COM90C66 Datasheet, PDF (11/76 Pages) SMSC Corporation – ARCNET Controller/Transceiver with AT Interface and On-Chip RAM

transmitted every 4.4 µS and the time to transmit
a message can be precisely determined. The line
idles in a spacing (logic "0") condition. A logic "0"
is defined as no line activity and a logic "1" is
defined as a negative pulse of 200 nS duration.
A transmission starts with an ALERT BURST
consisting of six unit intervals of mark (logic "1").
Eight-bit data characters are then sent with each
character preceded by two 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
04 HEX)
• Two (repeated) DID (Destination
IDentification) characters
ALERT
BURST EOT
DID
DID
Free Buffer Enquiries
A Free Buffer Enquiry is used to ask another
node if it is able to accept a packet of data and
is sent by the following sequence:
• An ALERT BURST
• An ENQ (ENQuiry--ASCII code 85 HEX)
• Two (repeated) DID (Destination
IDentification) characters
ALERT
BURST ENQ DID
DID
Data Packets
A Data Packet consists of the actual data being
sent to another node and is sent by the following
sequence:
• An ALERT BURST
• An SOH (Start Of Header--ASCII code 01
HEX)
• An SID (Source IDentification) character
• Two (repeated) DID (Destination
IDentification) characters
• A single COUNT character which is the 2's
complement of the number of data bytes to
follow if a short packet is being sent or 00
HEX followed by a COUNT character which
is the 2's complement of the number of data
bytes to follow if a long packet is being 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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