COM20051I Datasheet, PDF (42/74 Pages) SMSC Corporation – Integrated Microcontroller and ARCNET (ANSI 878.1) Interface

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
Setup 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 may be 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.
SMSC DS – COM20051I
Page 42
Rev. 03/27/2000