16
16
Data Packets (PAC)
A Data Packet consists of the actual data being
sent to another node. It is sent by the following
sequence:
• An ALERT BURST
• An PAC (Data Packet--ASCII code 01H)
• An SID (Source ID) character
• Two (repeated) DID (Destination ID)
characters
• A single COUNT character which is the
2's complement of the number of data
bytes to follow if a short packet is sent,
or
00Hex followed by a COUNT
character if a long packet is sent
• N data bytes where COUNT = 256-N (or
512-N for a long packet)
• Two CRC (Cyclic Redundancy Check)
characters.
The CRC polynomial used is:
X16 + X15 + X2 + 1.
Acknowledgements (ACK)
An Acknowledgement is used to acknowledge
reception of a packet or as an affirmative
response to FREE BUFFER ENQUIRIES and is
sent by the following sequence:
•
An ALERT BURST
•
An ACK (ACKnowledgement--ASCII code
86H) character
ALERT
BURST
ACK
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.
ALERT
BURST
PAC
SID
DID
DID
COUNT
data
data
CRC
CRC
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