COM20020_00 Datasheet, PDF (12/82 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

Free Buffer Enquiries
A Free Buffer Enquiry is used to ask another node
if it is able to accept a packet of data. It is sent by
the following sequence:
• An ALERT BURST
• An ENQ (ENQuiry: ASCII code 85H)
• Two (repeated) DID (Destination ID)
characters
ALERT
ENQ
DID
DID
BURST
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
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