Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 131)

Half-Duplex Protocol
Chapter 11
11Ć4
1775–KA is just one of several stations on a Data Highway, the STN
together with the DST identifies the 1775–KA station
DLE STX – separates the data link protocol information from the
network packet.
Link–layer data: (00–0F and 11–FF hex) – encodes the bytes of the
network packet.
DLE DLE – encodes the value 10 hex in the network packet. This is
necessary to distinguish a text code of 10 hex from a DLE control code
of 10 hex.
DLE ETX BCC – terminates a message or polling packet.
DLE ENQ – indicates the start of a polling packet.
Response codes from station receiving a message (or poll):
DLE ACK – signals that the receiver has successfully received the last
message sent.
DLE NAK – serves as a global link reset command. It causes all slaves
to cancel all messages they have ready to transmit to the master. The
1775–KA module responds to this by writing error code 84 into its
error word in the PC data table.
DLE EOT – is the response that a slave sends to a poll from the master
when the salve has no messages to send.
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 11.1 illustrates the formats of these packets. Note that the slave
message packet has the same format as the full–duplex message packet.
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.
At the end of each polling packet is a BCC byte. At the end of each
message packet is a one–byte BCC field.
Link-Layer Packets