5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
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.
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 a 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 by 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 the Tentative ID Register and monitor the New Next ID
bit to maintain an updated network map.
OSCILLATOR
The COM20020I contains circuitry which, in conjunction with an external parallel resonant crystal or TTL clock, forms an
oscillator.
If an external crystal is used, two capacitors are needed (one from each leg of the crystal to ground). No external
resistor is required, since the COM20020I contains an internal resistor. The crystal must have an accuracy of 0.020% or
better. The oscillation frequency range is from 10 MHz to 20 MHz.
SMSC COM20020I 3.3V
Page 43
Revision 12-06-06
DATASHEET
SMSC [ SMSC CORPORATION ]
