COM20020_00 Datasheet, PDF (10/82 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

INVITATIONS TO TRANSMIT are sent to all NIDs
(1-255).
Each COM20020 on the network will finally have
saved a NID value equal to the ID of the
COM20020 that it released control to. At this
point, control is passed directly from one node to
the next with no wasted INVITATIONS TO
TRANSMIT being sent to ID's not on the network,
until the next NETWORK RECONFIGURATION
occurs. When a node is powered off, the previous
node attempts to pass the token to it by issuing an
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".
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.
Response Time
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 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 15.5 μS translates to a
distance of about 2 miles. The flow chart in Figure
1 uses a value of 37.4 μS (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 μS. This 41 μS is
equal to the Response Time of 37.4 μS plus the
time it takes the COM20020 to start retransmitting
another message (usually another INVITATION
TO TRANSMIT).
EXTENDED TIMEOUT FUNCTION
Reconfiguration Time
If any node does not receive the token within the
Reconfiguration Time, the node will initiate a
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