COM20020 Datasheet, PDF (10/72 Pages) SMSC Corporation – COM20020 3.3V ULANC Universal Local Area Network Controller with 2K x 8 On-Board RAM

INVITATION TO TRANSMIT. Since this node
does not respond, the previous node times out
and transmits another INVITATION TO
TRANSMIT to an incremented ID and eventually a
response will be received.
The NETWORK RECONFIGURATION time
depends on the number of nodes in the network,
the propagation delay between nodes, and the
highest ID number on the network, but is typically
within the range of 12 to 30.5 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 4 illustrates the position of
each byte in the packet with the DID residing at
address 0X01 or 1 Hex of the current page
selected in the "Enable Transmit from Page fnn"
command. Each individual node has the ability to
ignore broadcast messages by setting the most
significant bit of the "Enable Receive to Page fnn"
command to a logic "0".
EXTENDED TIMEOUT FUNCTION
There are three timeouts associated with the
COM20020 operation. The values of these
timeouts are controlled by bits 3 and 4 of the
Configuration Register and bit 5 of the Setup 1
Register.
network topology. For a typical system using
RG62 coax in a baseband system, a one way
cable propagation delay of 15.5 "S translates to a
distance of about 2 miles. The flow chart in Figure
1 uses a value of 37.4 "S (15.5 + 15.5 + 6.4) 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. 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 41 "S. This 41 "S is
equal to the Response Time of 37.4 "S 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
2 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.
Response Time
LINE PROTOCOL
The Response Time determines the maximum
propagation delay allowed between any two
nodes, and should be chosen to be larger than
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
COM20020 to start sending a message in
response to a received message) which is
approximately 6.4 "S. The round trip propagation
delay is a function of the transmission media and
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 5
Mbps network, each byte takes exactly 11 clock
intervals of 200ns each. As a result, one byte is
transmitted every 2.2 "S and the time to transmit
a message can be precisely determined. The line
idles in a spacing (logic "0") condition. A logic "0"
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