Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 130)

Half-Duplex Protocol
Chapter 11
11Ć3
These ASCII control characters are extended to 8 bits by adding a zero for
bit 7. See ANSI X3.4, CCITT V.3, or ISO 646 for the standard definition
of these characters.
Additionally, a block check character (BCC) is used at the end of each
transmission packet for error checking. This byte can be any value from
00 to FF hex.
The term code means (in the following paragraphs) an indivisible
sequence of one or more bytes having a specific meaning to the protocol.
Indivisible means that the component bytes of a code must be sent one
after another with no other bytes inserted between them. It does not refer
to the timing of the bytes. (This definition has less significance than for
full–duplex protocol, since there is no multiplexing of transmission codes
in half–duplex protocol).
Half–duplex protocol uses the following control codes:
DLE SOH
DLE STX
DLE ETX BCC/CRC
DLE ACK
DLE NAK
DLE ENQ
DLE EOT
Half–duplex protocol also uses the following link–layer data codes:
Data (single bytes having values 00–0F and 11–FF hex)
DLE DLE (to represent the value 10 hex)
Link–layer address code
STN (station identifier)
We can group these codes into two classes according to their use:
1. message codes issued from a station sending a message (or poll)
2. response codes issued from a station receiving a message (or poll).
These codes are issued by a station transmitting a message (or poll):
DLE SOH – indicates the start of a message packet.
STN – helps to designate the station number. When the 1775–KA is
communicating with another station as a peer, the STN = DST. If the