Fieldbus Communication • 101MODBUS FunctionsWAGO-I/O-SYSTEM 750ETHERNET TCP/IP4.2.3.3 Function Code FC3 (Read multiple registers)This function reads the contents of holding registers from a slave device inword format.RequestThe request specifies the reference number (start register) and the word count(register quantity) of the registers to be read. The reference number of therequest is zero based, therefore, the first register starts at address 0.Example: Read registers 0 and 1:Byte Field name ExampleByte 0, 1 Transaction identifier 0x0000Byte 2, 3 protocol identifier 0x0000Byte 4, 5 length field 0x0006Byte 6 unit identifier 0x01 not usedByte 7 MODBUS function code 0x03Byte 8, 9 reference number 0x0000Byte 10, 11 Word count 0x0002ResponseThe reply register data is packed as 2 bytes per register. The first byte containsthe higher value bits, the second the lower values.Byte Field name Example.....Byte 7 MODBUS function code 0x03Byte 8 Byte count 0x04Byte 9, 10 Value Register 0 0x1234Byte 11, 12 Value Register 1 0x2345The contents of register 0 are displayed by the value 0x1234 and the contentsof register 1 is 0x2345.ExceptionByte Field name Example.....Byte 7 MODBUS function code 0x83Byte 8 Exception code 0x01 or 0x02
