Modbus

Modbus is an industrial messaging protocol. The protocol is specified for different types of networks or buses. Zephyr OS implementation supports communication over serial line and may be used with different physical interfaces, like RS485 or RS232. TCP support is not implemented directly, but there are helper functions to realize TCP support according to the application’s needs.

Modbus communication is based on client/server model. Only one client may be present on the bus. Client can communicate with several server devices. Server devices themselves are passive and must not send requests or unsolicited responses. Services requested by the client are specified by function codes (FCxx), and can be found in the specification or documentation of the API below.

Zephyr RTOS implementation supports both client and server roles.

More information about Modbus and Modbus RTU can be found on the website MODBUS Protocol Specifications.

Samples

Modbus RTU Server Sample and Modbus RTU Client Sample give the possibility to try out RTU server and RTU client implementation with an evaluation board.

Modbus TCP Server Sample is a simple Modbus TCP server.

Modbus TCP to serial line gateway sample is an example how to build a TCP to serial line gateway with Zephyr OS.

API Reference

group modbus

MODBUS transport protocol API.

Defines

MODBUS_MBAP_LENGTH

Length of MBAP Header

MODBUS_MBAP_AND_FC_LENGTH

Length of MBAP Header plus function code

Typedefs

typedef int (*modbus_raw_cb_t)(const int iface, const struct modbus_adu *adu, void *user_data)

ADU raw callback function signature.

Param iface

Modbus RTU interface index

Param adu

Pointer to the RAW ADU struct to send

Param user_data

Pointer to the user data

Retval 0

If transfer was successful

Enums

enum modbus_mode

Modbus interface mode.

Values:

enumerator MODBUS_MODE_RTU

Modbus over serial line RTU mode

enumerator MODBUS_MODE_ASCII

Modbus over serial line ASCII mode

enumerator MODBUS_MODE_RAW

Modbus raw ADU mode

Functions

int modbus_read_coils(const int iface, const uint8_t unit_id, const uint16_t start_addr, uint8_t *const coil_tbl, const uint16_t num_coils)

Coil read (FC01)

Sends a Modbus message to read the status of coils from a server.

Parameters

  • iface – Modbus interface index

  • unit_id – Modbus unit ID of the server

  • start_addr – Coil starting address

  • coil_tbl – Pointer to an array of bytes containing the value of the coils read. The format is: 

                    MSB                               LSB
                B7   B6   B5   B4   B3   B2   B1   B0
                -------------------------------------
coil_tbl[0]     #8   #7                            #1
coil_tbl[1]     #16  #15                           #9
     :
     :
    Note that the array that will be receiving the coil values must be greater than or equal to: (num_coils - 1) / 8 + 1

  • num_coils – Quantity of coils to read

Return values

0 – If the function was successful

int modbus_read_dinputs(const int iface, const uint8_t unit_id, const uint16_t start_addr, uint8_t *const di_tbl, const uint16_t num_di)

Read discrete inputs (FC02)

Sends a Modbus message to read the status of discrete inputs from a server.

Parameters

  • iface – Modbus interface index

  • unit_id – Modbus unit ID of the server

  • start_addr – Discrete input starting address

  • di_tbl – Pointer to an array that will receive the state of the discrete inputs. The format of the array is as follows: 

                  MSB                               LSB
              B7   B6   B5   B4   B3   B2   B1   B0
              -------------------------------------
di_tbl[0]     #8   #7                            #1
di_tbl[1]     #16  #15                           #9
     :
     :
    Note that the array that will be receiving the discrete input values must be greater than or equal to: (num_di - 1) / 8 + 1

  • num_di – Quantity of discrete inputs to read

Return values

0 – If the function was successful

int modbus_read_holding_regs(const int iface, const uint8_t unit_id, const uint16_t start_addr, uint16_t *const reg_buf, const uint16_t num_regs)

Read holding registers (FC03)

Sends a Modbus message to read the value of holding registers from a server.

Parameters

  • iface – Modbus interface index

  • unit_id – Modbus unit ID of the server

  • start_addr – Register starting address

  • reg_buf – Is a pointer to an array that will receive the current values of the holding registers from the server. The array pointed to by ‘reg_buf’ needs to be able to hold at least ‘num_regs’ entries.

  • num_regs – Quantity of registers to read

Return values

0 – If the function was successful

int modbus_read_input_regs(const int iface, const uint8_t unit_id, const uint16_t start_addr, uint16_t *const reg_buf, const uint16_t num_regs)

Read input registers (FC04)

Sends a Modbus message to read the value of input registers from a server.

Parameters

  • iface – Modbus interface index

  • unit_id – Modbus unit ID of the server

  • start_addr – Register starting address

  • reg_buf – Is a pointer to an array that will receive the current value of the holding registers from the server. The array pointed to by ‘reg_buf’ needs to be able to hold at least ‘num_regs’ entries.

  • num_regs – Quantity of registers to read

Return values

0 – If the function was successful

int modbus_write_coil(const int iface, const uint8_t unit_id, const uint16_t coil_addr, const bool coil_state)

Write single coil (FC05)

Sends a Modbus message to write the value of single coil to a server.

Parameters

  • iface – Modbus interface index

  • unit_id – Modbus unit ID of the server

  • coil_addr – Coils starting address

  • coil_state – Is the desired state of the coil

Return values

0 – If the function was successful

int modbus_write_holding_reg(const int iface, const uint8_t unit_id, const uint16_t start_addr, const uint16_t reg_val)

Write single holding register (FC06)

Sends a Modbus message to write the value of single holding register to a server unit.

Parameters

  • iface – Modbus interface index

  • unit_id – Modbus unit ID of the server

  • start_addr – Coils starting address

  • reg_val – Desired value of the holding register

Return values

0 – If the function was successful

int modbus_request_diagnostic(const int iface, const uint8_t unit_id, const uint16_t sfunc, const uint16_t data, uint16_t *const data_out)

Read diagnostic (FC08)

Sends a Modbus message to perform a diagnostic function of a server unit.

Parameters

  • iface – Modbus interface index

  • unit_id – Modbus unit ID of the server

  • sfunc – Diagnostic sub-function code

  • data – Sub-function data

  • data_out – Pointer to the data value

Return values

0 – If the function was successful

int modbus_write_coils(const int iface, const uint8_t unit_id, const uint16_t start_addr, uint8_t *const coil_tbl, const uint16_t num_coils)

Write coils (FC15)

Sends a Modbus message to write to coils on a server unit.

Parameters

  • iface – Modbus interface index

  • unit_id – Modbus unit ID of the server

  • start_addr – Coils starting address

  • coil_tbl – Pointer to an array of bytes containing the value of the coils to write. The format is: 

                    MSB                               LSB
                B7   B6   B5   B4   B3   B2   B1   B0
                -------------------------------------
coil_tbl[0]     #8   #7                            #1
coil_tbl[1]     #16  #15                           #9
     :
     :
    Note that the array that will be receiving the coil values must be greater than or equal to: (num_coils - 1) / 8 + 1

  • num_coils – Quantity of coils to write

Return values

0 – If the function was successful

int modbus_write_holding_regs(const int iface, const uint8_t unit_id, const uint16_t start_addr, uint16_t *const reg_buf, const uint16_t num_regs)

Write holding registers (FC16)

Sends a Modbus message to write to integer holding registers to a server unit.

Parameters

  • iface – Modbus interface index

  • unit_id – Modbus unit ID of the server

  • start_addr – Register starting address

  • reg_buf – Is a pointer to an array containing the value of the holding registers to write. Note that the array containing the register values must be greater than or equal to ‘num_regs’

  • num_regs – Quantity of registers to write

Return values

0 – If the function was successful

int modbus_read_holding_regs_fp(const int iface, const uint8_t unit_id, const uint16_t start_addr, float *const reg_buf, const uint16_t num_regs)

Read floating-point holding registers (FC03)

Sends a Modbus message to read the value of floating-point holding registers from a server unit.

Parameters

  • iface – Modbus interface index

  • unit_id – Modbus unit ID of the server

  • start_addr – Register starting address

  • reg_buf – Is a pointer to an array that will receive the current values of the holding registers from the server. The array pointed to by ‘reg_buf’ needs to be able to hold at least ‘num_regs’ entries.

  • num_regs – Quantity of registers to read

Return values

0 – If the function was successful

int modbus_write_holding_regs_fp(const int iface, const uint8_t unit_id, const uint16_t start_addr, float *const reg_buf, const uint16_t num_regs)

Write floating-point holding registers (FC16)

Sends a Modbus message to write to floating-point holding registers to a server unit.

Parameters

  • iface – Modbus interface index

  • unit_id – Modbus unit ID of the server

  • start_addr – Register starting address

  • reg_buf – Is a pointer to an array containing the value of the holding registers to write. Note that the array containing the register values must be greater than or equal to ‘num_regs’

  • num_regs – Quantity of registers to write

Return values

0 – If the function was successful

int modbus_iface_get_by_name(const char *iface_name)

Get Modbus interface index according to interface name.

If there is more than one interface, it can be used to clearly identify interfaces in the application.

Parameters

  • iface_name – Modbus interface name

Return values

Modbus – interface index or negative error value.

int modbus_init_server(const int iface, struct modbus_iface_param param)

Configure Modbus Interface as raw ADU server.

Parameters

  • iface – Modbus RTU interface index

  • param – Configuration parameter of the server interface

Return values

0 – If the function was successful

int modbus_init_client(const int iface, struct modbus_iface_param param)

Configure Modbus Interface as raw ADU client.

Parameters

  • iface – Modbus RTU interface index

  • param – Configuration parameter of the client interface

Return values

0 – If the function was successful

int modbus_disable(const uint8_t iface)

Disable Modbus Interface.

This function is called to disable Modbus interface.

Parameters

  • iface – Modbus interface index

Return values

0 – If the function was successful

int modbus_raw_submit_rx(const int iface, const struct modbus_adu *adu)

Submit raw ADU.

Parameters

  • iface – Modbus RTU interface index

  • adu – Pointer to the RAW ADU struct that is received

Return values

0 – If transfer was successful

void modbus_raw_put_header(const struct modbus_adu *adu, uint8_t *header)

Put MBAP header into a buffer.

Parameters

  • adu – Pointer to the RAW ADU struct

  • header – Pointer to the buffer in which MBAP header will be placed.

void modbus_raw_get_header(struct modbus_adu *adu, const uint8_t *header)

Get MBAP header from a buffer.

Parameters

  • adu – Pointer to the RAW ADU struct

  • header – Pointer to the buffer containing MBAP header

void modbus_raw_set_server_failure(struct modbus_adu *adu)

Set Server Device Failure exception.

This function modifies ADU passed by the pointer.

Parameters

  • adu – Pointer to the RAW ADU struct

int modbus_raw_backend_txn(const int iface, struct modbus_adu *adu)

Use interface as backend to send and receive ADU.

This function overwrites ADU passed by the pointer and generates exception responses if backend interface is misconfigured or target device is unreachable.

Parameters

  • iface – Modbus client interface index

  • adu – Pointer to the RAW ADU struct

Return values

0 – If transfer was successful

struct modbus_adu
#include <modbus.h>

Frame struct used internally and for raw ADU support.

Public Members

uint16_t trans_id

Transaction Identifier

uint16_t proto_id

Protocol Identifier

uint16_t length

Length of the data only (not the length of unit ID + PDU)

uint8_t unit_id

Unit Identifier

uint8_t fc

Function Code

uint8_t data[CONFIG_MODBUS_BUFFER_SIZE - 4]

Transaction Data

uint16_t crc

RTU CRC

struct modbus_user_callbacks
#include <modbus.h>

Modbus Server User Callback structure

Public Members

int (*coil_rd)(uint16_t addr, bool *state)

Coil read callback

int (*coil_wr)(uint16_t addr, bool state)

Coil write callback

int (*discrete_input_rd)(uint16_t addr, bool *state)

Discrete Input read callback

int (*input_reg_rd)(uint16_t addr, uint16_t *reg)

Input Register read callback

int (*input_reg_rd_fp)(uint16_t addr, float *reg)

Floating Point Input Register read callback

int (*holding_reg_rd)(uint16_t addr, uint16_t *reg)

Holding Register read callback

int (*holding_reg_wr)(uint16_t addr, uint16_t reg)

Holding Register write callback

int (*holding_reg_rd_fp)(uint16_t addr, float *reg)

Floating Point Holding Register read callback

int (*holding_reg_wr_fp)(uint16_t addr, float reg)

Floating Point Holding Register write callback

struct modbus_serial_param
#include <modbus.h>

Modbus serial line parameter.

Public Members

uint32_t baud

Baudrate of the serial line

enum uart_config_parity parity

parity UART’s parity setting: UART_CFG_PARITY_NONE, UART_CFG_PARITY_EVEN, UART_CFG_PARITY_ODD

enum uart_config_stop_bits stop_bits_client

stop_bits_client UART’s stop bits setting if in client mode: UART_CFG_STOP_BITS_0_5, UART_CFG_STOP_BITS_1, UART_CFG_STOP_BITS_1_5, UART_CFG_STOP_BITS_2,

struct modbus_server_param
#include <modbus.h>

Modbus server parameter.

Public Members

struct modbus_user_callbacks *user_cb

Pointer to the User Callback structure

uint8_t unit_id

Modbus unit ID of the server

struct modbus_raw_cb
#include <modbus.h>
struct modbus_iface_param
#include <modbus.h>

User parameter structure to configure Modbus interface as client or server.

Public Members

enum modbus_mode mode

Mode of the interface

uint32_t rx_timeout

Amount of time client will wait for a response from the server.

struct modbus_serial_param serial

Serial support parameter of the interface

struct modbus_raw_cb rawcb

Pointer to raw ADU callback function