Modbus Master

Example Usage

#include <proto/modbus.h>

/* init modbus RTU master, on port UART0  */
mb_init("/dev/ttyS0", MB_RTU, dirctl);

/* configure modbus at 115200bps, 8bits, No parity, 1 stopbit*/
mb_config(115200, "8N1");

/* send modbus request */
struct modbus_t mbreq;
uint8_t buffer[256];

/* set slave id */
mbreq.slaveid = 1;
/* Set function code as Read holding register */
mbreq.function = MB_FUNC_RD_HOLDING;
/* set start address to read from */
mbreq.op.read.start_add = 0x00;
/* read 1 register */
mbreq.op.read.no_regs = 1;
/* configure response buffer */
mbreq.resp_buf = buffer;
mbreq.resp_buflen = sizeof(buffer);
/* set response timeout in milliseconds */
mbreq.resp_timeout = 1000;
/* initiate request */
mb_request(&mbreq);

/* doing, Modbus transceiver direction control, this function is passed as argument to mb_init() */
void dirctl(int dir)
{
    if (dir == MB_DIR_RX) {
        /* receive, control GPIO or something */
    } else {
        /* transmit, change gpio again */
    }
}

API Reference

Header File

Source: include/proto/modbus.h

#include <proto/modbus.h>

Functions

int mb_init(const char *com, int mode, mb_dirctl func)

Initialize MODBUS

Parameters:

com – [in] UART device file (e.g. /dev/ttyS1)
mode – [in] Operation mode _mb_mode
func – [in] Direction control callback function

Returns:

For return value see mb_err_e

int mb_config(unsigned int baud, const char *com_para)

Configure MODBUS Parameters

Parameters:

baud – [in] Baudrate value
com_para –
[in] Communication parameter data bits, parity and stopbit in ascii format e.g. “8N1”, “8E2”

Data bits: 8, 7, 6, 5 (not supported on all platforms)

Parity: N - None, E - Even, O - Odd Parity

Stop Bits: 1 or 2

Returns:

For return value see mb_err_e

int mb_request(struct modbus_t *mbus)

Perform a MODBUS request

Parameters:: mbus – [in] Modbus handle from mb_init
Returns:: For return value see mb_err_e

unsigned short mb_crc(unsigned char *msg, int len)

Calculate MODBUS CRC

Parameters:

msg – [in] Input buffer
len – [in] Length of data in buffer

Returns:

CRC value

unsigned char mb_lrc(unsigned char *msg, int len)

Calculate MODBUS LRC

Parameters:

msg – [in] Input buffer
len – [in] Length of data in buffer

Returns:

LRC value

char *mb_strerror(uint8_t errnum)

MODBUS exception to string

Parameters:: errnum – [in] Exception code
Returns:: error description as string

Unions

union mb_op

#include <modbus.h>

Modbus operation

Public Members

struct _mb_read read: Read operation, see _mb_read

struct _mb_write write: Write operation, see _mb_write

Structures

struct _mb_read

Read operation structure

Public Members

uint32_t start_add: Start address

uint16_t no_regs: Number of registers (16-bit) to read

struct _mb_write

Write operation structure

Public Members

uint32_t wr_add: Start address

uint16_t no_regs: Number of registers (16-bit) to write. For writing a 32-bit value no_regs = 2

int len: Length of data in data buffer

uint8_t *data: buffer containing data to write

struct modbus_t

MODBUS request structure

Public Members

uint8_t slaveid: Slave ID

uint8_t function: Function code

union mb_op op: Modbus Operation, see mb_op

uint8_t *resp_buf: Buffer pointer to store MODBUS response. In MODBUS ASCII mode, data will be stored after conversion from ASCII to hex

uint16_t resp_buflen: Length of response buffer, On return this value will be updated with actual number of bytes stored in response buffer

uint8_t resp_timeout: MODBUS response timeout in milliseconds

int reserved: Reserved Unused

Type Definitions

typedef void (*mb_dirctl)(int dir)

MODBUS direction control callback function

Param dir:: Direction request flag _mb_dir

Enumerations

enum mb_err_e

Modbus Error codes

Values:

enumerator MB_OK: No error.

enumerator MB_ERR_INVALID_ARG: Invalid argument

enumerator MB_ERR_NOTSUP: Mode or Function code not supported

enumerator MB_ERR_TIMEOUT: Modbus request timed out

enumerator MB_ERR_INVALID_DATA: Invalid response received from slave

enumerator MB_ERR_CRC_FAIL: Checksum failed, CRC (for RTU) or LRC (for ASCII)

enumerator MB_ERR_INIT_FAIL: Fail to initialize port or Port not initialized

enumerator MB_ERR_CONF_FAIL: Fail to configure port

enum _mb_func

MODBUS function code

Values:

enumerator MB_FUNC_INVAL: Invalid function code

enumerator MB_FUNC_RD_COIL: Read coil

enumerator MB_FUNC_RD_DISC_INPUT: Read discrete input

enumerator MB_FUNC_RD_HOLDING: Read holding register

enumerator MB_FUNC_RD_INPUT: Read input register

enumerator MB_FUNC_WR_COIL: Write coil

enumerator MB_FUNC_WR_REG: Write single holding register

enumerator MB_FUNC_RD_EXP: Read exception status

enumerator MB_FUNC_DIAG: Diagnostic

enumerator MB_FUNC_COMM_EVT_COUNT: Get COM event counter

enumerator MB_FUNC_COMM_EVT_LOG: Get COM event log

enumerator MB_FUNC_WR_MUL_COIL: Write multiple coils

enumerator MB_FUNC_WR_MUL_REG: Write multiple holding register

enumerator MB_FUNC_REP_SERV_ID: Report slave ID

enumerator MB_FUNC_RD_FILE_REC: Read file record

enumerator MB_FUNC_WR_FILE_REC: Write file record

enumerator MB_FUNC_MASK_WR_REG: Mask write register

enumerator MB_FUNC_RDWR_MUL_REG: Read/Write multiple register

enumerator MB_FUNC_RD_FIFO_Q: Read FIFO Queue

enum _mb_mode

MODBUS operation mode

Values:

enumerator MB_INVALID: Invalid mode

enumerator MB_ASCII: MODBUS ASCII

enumerator MB_RTU: MODBUS RTU

enum _mb_dir

MODBUS direction request flag

Values:

enumerator MB_DIR_RX: Enable read request

enumerator MB_DIR_TX: Enable write request