com90c66 Standard Microsystems Corp., com90c66 Datasheet - Page 11

• Manufacturer Part Number: com90c66
• Description: Arcnet Controller/transceiver With At Interface And On-chip Ram Corporation
• Manufacturer: Standard Microsystems Corp.

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").
character preceded by two unit intervals of mark
and one unit interval of space.
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:
Free Buffer Enquiries
A Free Buffer Enquiry is used to ask another
node if it is able to accept a packet of data and
Eight-bit data characters are then sent with each
An ALERT BURST
An EOT (End Of Transmission--ASCII code
Two
04 HEX)
IDentification) characters
BURST
ALERT
(repeated)
EOT
DID
DID
Five types of
(Destination
DID
11
is sent by the following sequence:
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 ENQ (ENQuiry--ASCII code 85 HEX)
Two
An ALERT BURST
An SOH (Start Of Header--ASCII code 01
An SID (Source IDentification) character
Two
A single COUNT character which is the 2's
N data bytes where COUNT = 256-N (or
Two
IDentification) characters
HEX)
IDentification) characters
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
512-N for a long packet)
characters. The CRC polynomial used is X
+ X
BURST
ALERT
15
+ X
CRC
(repeated)
(repeated)
2
+ 1.
ENQ
(Cyclic
DID
Redundancy
DID
DID
DID
(Destination
(Destination
Check)
16