COM20019 Datasheet, PDF (10/81 Pages) SMSC Corporation – Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM

NETWORK RECONFIGURATION
A significant advantage of the COM20019 is its
ability to adapt to changes on the network.
Whenever a new node is activated or deactivated,
a NETWORK RECONFIGURATION is performed.
When a new COM20019 is turned on (creating a
new active node on the network), or if the
COM20019 has not received an INVITATION TO
TRANSMIT for 6.72S, or if a software reset
occurs, the COM20019 causes a NETWORK
RECONFIGURATION by sending a
RECONFIGURE BURST consisting of eight
marks and one space repeated 765 times. The
purpose of this burst is to terminate all activity on
the network. Since this burst is longer than any
other type of transmission, the burst will interfere
with the next INVITATION TO TRANSMIT,
destroy the token and keep any other node from
assuming control of the line.
Each COM20019 on the network will finally have
saved a NID value equal to the ID of the
COM20019 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 192 to 488 mS.
When any COM20019 senses an idle line for
greater than 656 S, which occurs only when the
token Is lost, each COM20019 starts an internal
timeout equal to 1.168mS times the quantity 255
minus its own ID. The COM20019 starts network
reconfiguration by sending an invitation to transmit
first to itself and then to all other nodes by
decrementing the destination Node ID. If the
timeout expires with no line activity, the
COM20019 starts sending INVITATION TO
TRANSMIT with the Destination ID (DID) equal to
the currently stored NID. Within a given network,
only one COM20019 will timeout (the one with the
highest ID number). After sending the
INVITATION TO TRANSMIT, the COM20019
waits for activity on the line. If there is no activity
for 597.6 S, the COM20019 increments the NID
value and transmits another INVITATION TO
TRANSMIT using the NID equal to the DID. If
activity appears before the 597.6 S timeout
expires, the COM20019 releases control of the
line. During NETWORK RECONFIGURATION,
INVITATIONS TO TRANSMIT are sent to all NIDs
(1-255).
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
COM20019 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.
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