Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 46)

Asynchronous Link Protocols
Chapter 4
4-3
Data Codes:
- Data (single characters having values 00-0F and 11-FF hex)
- DLE DLE (to represent the value 10 hex)
Codes are grouped into two classes: message codes, sent from transmitter
to receiver; and response codes, sent from receiver to transmitter.
DLE STX is a message code used to indicate the start of a message.
DLE ETX BCC/CRC is a message code used to terminate a message.
Data 00-0F and 11-FF hex are message codes used to encode the
corresponding values in the message itself. DLE DLE is a message
used to encode the occurrence of the value 10 in the message.
DLE ENQ is a message code. It requests the retransmission of the last
received code.
DLE ACK, a response code, signals that a message has been
successfully received.
DLE NAK, also a response code, signals that an attempt to transfer a
message has failed.
Link-Layer Message Packets
A link-layer message packet starts with a DLE STX and ends in a DLE
ETX BCC/CRC, with data codes in between. Data codes can occur only
inside a message packet. Response codes can also occur between a DLE
STX and a DLE ETX BCC/CRC, but such response codes are not part of
the message packet. They are referred to as embedded responses.
Figure 4.1 shows the format of a link-layer message packet for full-duplex
protocol, and the layer at which each portion should be implemented. At
the end of each message packet is the one-byte BCC field.