Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 153)

Network and Application Layer Protocols
Chapter 12
12Ć6
message. During an upload or download, the TNS value is the only way
to distinguish between the physical read or write reply messages.
Recall from chapter nine that the application layer provides the Data
Highway commands that you use to transfer data and manage the
network. This function is provided by the command initiators and
command executors.
At the application layer, the command initiators are responsible for:
creating a message packet and submitting that packet to the network
layer
maintaining the sequence number and the timeout
accepting the reply
canceling the timeout and sequence number
destroying the reply message packet when it is no longer needed
At the application layer, the command executors must:
create the reply message packet
copy over certain information from the command
fill in any reply information
submit the packet to the network
destroy the command packet
Application programs communicate by sending information back and
forth in the command, status, and data fields of network packets.
Application protocols may vary depending on the types of application
programs that are communicating.
Application Message Fields
Figure 12.2 shows the general format of the application fields for a
command message. Not all command messages have FNC, ADDR, or
DATA bytes.
Figure 12.3 shows the general format of the application fields for a reply
message. Not all reply messages have DATA bytes.
In addition to the application layer fields shown in these figures, some of
the PLC–3 command messages also contain these application layer fields:
EXT STS (extended status)
Packet Offset
Application Layer