Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 58)

4–4 Using Full-duplex Protocol to Send and Receive Messages
Publication 1770Ć6.5.16 - October 1996
Figure 4.3 shows the protocol environment for message symbols
from transmitter A to receiver B (path 1) and response codes from
receiver B to transmitter A (path 2).
Figure 4.3
Protocol Environment
Transmitter A Sink Receiver B Source
Packet
Sink Full Packet Status
Path 1
Path 2
Packet
I deally, the data-link-layer protocol is not concerned with the content
or form of the message packet (link-layer data) it is transferring.
However, full-duplex protocol places the following restrictions on
link-layer data submitted to it for transfer:
•
minimum size of valid link-layer data is 6 bytes
•
maximum size of valid link-layer data depends on the
application-layer command
•
some protocol implementations (e.g., point-to-point links to a
1771-KG module) require that the first byte of the link-layer data
match the node address
The receiver ignores messages that do not contain the correct
address.
•
as part of the duplicate message detection algorithm, the receiver
compares the second, third, fifth, and sixth bytes of the link-layer
data with the same bytes in the previous message
If there is no difference between the sets of bytes, the message is
classified as a re-transmission of the previous message. You can
set some Allen-Bradley interface modules so that they do not
implement duplicate-message detection.
In full-duplex protocol, messages are sent from the source (part of
software that supplies message packet) through a transmitter
(device that sends data) and then a receiver (device that receives
data) to the sink (part of the software that accepts the received data):
For See page
structured text on how the transmitter operates 4-5
a flowchart of how the transmitter operates 4-6
structured text on how the receiver operates 4-7
a flowchart of how the receiver operates 4-9
Message Characteristics
Transmitter and Receiver
Message Transfer