modbus库功能码和函数对应关系

/* Function codes */

#define _FC_READ_COILS                0x01            

#define _FC_READ_DISCRETE_INPUTS      0x02

#define _FC_READ_HOLDING_REGISTERS    0x03

#define _FC_READ_INPUT_REGISTERS      0x04

#define _FC_WRITE_SINGLE_COIL         0x05

#define _FC_WRITE_SINGLE_REGISTER     0x06

#define _FC_READ_EXCEPTION_STATUS     0x07

#define _FC_WRITE_MULTIPLE_COILS      0x0F

#define _FC_WRITE_MULTIPLE_REGISTERS  0x10

#define _FC_REPORT_SLAVE_ID           0x11

#define _FC_WRITE_AND_READ_REGISTERS  0x17

0x01:读取线圈状态取得一组逻辑线圈的当前状态（ON/OFF)  
/* Reads the boolean status of bits and sets the array elements

   in the destination to TRUE or FALSE (single bits). */

int modbus_read_bits(modbus_t *ctx, int addr, int nb, uint8_t *dest)

0x02:

读取输入状态 取得一组开关输入的当前状态（ON/OFF)  

/* Same as modbus_read_bits but reads the remote device input table */

int modbus_read_input_bits(modbus_t *ctx, int addr, int nb, uint8_t *dest)

0x03:读取保持寄存器 在一个或多个保持寄存器中取得当前的二进制值 
/* Reads the holding registers of remote device and put the data into an

   array */

int modbus_read_registers(modbus_t *ctx, int addr, int nb, uint16_t *dest)

0x04:读取输入寄存器 在一个或多个输入寄存器中取得当前的二进制值 
/* Reads the input registers of remote device and put the data into an array */

int modbus_read_input_registers(modbus_t *ctx, int addr, int nb,

                                uint16_t *dest)

0x05:强置单线圈 强置一个逻辑线圈的通断状态
/* Turns ON or OFF a single bit of the remote device */

int modbus_write_bit(modbus_t *ctx, int addr, int status)

0x06:预置单寄存器 把具体二进值装入一个保持寄存器
/* Writes a value in one register of the remote device */

int modbus_write_register(modbus_t *ctx, int addr, int value)

 0x07:读取异常状态 取得8个内部线圈的通断状态，这8个线圈的地址由控制器决定，用户逻辑可以将这些线圈定义，以说明从机状态，短报文适宜于迅速读取状态  

0x0F:强置多线圈 强置一串连续逻辑线圈的通断
/* Write the bits of the array in the remote device */

int modbus_write_bits(modbus_t *ctx, int addr, int nb, const uint8_t *src)

0x10:控询（只用于484） 可使主机与一台正在执行长程序任务从机通信，探询该从机是否已完成其操作任务，仅在含有功能码9的报文发送后，本功能码才发送

/* Write the values from the array to the registers of the remote device */

int modbus_write_registers(modbus_t *ctx, int addr, int nb, const uint16_t *src)

0x17:报告从机标识 可使主机判断编址从机的类型及该从机运行指示灯的状态

/* Write multiple registers from src array to remote device and read multiple

   registers from remote device to dest array. */

int modbus_write_and_read_registers(modbus_t *ctx,

                                    int write_addr, int write_nb, const uint16_t *src,

                                    int read_addr, int read_nb, uint16_t *dest)