COM20020 Datasheet, PDF (47/72 Pages) SMSC Corporation – COM20020 3.3V ULANC Universal Local Area Network Controller with 2K x 8 On-Board RAM

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 420mS (or 840mS 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
420mS 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
node with the same ID does not exist on the
network. Once it is determined that the ID in the
Node ID Register is unique, the software should
write a logic "1" to bit 5 of the Configuration
Register to enable the basic transmit function.
This allows the node to join the network.
The Receive Activity (RCVACT) bit of the
Diagnostic Status Register will be set to a logic "1"
whenever activity (logic "1") is detected on the
RXIN pin.
The Token Seen (TOKEN) bit is set to a logic "1"
whenever any token has been seen on the
network (except those tokens transmitted by this
node).
The RCVACT and TOKEN bits may help the user
to troubleshoot the network or the node. If unusual
events are occurring on the network, the user may
find it valuable to use the TXEN bit of the
Configuration Register to qualify events. Different
combinations of the RCVACT, TOKEN, and TXEN
bits, as shown indicate different situations:
Normal Results:
RCVACT=1, TOKEN=1, TXEN=0: The node is
not part of the network. The network is operating
properly without this node.
RCVACT=1, TOKEN=1, TXEN=1: The node sees
receive activity and sees the token. The basic
transmit function is enabled. Network and node
are operating properly.
MYRECON=0, DUPID=0, RCVACT=1, TXEN=0,
TOKEN=1: Single node network.
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