECORD

#define SYN_MB_FC_READ_FILE_RECORD `0x14`

Read file record

define SYN_MB_FC_READ_HOLDING

#define SYN_MB_FC_READ_HOLDING `0x03`

Read holding registers

define SYN_MB_FC_READ_INPUT

#define SYN_MB_FC_READ_INPUT `0x04`

Read input registers

define SYN_MB_FC_READ_WRITE_MULTIPLE

#define SYN_MB_FC_READ_WRITE_MULTIPLE `0x17`

Read/Write multiple regs

define SYN_MB_FC_REPORT_SERVER_ID

#define SYN_MB_FC_REPORT_SERVER_ID `0x11`

Report server ID

define SYN_MB_FC_WRITE_FILE_RECORD

#define SYN_MB_FC_WRITE_FILE_RECORD `0x15`

Write file record

define SYN_MB_FC_WRITE_MULTIPLE

#define SYN_MB_FC_WRITE_MULTIPLE `0x10`

Write multiple registers

define SYN_MB_FC_WRITE_MULTIPLE_COILS

#define SYN_MB_FC_WRITE_MULTIPLE_COILS `0x0F`

Write multiple coils (bits)

define SYN_MB_FC_WRITE_SINGLE

#define SYN_MB_FC_WRITE_SINGLE `0x06`

Write single register

define SYN_MB_FC_WRITE_SINGLE_COIL

#define SYN_MB_FC_WRITE_SINGLE_COIL `0x05`

Write single coil (bit)

define SYN_MB_MEI_TYPE_READ_DEVICE_ID

#define SYN_MB_MEI_TYPE_READ_DEVICE_ID `0x0E`

Read Device Identification

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