COM20020-5 Datasheet, PDF (11/58 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

Data Packets
A Data Packet consists of the actual data being
sent to another node. It is sent by the following
sequence:
• An ALERT BURST
• An SOH (Start Of Header--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 00H
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.
ALERT
SOH SID DID DID COUNT
data
BURST
data CRC CRC
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
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