COM20020_00 Datasheet, PDF (11/82 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

NETWORK RECONFIGURATION. The ET2 and
ET1 bits of the Configuration Register allow the
network to operate over longer distances than the
2 miles stated earlier. The logic levels on these
bits control the maximum distances over which the
COM20020 can operate by controlling the three
timeout values described above. For proper
network operation, all COM20020's connected to
the same network must have the same Response
Time, Idle Time, and Reconfiguration Time.
LINE PROTOCOL
The ARCNET line protocol is considered
isochronous because each byte is preceded by a
start interval and ended with a stop interval. Unlike
asynchronous protocols, there is a constant
amount of time separating each data byte. On a 5
Mbps network, each byte takes exactly 11 clock
intervals of 200ns each. As a result, one byte is
transmitted every 2.2 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 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
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