Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 75)

Asynchronous Link Protocols
Chapter 4
4-32
When a DLE NAK is received, the transceiver takes messages from the
source until it is empty. Each message will be discarded with an error
code sent back to the message source. This can be used by the master to
clear up the message source buffers of all slaves after the master has been
down.
Half-Duplex Protocol Diagrams
The following figures show some of the events that occur on various
interfaces. Time is represented as increasing from the top of the figure to
the bottom.
Figure 4.17
Normal Message Transfer
Not Full
OK
xxxx
xxxx
SINK MASTER LINK SLAVE SINK
11567
DLE ACK
DLE STX xxxx DLE ETX BCC
SOURCE/ SOURCE/
DLE SOH STN
Figure 4.18
Message Transfer with Invalid BCC
Not Full
OK
xxxx
xxxx
SINK MASTER LINK SLAVE SINK
11568
DLE ACK
DLE STX ??? DLE ETX BCC
SOURCE/ SOURCE/
DLE SOH STN
DLE STX xxxx DLE ETX BCC DLE SOH STN
(Timeout)