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.
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
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
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