User manual

PAT700 Total Organic Carbon Analyzer – Modbus Protocol Page 147
Anatel Operator Manual
Data Encoding
Modbus uses a 'big-endian' representation for addresses and data items. This means that
when a numerical quantity larger than a single byte is transmitted, the MOST significant byte
is sent first. The following sub-topics describe the different types of encoding and show how
the data is encoded as it is within the Modbus/TCP packet. Most client drivers will extract the
data from the packet in the correct format for use/display within the client environment.
Binary
A binary item is represented as a single bit within a data word. All binary data is packed in to
16-bit data words, which are accessed using FC3 therefore, a single register contains 16 bits
of binary data, each having a specific meaning.
Example (Decimal value): 43,605
16-bit Word (short)
A 16-bit word item is transmitted with the MOST significant byte first. FC3 reads 16-bit items
at a time; therefore, each of these data items will fit within one register that is read.
Example (Decimal value): 4,660
32-bit Word (int)
A 32-bit word item is transmitted with the MOST significant byte first, then the next MOST
significant, until all the bytes are transmitted. FC3 reads 16-bit items at a time; therefore, two
registers are required to read each 32-bit data item.
Example (Decimal value): 305,419,896
Value (Hex) 1st Register 2nd Register
0xAA55 0xAA 0x55
(101010100101) (10101010) (01010101)
Value (Hex) 1st Register 2nd Register
0x1234 0x12 0x34
1st Register 2nd Register
Value (Hex) 1st 2nd 1st 2nd
0x12345678 0x12 0x34 0x56 0x78