Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 57)

4–3 Using Full-duplex Protocol to Send and Receive Messages
Publication 1770Ć6.5.16 - October 1996
Figure 4.2
Software implementation of data paths
Software
Multiplexer
• To implement four logical paths with two physical circuits, a software multiplexer is needed to combine the message symbols with
the response symbols going in the same direction.
• At the other end of the link, a software separator divides the message symbols from the response symbols.
The internal software sends the message symbols to the appropriate receiver, and the response symbols to the appropriate
transmitter.
• Although message symbols and response symbols on the same circuit operate independently of each other,
there is some interaction.
• For example, a message on physical circuit AB will be delayed if a response symbol from receiver A is inserted in a stream of
message symbols from transmitter A (embedded response).
• Also, any hardware problems that affect message symbols traveling over a circuit will also affect response symbols
on the same circuit.
Transmitter A Receiver B
Path 1 Path 2
Physical Circuit AB
Path 2
Path 1
Physical Circuit BA
Software
Multiplexer
Path 3 Path 4 Path 4 Path 3
Transmitter B
Software
Separator
Receiver A
Software
Separator
Paths 1, 2, 3, and 4
Paths 1 (a message symbol sent from node A to node B)
Software
Multiplexer
Transmitter A
Software
Separator
Receiver B
Path 1 Path 1
Physical Circuit
To define the environment of the protocol:
•
the transmitter needs to know where to get the message it sends,
the message source. We assume the message source:
– supplies one message at a time upon request from the
transmitter
– requires notification of the success or failure of the transfer
before supplying the next message
•
the receiver must have a means of disposing of messages,
the message sink
If the Then the
message source is empty
transmitter waits in an inactive state until a
message is available
receiver has received a message
successfully
receiver attempts to give it to the message
sink. If the message sink is full, the
receiver must be notified
FullĆduplex Protocol
Environment