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C
C-11GFK–0804B Appendix C RTU Protocol
Message (03): Read Registers
Format:
Address Func Error Check
03
Query
Address Func Error Check
03
Normal Response
Register No. Registers
Starting Number of
Data
Hi Lo Hi Lo
Byte
Count
Register
First
Hi Lo Hi Lo
Query:
An address of 0 is not allowed as this request cannot be a broadcast request.
The function code is equal to 03.
The starting register number is two bytes in length. The starting register number
may be any value less than the highest register number available in the Micro PLC.
It is equal to one less than the number of the first register returned in the normal
response to this request.
The number of registers value is two bytes in length. It must contain a value from 1
to 125 inclusive. The sum of the starting register value and the number of registers
value must be less than or equal to the highest register number available in the
Micro PLC. The high order byte of the starting register number and number of
registers fields is sent as the first byte in each of these fields. The low order byte is
the second byte in each of these fields.
Response:
The byte count is a binary number from 2 to 250 inclusive. It is the number of bytes
in the normal response following the byte count and preceding the error check.
Note that the byte count is equal to two times the number of registers returned in
the response. A maximum of 250 bytes (125) registers is set so that the entire
response can fit into one 256 byte data block.
The registers are returned in the data field in order of number with the lowest
number register in the first two bytes and the highest number register in the last two
bytes of the data field. The number of the first register in the data field is equal to
the starting register number plus one. The high order byte is sent before the low
order byte of each register.