Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 107)

Full-Duplex Protocol
Chapter 10
10Ć3
Additionally, a block check character (BCC) is used at the end of each
packet for error checking. These bytes can be any value from 00 to FF
hex.
In the following paragraphs we use the term code to mean an indivisible
sequence of ASCII characters or values having specific meaning to the
protocol. Indivisible means that the component bytes of a code must be
sent one after another with no other bytes between them. It does not refer
to the timing of the bytes.
Full–duplex protocol uses these codes:
Control codes:
DLE STX
DLE ETX BCC
DLE ACK
DLE NAK
DLE ENQ
Link–layer data codes:
Data (single bytes having values 00–0F and10–FF hex)
DLE DLE (to represent the value 10 hex)
We can also group codes into two classes according to their use:
1. Message codes issued from a station sending a message.
2. Response codes issued from a station receiving a message.
The full–duplex codes sent by the station transmitting a message are:
DLE STX – indicates the start of a message packet.
Link–layer data (00–OF and10–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 packet.
DLE ENQ – requests the retransmission of the last received
transmission.
The full–duplex response codes sent by a station receiving a message are: