Chapter 6. Serial I/O, SNP & RTU Protocols
238
PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual
GFK-2222AD
Message (03): Read Registers
Format:
Address
Starting
Register No.
Number of
Registers
s
Error
Check
Func
03
Query
Hi
Hi
Lo
Lo
Address
Data
Error
Check
Func
03
Normal Response
Byte
Count
First Register
Hi
Hi
Lo
Lo
Figure 54: RTU Read Registers Message Format
Query:
▪
An address of 0 is not allowed as this request cannot be a broadcast request.
▪
The function code is equal to 3.
▪
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 attached CPU. 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 attached CPU. 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.
