Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM
This 656 μS is equal to the Response Time of 597.6 μS plus the time it takes the COM20019I to start
retransmitting another message (usually another INVITATION TO TRANSMIT).
4.5.3 Reconfiguration Time
If any node does not receive the token within the Reconfiguration Time, the node will initiate a NETWORK
RECONFIGURATION. The ET2 and ET1 bits of the Configuration Register allow the network to operate
over longer distances than the 32 miles stated earlier. Thelogic levels on these bits control the maximum
distances over which the COM20019I can operate by controlling the three timeout values described above.
For proper network operation, all COM20019I's connected to the same network must have the same
Response Time, Idle Time, and Reconfiguration Time.
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 312.5 Kbps network, each byte takes exactly 11 clock intervals of 3.2 μS
each. As a result, one byte is transmitted every 35.2 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 1.6 S 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:
SMSC COM20019I
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Rev. 03-07-06
DATASHEET
SMSC [ SMSC CORPORATION ]
