PACSystems™ RX3i and RSTi-EP CPU Reference Manual Section 6
GFK-2222AK October 2019
Serial I/O, SNP & RTU Protocols 172
▪ 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.
6.3.3.2.3 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.
6.3.3.3 Message (03): Read Registers
6.3.3.3.1 Format:
Figure 25: RTU Read Registers Message Format
6.3.3.3.2 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.
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