41
41
the receiving node to reply to the transmitting
node. The ARCNET core 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 1 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 in order to
make the packet fit into a long packet. Note that
only the number of bytes specified in the byte
count plus the three-byte header are transmitted.
For example, if the byte count is equal to 253,
only three bytes of data will be transmitted plus
the header (SID, DID, Byte Count) for a total of
six bytes.
Once the packet is written into the buffer, the
microcontroller 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.
The conclusion of the
Transmit command will generate an interrupt if
the Interrupt Mask allows it.
If the device is
configured for the Command Chaining operation,
please see the Command Chaining section for
further detail on the transmit sequence.
Once
the
TA
bit
becomes
a
logic
"1",
the
microcontroller may issue the "Enable Transmit
from Page fnn" command, which resets the TA
and TMA bits to logic "0". If the message is not a
BROADCAST, the ARCNET core automatically
sends a FREE BUFFER
ENQUIRY to the
destination node in order to send the message.
At this point, one of four possibilities may occur.
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 ARCNET core 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.
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