Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 62)

Asynchronous Link Protocols
Chapter 4
4-19
Figure 4.13
Message Transfer with Message Sink Full
Not Full
OK
xxxx
xxxx
SOURCE XMTR LINK RCVR SINK
11565
DLE NAK
DLE STX xxxx DLE EXT BCC
DLE STX xxxx DLE EXT BCC
DLE ACK
DLE STX xxxx DLE ETX BCC
DLE NAK
Full
Full
Retransmission occurs when noise hits both sides of the line. This type of
noise destroys the DLE ACK while also producing invalid characters at
the receiver. The result is that the receiver changes its last response to
NAK and the transmitter retransmits the original message. Note that this
is detected as a duplicate message if Switch 1 of Switch Group SW-1 is
on.