Rockwell-automation 1770-KF2 Data Highway or Highway Plus Interface Mo Instrukcja Użytkownika Pobierz pdf (Strona 52)

Asynchronous Link Protocols

Chapter 4

4-9

Although the protocols on each subsystem operate independently of each

other, there will be a slight interaction as transmission of a message is

delayed when a two-character response code is inserted in a stream of

message codes. Also, any hardware problem that affects message codes

traveling over a hardware circuit will also likely affect response codes on

the same circuit.

Protocol Environment Definition

To fully define the environment of the protocol, the transmitter needs to

know where to get the message it sends; and the receiver must have a

means of disposing of messages. These are implementation dependent

functions which shall respectively be called the message source and the

message sink.

We assume that the message source supplies one message at a time upon

request from the transmitter and requires notification of the success or

failure of the transfer to Station B before supplying the next message.

When the message source is empty, the transmitter waits in an inactive

state until a message is available. Whenever the receiver has received a

message successfully, it attempts to give it to the message sink. The

possibility exists that the message sink will be full. The receiver requires

an indication of this.

Figure 4.6 shows this representation of the protocol environment.

Figure 4.6

Protocol

Environment

SOURCE SINK

Transmitter

Receiver

Network

Packet

Status Path 2 Full

Path 1

Network

Packet

Software Software

Hardware

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Rockwell-automation 1770-KF2 Data Highway or Highway Plus Interface Mo Instrukcja Użytkownika Strona 52