COM20019IP datasheet(49/81 Pages) SMSC

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The

Receive

Activity

(RCVACT)

bit

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.

Abnormal Results:

RCVACT=1, TOKEN=0, TXEN=X:

The node

sees receive activity, but does not see the token.

Either no other nodes exist on the network, some

type of data corruption exists, the media driver is

malfunctioning, the topology is set up incorrectly,

there is noise on the network, or a reconfiguration

is occurring.

RCVACT=0, TOKEN=0, TXEN=1:

No receive

activity is seen and the basic transmit function is

enabled. The transmitter and/or receiver are not

functioning properly.

RCVACT=0, TOKEN=0, TXEN=0:

No receive

activity and basic transmit function disabled. This

node is not connected to the network.

The Excessive NAK (EXCNAK) bit is used to

replace

timeout

function

traditionally

implemented in software.

This function is

necessary to limit the number of times a sender

issues a FBE to a node with no available buffer.

When the destination node replies to 128 FBEs

with 128 NAKs or 4 FBEs with 4 NAKs, the

EXCNAK bit of the sender is set, generating an

interrupt. At this point the software may abandon

the transmission via the "Disable Transmitter"

command. This sets the TA bit to logic "1" when

the node next receives the token, to allow a

different transmission to occur. The timeout value

for the EXNACK bit (128 or 4) is determined by

the FOUR-NAKS bit on the Setup1 Register.

The user may choose to wait for more NAK's

before

disabling

the

transmitter

taking

advantage of the wraparound counter of the

EXCNAK bit. When the EXCNAK bit goes high,

indicating 128 or 4 NAKs, the "POR Clear Flags"

command maybe issued to reset the bit so that it

will go high again after another count of 128 or 4.

The software may count the number of times the

EXCNAK bit goes high, and once the final count is

reached, the "Disable Transmitter" command may

be issued.

The New Next ID bit permits the software to

detect the withdrawal or addition of nodes to the

network.

The Tentative ID bit allows the user to build a

network map of those nodes existing on the

network.

This feature is useful because it

minimizes the need for human intervention. When

a value placed in the Tentative ID Register

matches the Node ID of another node on the

network, the TENTID bit is set, telling the software

that this NODE ID already exists on the network.

The software should periodically place values in

COM20019IP Arkusz danych(PDF) 49 Page - SMSC Corporation

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