Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 67)

Asynchronous Link Protocols
Chapter 4
4-24
Link-Layer Packets
Half-duplex protocol uses three types of transmissions:
Polling Packet
Master Message Packet
Slave Message Packet
The master station transmits both polling packets and master message
packets, while slave stations transmit slave message packets.
Figure 4.14 illustrates the formats of these packets. Note that the slave
message packet has the same format as the full-duplex message packet
(see “Link-Layer Message Packets”). The master message packet is the
same as the slave message packet except that it is prefixed with DLE SOH
and an address code to specify a slave station number.
Figure 4.14
Packet Formats for Half-Duplex Protocol
DLE ENQ STN BCC
A. Polling Packet
CMD STS Command/Response Block
DST SRC CMD STS TNS
Data
(From Application Layer)
DLE STX
Data
(From Network Layer)
DLE ETX BCC
B. Slave Message Link Packet
From
Application
Layer
From
Network
Layer
From
Application
Layer
From
Network
Layer
CMD STS Command/Response Block
DST SRC CMD STS TNS
Data
(From Application Layer)
DLE SOH STN DLE STX
Data
(From Network Layer) DLE ETX BCC
C. Master Message Link Packet
11327
Data Link
Layer Packet
Data Link
Layer Packet