COM20051I Datasheet, PDF (17/74 Pages) SMSC Corporation – Integrated Microcontroller and ARCNET (ANSI 878.1) Interface

Negative Acknowledgements (NAK)
A Negative Acknowledgement is used as a negative response to FREE BUFFER ENQUIRIES and is sent by the
following sequence:
!" An ALERT BURST
!" A NAK (Negative Acknowledgement--ASCII code 15H) character
ALERT
BURST
NAK
Figure 5 illustrates the flow of events on a 5-node network where a node with the NID=1 transmits a data packet to
a node with the NID=5, and a node with the NID=3 tries to transmit a data to a node with the NID=4 but node 4
cannot accept it. All other nodes are just passing the tokens.
#5 ITT to #1
#1 FBE to #5
#5 ACK to #1
#1 PAC to #5
#5 ACK to #1
#3 ITT to #4
#4 NAK to #3
#3 FBE to #4
#2 ITT to #3
#1 ITT to #2
#4 ITT to #5
SEQUENCE OF LINE EVENTS
1) NODE 1 RECEIVES TOKEN FROM NODE 5
2) NODE 1 TRANSMITS TO NODE 5
A) ISSUES FBE TO NODE 5
B) NODE 5 IS READY TO RECEIVE SO IT ISSUES AN ACK
C) NODE 1 NOW TRANSMITS THE DATA
D) NODE 5 RECEIVES THE DATA ERROR FREE AND ISSUES AN ACK
3) NODE 1 PASSES TOKEN TO NODE 2
4) NODE 2 DOES NOT NEED TO TRANSMIT AND PASSES THE TOKEN TO NODE 3
5) NODE 3 NEEDS TO TRANSMIT TO NODE 4
A) ISSUES AN FBE TO NODE 4
B) NODE 4 IS NOT READY TO RECEIVE AND IT ISSUES A NAK
6) NODE 3 PASSES THE TOKEN TO NODE 4
7) NODE 4 PASSES THE TOKEN TO NODE 5
8) GO TO STEP 1
FIGURE 5 – AVERAGE SEQUENCE OF LINE EVENTS FOR A FIVE-NODE NETWORK
SMSC DS – COM20051I
Page 17
Rev. 03/27/2000