Allen-Bradley 1779-KFMR User Manual page 93

Chapter 6
HDLC Data Link Layer
SOURCE
Packet
3 OK
SOURCE
Packet
Packet
3 OK
Packet
4 OK
6 24
Figure 6.6 shows a receive not ready (RNR) response to the initial
message transmission because the message sink is full. After the message
sink is no longer full, a retransmission of the message causes a receive
ready (RR) response.
Figure 6.6
Message Transfer with Message Sink Full
XMTR LINK
1,D3,7
1,RNR3
3,RR3
1,D3,7
1,RR4
Figure 6.7 shows a reject (REJ) response to a data frame because it's out
of sequence. The frame is out of sequence because the receiver had to
discard the previous data frame. After the transmitter sends the frame
with the proper send sequence number, the receiver sends an RR response.
Figure 6.7
Message Transfer with Data Frame Out of Sequence
XMTR LINK
1,D3,???
1,D4,7
1,REJ3
1,D3,7
1,RR4
1,D4,7
1,RR5
RCVR SINK
Full
Not Full
Not Full
Packet
RCVR SINK
Not Full
Packet
Not Full
Packet