COM20020D Datasheet, PDF (11/72 Pages) SMSC Corporation – ULANC Revision D Universal Local Area Network Controller with 2K x 8 On-Board RAM

is defined as no line activity and a logic "1" is
defined as a negative pulse of 100nS duration. A
transmission starts with an ALERT BURST
consisting of 6 unit intervals of mark (logic "1").
Eight bit data characters are then sent, with each
character preceded by 2 unit intervals of mark and
one unit interval of space. Five types of
transmission can be performed as described
below:
Invitations To Transmit
An Invitation To Transmit is used to pass the
token from one node to another and is sent by the
following sequence:
• An ALERT BURST
• An EOT (End Of Transmission: ASCII code
04H)
• Two (repeated) DID (Destination ID)
characters
ALERT
EOT
DID
DID
BURST
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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