Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 83)

Message Packet Formats
Chapter 5
5-4
The meanings of the bytes are:
DST — Destination Station for the Message
SRC — Source Station of the Message
CMD — Command Code
STS — Status Code
TNS — Transaction Number (2 Bytes)
FNC — Function Code
EXT STS — Extended Status Code
ADDR — Address of Memory Location (2 Bytes Minimum)
SIZE — Number of Bytes to Be Transferred
DATA — Data Values Being Transferred by the Message
These bytes are described in more detail below.
DST and SRC
The DST (destination) byte contains the station number of the station that
is the ultimate destination of the message. The SRC (source) byte is the
station number of the station that originated the message.
The network layer supplies the DST and SRC values. Allowed values for
these bytes are 0 to 254 decimal (0 to 376 octal). Note that you can form
the DST and SRC bytes of a reply message simply by interchanging the
DST and SRC bytes of the corresponding command message.
In application programming of a computer connected to the network by a
KF2, the SRC byte should be set to zero. The module will set the byte to
its own station number.
CMD and FNC
The CMD (command) and FNC (function) bytes together define the
activity to be performed by the command message at the destination
station. CMD defines the command type; and FNC, if used, defines the
specific function under that command type.
Table 5.A lists all the available CMD and FNC codes in hexadecimal
notation. The exact format for a particular message depends on the CMD
and FNC values. “Message Formats” explains the use of each command
or function and gives a detailed description of the message text format for
each.