Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 154)

Network and Application Layer Protocols
Chapter 12
12Ć7
Word Offset
TOTAL TRANS (total transaction size)
Appendix A lists the message formats (command and reply) of every
command the PLC–3 can send or receive.
CMD and FNC (command and function)
For these message formats which include an FNC byte, the low nibble of
the CMD (command) byte together with the FNC (function) byte define
what action the command executor at the destination station will perform.
For those message formats which do not include an FNC byte, the CMD
byte alone defines what action the command executor at the destination
station will perform. The hexidecimal values of the CMD and FND bytes
are listed in Table 12.A for each of the types of messages that can be
transmitted across this link.
Bits 0 through 3 of the CMD byte must be the same in the reply message
as it is in the corresponding command message. In current
implementations this is either a command code or a command executor
selector. The application program must always copy this field from the
command to the reply message.