COM20020-5 Datasheet, PDF (10/58 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

a typical system using RG62 coax in a
baseband system, a one way cable propagation
delay of 15.5µS translates to a distance of about
2 miles. The flow chart in Figure 1 uses a value
of 37.35µS (15.5 + 15.5 + 6.35) to determine if
any node will respond.
Idle Time (ET1, ET2, ET3)
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 41µS. This
41µS is equal to the Response Time of 37.35µS
plus the time it takes the COM20020-5 to start
retransmitting another message (usually another
INVITATION TO TRANSMIT).
Reconfiguration Time (ET1, ET2)
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 2 miles stated earlier. The logic levels
on these bits control the maximum distances
over which the COM20020-5 can operate by
controlling the three timeout values described
above. For proper network operation, all
COM20020-5'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. Each byte takes exactly 11 clock intervals
that are defined by nPULSE1 and nPULSE2
signals (at 5 Mbps one byte is transmitted every
2.2µS). 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 (at 5 Mbps). 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 (ITT)
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
Free Buffer Enquiries (FBE)
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 ENQ DID DID
BURST
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