Rockwell Automation 1775-S5_SR5,D17756.5.5 User Manual PLC-3 FAMILY I/0 - Manual (Page 135)

Binary Command Language
Appendix A
A-7
Command Block
With the exception of the CCR, all commands sent from the master to the
scanner are in the form of a command block. The command is always
structured as follows:
Command Data Command Command ETX
Checksum
STX
(02)
Command
Block Seq
Count Buffer Size (03)
command buffer
Command: Description:
STX (start of text) the first byte of the control block (02 hex).
Command block sequence count Command block sequence count (0 to 7F hex)
Command buffer size Number of bytes to follow in the command buffer. The
buffer includes all bytes between this buffer size byte and
the ETX.
Command The fourth byte in the command block contains the hex
code corresponding to the desired command.
Command data The command data contains all parameters associated
with the indicated command. When the command data
contains data in word format, the lower byte must be sent
first.
Important: The master can send multiple
commands in one command buffer by including another
command and command data after the previous one.
Each command must have all necessary parameters and
the entire command block cannot be larger than the
scanner is capable of receiving. This maximum size
should be one less than is reported in a CCR response.
One word is reserved for scanner use.
ETX (End of text) Following the command buffer is the ETX code (03 hex).
Checksum The last byte is a checksum which is the sum of all
previous bytes from the STX (02) through ETX (03)
inclusively.