Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 113)

Full-Duplex Protocol
Chapter 10
10Ć9
Whenever the receiver has received a link packet successfully, it attempts
to give the network packet portion (link level data) to the message sink. If
the message sink is full, it must notify the receiver.
Figure 10.6 represents the protocol environment.
Figure 10.6
Protocol Environment
Software Software
Hardware
SOURCE
Network
Packet
Network
Packet Path 1
Path 2 Status Full
Transmitter
A
Receiver
B
SINK
10043-I
Full–duplex protocol places the following restrictions on the network
packet that is submitted to the link layer for transfer:
The size of a valid network packet is 6 bytes minimum, and 250 bytes
maximum.
The first byte of a network packet must be the station number of the
receiver station. The receiver ignores messages that do not contain the
correct station number.
As part of the duplicate message detection algorithm, the receiver
checks the second, third, fifth, and sixth bytes of each network packet.
(Recall that a network packet consists of the source, command, and
transaction fields.) At least one of these bytes of the current network
packet must differ from the corresponding byte of the previous network
packet in order for the receiver to accept the current network packet.
Otherwise, the receiver assumes that the current packet is a
retransmission of the previous packet, so it discards the current packet.
Transmitter Actions
Whenever the message source can supply a packet and the transmitter is
not busy, transmitter A sends a link packet on path 1. It then starts a