COM90C66 Datasheet, PDF (10/76 Pages) SMSC Corporation – ARCNET Controller/Transceiver with AT Interface and On-Chip RAM

The time required to do a NETWORK
RECONFIGURATION depends on the number of
nodes in the network, the propagation delay
between nodes, and the highest ID number on
the network, but will be in the range of 24 to 61
mS.
BROADCAST MESSAGES
Broadcasting gives a particular node the ability to
transmit a data packet to all nodes on the
network simultaneously. ID zero is reserved for
this feature and no node on the network can be
assigned ID zero. To broadcast a message, the
transmitting node's processor simply loads the
RAM buffer with the data packet and sets the DID
equal to zero. Figure 9 illustrates the position of
each byte in the packet with the DID residing at
address 01 HEX of the current page selected in
the TRANSMIT command. Each individual node
has the ability to ignore broadcast messages by
setting the most significant bit of the ENABLE
RECEIVE TO PAGE nn command (see Table 7)
to logic "0".
EXTENDED TIMEOUT FUNCTION
There are three timeouts associated with the
COM90C66 operation:
Response Time
The Response Time is equal to the round trip
propagation delay between the two furthest
nodes on the network plus the maximum turn
around time (the time it takes a particular
COM90C66 to start sending a message in
response to a received message), which is
approximately 12.7 µS. The round trip
propagation delay is a function of the
transmission media and network topology. For a
typical system using RG62 coax in a baseband
system, a one-way cable propagation delay of 31
µS translates to a distance of about four miles.
The flow chart in Figure 1 uses a value of 74.7
µS (31 + 31 + 12.7) to determine if any node will
respond.
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.
Every other node on the network must distinguish
between this operation and an entirely idle line.
During NETWORK RECONFIGURATION,
activity will appear on the line every 82 µS. This
82 µS is equal to the Response Time of 74.7 µS
plus the time it takes the COM90C66 to
retransmit 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 (bits 3 and 4 of the
Configuration Register) allow the network to
operate over longer distances than the four miles
stated earlier. The logic levels on these bits
control the maximum distances over which the
COM90C66 can operate by controlling the three
timeout values described above. See the
description of the ET1 and ET2 bits, found in
Table 8, for the table containing the
combinations of these bits. It should be noted
that for proper network operation, all
COM90C66s connected to the same network
must have the same Response Time, Idle Time,
and Reconfiguration Time.
LINE PROTOCOL
The ARCNET line protocol can be described as
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 up exactly 11 clock
intervals with a single clock interval being 400 nS
in duration. As a result, one byte is
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