Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 66)

Asynchronous Link Protocols
Chapter 4
4-23
The term “code” means (in the following paragraphs) an indivisible
sequence of one or more bytes having a specific meaning to the link
protocol. “Indivisible” means that the component characters of a code
must be sent one after another with no other characters inserted between
them. It does not refer to the timing of the characters. (This definition
has less significance than for full-duplex protocol since there is no
multiplexing of transmission codes in half-duplex protocol.)
These codes are used in half-duplex protocol:
Control Codes:
- DLE SOH
- DLE STX
- DLE ETX BCC/CRC
- DLE ACK
- DLE NAK
- DLE ENQ
- DLE EOT
Data Codes:
- DATA (single characters having values 00-0F and 11-FF hex)
- DLE DLE (to represent the value 10 hex)
DLE SOH indicates the start of a message.
DLE STX separates the link level header from the data field of a message.
DLE ETX BCC/CRC terminates a message.
DATA 00-0F and 11-FF encode the corresponding values in the message
itself. DLE DLE encodes the occurrence of the value 10 in the message.
DLE ACK signals that a message has been successfully received.
DLE NAK is used as a global link reset command and can be issued only
by the master. It causes all slaves to cancel all messages that are ready to
be transmitted to the master. Typically the slave will return the messages
to the command originator with an error code.
DLE ENQ starts a poll command.
DLE EOT is used by slaves as a response to a poll when they have no
messages to send.