Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 70)

Asynchronous Link Protocols
Chapter 4
4-27
transceiver is illustrated in Figure 4.15, and the master transceiver is
illustrated in Figure 4.16.
To fully define the environment of the protocol, the transceiver needs to
know where to get the messages it sends and must have a means of
disposing of messages it receives. These are implementation-dependent
functions that shall respectively be called the message source and the
message sink.
We assume that the message source will supply one message at a time
upon request from the transceiver and will require notification of the
success or failure of transfer before supplying the next message.
When the transceiver has received a message successfully, it will attempt
to give it to the message sink. The possibility exists that the message sink
will be full. The transceiver requires an indication of this.
Figure 4.15
Slave Transceiver
SOURCE SOURCE
MASTER
TRANSCEIVER
SLAVE
RECEIVER
Network
Packet
OK OK
Link
Network
Packet
Software Software
Hardware
11566
SINK SINK
Network
Full Full
To Other Slaves
Packet
Network
Packet
Message Characteristics
Ideally, the link protocol should not be at all concerned with the content or
form of the messages it is transferring. However, half-duplex protocol
places the following restrictions on the messages that are submitted to it
for transfer:
1. Minimum size of a valid message is 6 bytes, maximum is 250 bytes.