Fieldbus Communication • 99MODBUS FunctionsWAGO-I/O-SYSTEM 750ETHERNET TCP/IP4.2.3.1 Function Code FC1 (Read Coils)This function reads the status of the input and output bits (coils) in a slavedevice.RequestThe request specifies the reference number (starting address) and the bit countto read.Example: Read output bits 0 to 7.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 0x01Byte 8, 9 reference number 0x0000Byte 10, 11 Bit count 0x0008ResponseThe current values of the response bits are packed in the data field. A binary 1corresponds to the ON status and a 0 to the OFF status. The lowest value bit ofthe first data byte contains the first bit of the request. The others follow inascending order. If the number of inputs is not a multiple of 8, the remainingbits of the last data byte are filled with zeroes (truncated).Byte Field name Example.....Byte 7 MODBUS function code 0x01Byte 8 Byte count 0x01Byte 9 Bit values 0x12The status of the inputs 7 to 0 is shown as byte value 0x12 or binary 00010010. Input 7 is the bit having the highest significance of this byte and input 0the lowest value.The assignment is thus made from 7 to 0 with OFF-OFF-OFF-ON-OFF-OFF-ON-OFF.Bit: 0 0 0 1 0 0 1 0Coil: 7 6 5 4 3 2 1 0ExceptionByte Field name Example.....Byte 7 MODBUS function code 0x81Byte 8 Exception code 0x01 or 0x02
