COM90C66 Datasheet, PDF (30/76 Pages) SMSC Corporation – ARCNET Controller/Transceiver with AT Interface and On-Chip RAM

represents the message length. The SID in
Address 0 is used by the receiving node to reply
to the transmitting node. The COM90C66 puts
the local ID in this location, therefore, it is not
necessary to write into this location.
Please note that a short packet may contain
between one and 253 data bytes, while a long
packet may contain between 257 and 508 data
bytes. A minimum value of 257 exists on a long
packet so that the COUNT is expressable in eight
bits. This leaves three exception packet lengths
which do not fit into either a short or long packet;
packet lengths of 254, 255, or 256 bytes. If
packets of these lengths must be sent, the user
must add dummy bytes to the packet (00's) in
order to make the packet fit into a long packet.
Once the buffer is written into, the
microprocessor awaits a logic "1" on the TA bit,
indicating that a previous transmit command has
concluded and another may be issued. Each
time the message is loaded and a transmit
command issued, it will take a variable amount
of time before the message is transmitted,
depending on the traffic on the network and the
location of the token at the time the transmit
command was issued. Typically, the conclusion
of the transmit command, which is flagged when
TA becomes a logic "1", generates an interrupt.
If the device is configured for the Command
Chaining operation, please see the Command
Chaining section of this document for further
detail on the software.
Once the TA bit becomes a logic "1", the
microprocessor issues the ENABLE TRANSMIT
FROM PAGE nn command, which resets the TA
and TMA bits to logic "0". If the message is not a
BROADCAST, the COM90C66 automatically
issues a FREE BUFFER ENQUIRY to the
destination node in order to send the message.
At this point, one of three possibilities may occur:
The first possibility is if a free buffer is available
at the designation note, in which case it responds
with an ACKnowledgement. At this point, the
COM90C66 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 is 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 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 try to retransmit a FREE BUFFER
ENQUIRY. If a NAK is again received, the token
is again passed onto the next node. A software
timeout on the TA bit is used to prevent this from
happening endlessly. That is, if no software
timeout existed, the transmitting node would
keep trying to issue a Free Buffer Enquiry, even
though it keeps receiving NAKs as a response.
The software timeout on the TA bit is used to tell
the microprocessor to disable the transmitter via
the DISABLE TRANSMITTER command, which
will cause 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".
The third possibility which may occur after a
FREE BUFFER ENQUIRY is issued is if no
response is received from the destination node.
In this case, the TA bit is set to a logic "1" in
preparation for the next time the token is
received, while the TMA bit remains at a logic
"0". The user should determine whether the node
will try to retransmit the next time it receives the
token.
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