Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 135)

Half-Duplex Protocol
Chapter 11
11Ć8
Figure 11.2
Slave Transceiver
Software Software
Hardware
SOURCE
Network
Packet
Network
Packet
OK
Full
TRANSMITTER
RECEIVER
SINK
10052-I
SINK
Network
Packet
MASTER
SLAVE
SOURCE
Network
Packet
Full
OK
Link
To Other Slaves
Message Characteristics
Half–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 (see DST in chapter 12). The receiver ignores
messages that do not contain the correct station number.
As part of the duplicate message detection algorithm, the transceiver
checks the second, third, fifth, and sixth bytes of each network packet.
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 transceiver to act upon the current network packet. Otherwise, the
transceiver assumes that the current network packet is a retransmission
of the previous network packet, so it discards the current network
packet.
Master Polling Responsibilities
You may vary the master polling algorithm, depending on how much
activity you expect on your network.