Rockwell-automation 1775-KA PLC-3 Communication Adapter Module User Ma Manual do Utilizador descarregar pdf (Página 112)

Full-Duplex Protocol

Chapter 10

108

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

Receiver

Path 1

Path 2

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.

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Rockwell-automation 1775-KA PLC-3 Communication Adapter Module User Ma Manual do Utilizador Página 112