Chapter 6. Serial I/O, SNP & RTU Protocols
GFK-2222AD
April 2018
237
Message (02): Read Input Table
Format:
Address
Starting
Point No.
Number of
Points
Error
Check
Func
02
Query
Hi
Hi
Lo
Lo
Address
Data
Error
Check
Func
02
Normal Response
Byte
Count
Figure 53: RTU Read Input Table Message Format
Query:
▪
An address of 0 is not allowed as this cannot be a broadcast request.
▪
The function code is 02.
▪
The starting point number is two bytes in length and may be any value less than the highest input
point number available in the attached CPU. The starting point number is equal to one less than the
number of the first input point returned in the normal response to this request.
▪
The number of points value is two bytes in length. It specifies the number of input points returned in
the normal response. The sum of the starting point value and the number of points value must be
less than or equal to the highest input point number available in the attached CPU. The high order
byte of the Starting Point Number and Number Of Bytes fields is sent as the first byte. The low order
byte is the second byte in each of these fields.
Response:
▪
The byte count is a binary number from 1 to 256 (0 = 256). It is the number of bytes in the normal
response following the byte count and preceding the error check.
▪
The Data field of the normal response is packed input status data. Each byte contains eight input
point values. The least significant bit (LSB) of the first byte contains the value of the input point whose
number is equal to the starting point number plus one. The values of the input points are ordered by
number starting with the LSB of the first byte of the Data field and ending with the most significant
bit (MSB) of the last byte of the Data field. If the number of points is not a multiple of 8, then the last
data byte contains zeroes in one to seven of its highest order bits.
