Rockwell-automation 1775-KA PLC-3 Communication Adapter Module User Ma Manual de usuario descargar pdf (Pagina 134)

Half-Duplex Protocol

Chapter 11

117

Each station on the multidrop link must contain a software routine, known

as a transceiver, that can both transmit and receive message packets. The

1775–KA module already contains a slave transceiver routine, so it will

function as a slave station if you select Polled–Subscriber Mode with

LIST (chapter 2). To establish master station, you have to program a

transceiver routine at a computer. In addition to transmitting and

receiving message packets, the master transceiver must also be able to

transmit polling packets.

Note that you can program separate transmitter and receiver routines

instead of a single transceiver. For purposes of the discussion here,

however, we assume that the transceiver is a single software routine.

Figure 11.2 illustrates the operation of master and slave transceivers. To

fully define the protocol environment, you must tell the master transceiver

where to get the messages it sends and how to dispose of messages it

receives. These are implementation–dependent functions that we call the

message source and the message sink respectively.

We assume that the message source supplies one network packet at a time

upon request from the transceiver, and that the source has to be notified

about the success or failure of transfer before supplying the next.

Whenever the transceiver has received a link packet successfully, it

attempts to give the network packet portion to the message sink. The

message sink may be full. The message sink must notify the transceiver

when it is full.

Protocol Environment Definition

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Rockwell-automation 1775-KA PLC-3 Communication Adapter Module User Ma Manual de usuario Pagina 134