Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 134)

Half-Duplex Protocol
Chapter 11
11Ć7
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