10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Board RAM
Datasheet
4.6
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 10 Mbps network, each byte takes exactly 11 clock intervals of 100ns
each. As a result, one byte is transmitted every 1.1 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 50nS 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:
4.6.1 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
BURST
EOT
DID
DID
4.6.2 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
BURST
ENQ
DID
DID
4.6.3 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
data
CRC
CRC
BURST
Revision 02-27-06
Page 14
SMSC COM20022I
DATASHEET
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