Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 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
A
Receiver
B
Network
Packet
Status Path 2 Full
Path 1
Network
Packet
Software Software
Hardware
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