modbus_read_registers - rd_reg - 0x03

modbus_write_register - wr_reg - 0x06

modbus_write_bit - fcoil - 0x05 

modbus_write_bits - fcoil_mult - 0x0F



libmodbus to freemodbus mapping 

01 (0x01) Read Coils - int modbus_read_bits(modbus_t *ctx, int addr, int nb, uint8_t *dest);
eMBRegCoilsCB

05 (0x05) Write Single Coil - int modbus_write_bit(modbus_t *ctx, int addr, int status);
eMBRegCoilsCB

15 (0x0F) Write Multiple Coils - int modbus_write_bits(modbus_t *ctx, int addr, int nb, const uint8_t *src);
eMBRegCoilsCB

04 (0x04) Read Input Registers - int modbus_read_input_registers(modbus_t *ctx, int addr, int nb, uint16_t *dest);
eMBRegInputCB

03 (0x03) Read Holding Registers - int modbus_read_registers(modbus_t *ctx, int addr, int nb, uint16_t *dest);
eMBRegHoldingCB

06 (0x06) Write Single Register - int modbus_write_register(modbus_t *ctx, int addr, int value);
eMBRegHoldingCB

16 (0x10) Write Multiple registers - int modbus_write_registers(modbus_t *ctx, int addr, int nb, const uint16_t *src);
eMBRegHoldingCB

23 (0x17) Read/Write Multiple registers - 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, const uint16_t *dest);
eMBRegHoldingCB