Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 148)

Chapter
12
12Ć1
The Network and Application Layer Protocol
The network protocol defines a network packet format for interaction
between application programs. The link protocol merely serves to carry
data blocks between two applications, regardless of which data link
protocol (half or full– duplex) you use. The application programs may be
located at opposite ends of a point–to–point full duplex link, or at
different points on a multidrop half– duplex link. The network protocol
can even handle the transfer of messages between application programs in
the same device.
The network layer ignores the internal functioning of data link protocols.
It requires that the data link driver accepts a message for delivery, tries to
send it, and indicates whether it was delivered.
Program And Message Types
The network protocol was designed on the assumption that application
programs are of two types: command initiators and command executors.
Corresponding to this division there are two message types:
Command messages – initiated by command initiators and carried over
the network to a command executor.
Reply messages – the replies that command executors send to
command initiators.
For each command message there is normally one and only one reply. (A
rare exception occurs when the data link delivers a message but receives
no acknowledgement to verify delivery. At the command initiator, the
network layer sends a reply command executor receives and executes the
command packet, and sends a single reply message to the command
initiator. In any case, the command executor generates only one reply
message for each command it receives.)
If the network layer of the command initiator station cannot deliver a
command to another station, it generates a reply message with an error
code in its own application layer. If a reply cannot be delivered, the
network layer destroys it.
Network Layer