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INITIALIZATION SEQUENCE
Bus Determination
Writing to and reading from an odd address
location from the COM20020's address space
causes the COM20020 to determine the
appropriate bus interface. When the COM20020
is powered on the internal registers may be
written to. Since writing a non-zero value to the
Node ID Register wakes up the core, the Setup
Register should be written to before the Node ID
Register. Until a non-zero value is placed into
the NID Register, no microcode is executed, no
tokens are passed by this node, and no
reconfigurations are generated by this node.
Once a non-zero value is placed in the register,
the core wakes up, but the node will not attempt
to join the network until the TX Enable bit of the
Configuration Register is set.
Before setting the TX Enable bit, the software
may make some determinations. The software
may first observe the Receive Activity and the
Token Seen bits of the Diagnostic Status
Register to verify the health of the receiver and
the network.
Next, the uniqueness of the Node ID value
placed in the Node ID Register is determined.
The TX Enable bit should still be a logic "0" until
it is ensured that the Node ID is unique. If this
node ID already exists, the Duplicate ID bit of
the Diagnostic Status Register is set after a
maximum of 840mS (or 1680mS if the ET1 and
ET2 bits are other than 1,1). To determine if
another node on the network already has this ID,
the COM20020 compares the value in the Node
ID Register with the DID's of the token, and
determines whether there is a response to it.
Once the Diagnostic Status Register is read, the
DUPID bit is cleared. The user may then
attempt a new ID value, wait 840mS before
checking the Duplicate ID bit, and repeat the
process until a unique Node ID is found. At this
point, the TX Enable bit may be set to allow the
node to join the network. Once the node joins
the network, a reconfiguration occurs, as usual,
thus setting the MYRECON bit of the Diagnostic
Status Register.
The Tentative ID Register may be used to build
a network map of all the nodes on the network,
even once the COM20020 has joined the
network. Once a value is placed in the
Tentative ID Register, the COM20020 looks for
a response to a token whose DID matches the
Tentative ID Register. The software can record
this information and continue placing Tentative
ID values into the register to continue building
the network map. A complete network map is
only valid until nodes are added to or deleted
from the network. Note that a node cannot
detect the existence of the next logical node on
the network when using the Tentative ID. To
determine the next logical node, the software
should read the Next ID Register.
IMPROVED DIAGNOSTICS
The COM20020 allows the user to better
manage the operation of the network through
the use of the internal Diagnostic Status
Register.
A high level on the My Reconfiguration
(MYRECON) bit indicates that the Token
Reception Timer of this node expired, causing a
reconfiguration by this node. After the
Reconfiguration (RECON) bit of the Status
Register interrupts the microcontroller, the
interrupt service routine will typically read the
MYRECON bit of the Diagnostic Status
Register. Reading the Diagnostic Status
Register resets the MYRECON bit. Successive
occurrences of a logic "1" on the MYRECON bit
indicates that a problem exists with this node.
At that point, the transmitter should be disabled
so that the entire network is not held down
while the node is being evaluated.
The Duplicate ID (DUPID) bit is used before the
node joins the network to ensure that another