Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 151)

Network and Application Layer Protocols
Chapter 12
12Ć4
Figure 12.3
Reply Message Packet Format
CMD STS DATA
DST SRC CMD STS TNS
Data
(From Application Layer)
From
Application
Layer
10064–I
x
4 x
Network
Layer
Packet
Legend: x = low hex digit of CMD byte supplied by application layer
DST and SRC
The DST (destination) byte is the number of the station that receives the
network packet. The SRC (source) byte is the number of the station that
sent the packet. There are 255 possible station numbers from 0 to 254
decimal. You reverse the DST and SRC of the command message to form
the DST and SRC of the corresponding reply message.
CMD (High Nibble)
The high nibble of the CMD (command) byte is supplied by the network
layer. Bit 6 (26 value) of the CMD byte is the command/reply indicator.
It is 0 for command messages and 1 for reply messages. Therefore the
high hex digit of the command byte is 0 for command messages and 4 for
reply messages. (The low nibble comes from the application layer).
STS (Low Nibble)
The low nibble of the STS (status) byte is supplied by the network layer.
In a command message, this field is set to zero. In a reply message
reporting no error or a remote error, this field is also set to zero. (The
high nibble comes from the application layer.)