Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 78)

Asynchronous Link Protocols
Chapter 4
4-35
Figure 4.24
Message Sink Full, Case 2
Full
OK
xxxx
xxxx
SINK MASTER LINK SLAVE SINK
11575
DLE ACK
SOURCE/ SOURCE/
BCC DLE SOH STN
DLE STX
BCC DLE ENQ STN
Not Full
(Timeout)
DLE STX xxxx DLE ETX
xxxx
Not Full
xxxx
xxxx DLE ETX BCC
BCC DLE SOH STN DLE STX xxxx DLE ETX
DLE ACK
The message sent for the second poll is the same as the first, since the first
transmission was not ACKed. Also, the master should ACK and discard
the second message, since it was a retransmission. This means that the
master must either poll a station repeatedly until it receives a DLE EOT,
without polling any other stations, to be sure it has detected all
retransmissions.
If each station is polled only once per cycle, the master must keep a
record of the last header from each station, since other stations may
transfer messages between retransmissions from a station.
When a station times out, it is a good idea to poll it to see if it is there. If
it answers with a DLE EOT to a message, but consistently fails to a ACK
a message, its message sink is probably full. If it responds DLE EOT to a
poll, you must wait for the buffers to clear.
When a station’s message source and sink share a common memory pool,
it may be that the message sink “full” indication results from an
abundance of messages in the message source, which use up all free pool
memory. In this case, the memory can be freed up by receiving messages
from the station. Waiting for the memory to clear by the action of the