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ALERT
BURST
EOT
DID
DID
ALERT
BURST
ENQ
DID
DID
Idle Time
The Idle Time is associated with a NETWORK
RECONFIGURATION. Figure 1 illustrates that
during a NETWORK RECONFIGURATION one
node will continually transmit INVITATIONS TO
TRANSMIT until it encounters an active node.
All other nodes on the network must distinguish
between this operation and an entirely idle line.
During NETWORK RECONFIGURATION,
activity will appear on the line every 82µS. This
82µS is equal to the Response Time of 74.7µS
plus the time it takes the COM20020 to start
retransmitting another message (usually another
INVITATION TO TRANSMIT).
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 4 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 2.5 Mbps network, each byte takes
exactly 11 clock intervals of 400ns each. As a
result, one byte is transmitted every 4.4µS 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 200nS 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
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