Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 112)

Full-Duplex Protocol
Chapter 10
10Ć8
We could show paths 2, 3, and 4 in a similar way.
The full–duplex protocol is symmetrical; that is, anything that we can say
about transmitter A, receiver B, and paths 1 and 2 applies equally to
transmitter B, receiver A, and paths 3 and 4. There are actually two
independent instances of the protocol operating simultaneously. For
simplicity, we define the link protocol on the subsystem that carries
messages from A to B, with reference to figure 10.5
Figure 10.5
Message Transmission from A to B
Transmitter
B
A
Receiver
Path 1 Path 2
M
S
S
M
Path 1 Path 2
Path 1
Path 2
10042-I
Although the protocols on each subsystem operate independently, there is
a slight delay when you transmit a response code in the middle of a stream
of message codes. Also, any non–transient hardware problem that affects
message codes traveling over a hardware circuit affects response codes on
the same circuit.
Message Characteristics
In the network layer (chapter 12) the message source provides the
transmitter with the messages it sends. The message sink tells the
receiver what to do with the messages it receives.
Upon request from the transmitter, the message source supplies one
network packet at a time. It must be notified about the success or failure
of the transfer to the receiver before supplying the next message. When
the message source is empty, the transmitter waits in an inactive state until
a message is available.