5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
The first possibility is if a free buffer is available at the destination node, in which case it responds with an
ACKnowledgement. At this point, the COM20020I fetches the data from the Transmit Buffer and performs the transmit
sequence. If a successful transmit sequence is completed, the TMA bit and the TA bit are set to logic "1". If the packet
was not transmitted successfully, TMA will not be set. A successful transmission occurs when the receiving node
responds to the packet with an ACK. An unsuccessful transmission occurs when the receiving node does not respond
to the packet.
The second possibility is if the destination node responds to the Free Buffer Enquiry with a Negative AcKnowledgement.
A NAK occurs when the RI bit of the destination node is a logic "1". In this case, the token is passed on from the
transmitting node to the next node. The next time the transmitter receives the token, it will again transmit a FREE
BUFFER ENQUIRY. If a NAK is again received, the token is again passed onto the next node. The Excessive NAK bit
of the Diagnostic Status Register is used to prevent an endless sending of FBE's and NAK's. If no limit of FBE-NAK
sequences existed, the transmitting node would continue issuing a Free Buffer Enquiry, even though it would
continuously receive a NAK as a response. The EXCNAK bit generates an interrupt (if enabled) in order to tell the
microcontroller to disable the transmitter via the "Disable Transmitter" command. This causes the transmission to be
abandoned and the TA bit to be set to a logic "1" when the node next receives the token, while the TMA bit remains at a
logic "0". Please refer to the Improved Diagnostics section for further detail on the EXCNAK bit.
The third possibility which may occur after a FREE BUFFER ENQUIRY is issued is if the destination node does not
respond at all. In this case, the TA bit is set to a logic "1", while the TMA bit remains at a logic "0". The user should
determine whether the node should try to reissue the transmit command.
The fourth possibility is if a non-traditional response is received (some pattern other than ACK or NAK, such as noise).
In this case, the token is not passed onto the next node, which causes the Lost Token Timer of the next node to time
out, thus generating a network reconfiguration.
The "Disable Transmitter" command may be used to cancel any pending transmit command when the COM20020I next
receives the token. Normally, in an active network, this command will set the TA status bit to a logic "1" when the token
is received. If the "Disable Transmitter" command does not cause the TA bit to be set in the time it takes the token to
make a round trip through the network, one of three situations exists. Either the node is disconnected from the network,
or there are no other nodes on the network, or the external receive circuitry has failed. These situations can be
determined by either using the improved diagnostic features of the COM20020I or using another software timeout which
is greater than the worst case time for a round trip token pass, which occurs when all nodes transmit a maximum length
message.
Receive Sequence
A receive sequence begins with the RI status bit becoming a logic "1", which indicates that a previous reception has
concluded. The microcontroller will be interrupted if the corresponding bit in the Interrupt Mask Register is set to logic
"1". Otherwise, the microcontroller must periodically check the Status Register. Once the microcontroller is alerted to the
fact that the previous reception has concluded, it may issue the "Enable Receive to Page fnn" command, which resets
the RI bit to logic "0" and selects a new page in the RAM buffer. Again, the appropriate buffer size is specified in the
"Define Configuration" command. Typically, the page which just received the data packet will be read by the
microcontroller at this point. Once the "Enable Receive to Page fnn" command is issued, the microcontroller attends to
other duties. There is no way of knowing how long the new reception will take, since another node may transmit a
packet at any time. When another node does transmit a packet to this node, and if the "Define Configuration" command
has enabled the reception of long packets, the COM20020I interprets the packet as either a long or short packet,
depending on whether the content of the buffer location 2 is zero or non-zero. The format of the buffer is shown in
Figure 10. Address 0 contains the Source Identifier (SID), Address 1 contains the Destination Identifier (DID), and
Address 2 contains, for short packets, the value 256-N, where N represents the message length, or for long packets, the
value 0, indicating that it is indeed a long packet. In the latter case, Address 3 contains the value 512-N, where N
represents the message length. Note that on reception, the COM20020I deposits packets into the RAM buffer in the
same format that the transmitting node arranges them, which allows for a message to be received and then
retransmitted without rearranging any bytes in the RAM buffer other than the SID and DID. Once the packet is received
and stored correctly in the selected buffer, the COM20020I sets the RI bit to logic "1" to signal the microcontroller that
the reception is complete.
SMSC COM20020I 3.3V
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Revision 12-06-06
DATASHEET
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