com20020-5 Standard Microsystems Corp., com20020-5 Datasheet - Page 33

• Manufacturer Part Number: com20020-5
• Description: Com20020-5 Ulanc Universal Local Area Network Controller With 2k X 8 On-board Ram
• Manufacturer: Standard Microsystems Corp.

buffer address 2 contains a zero or non-zero
value.
Figure 7.
Identifier
Destination Identifier (DID); Address 2 (COUNT)
contains, for short packets, the value 256-N,
where N represents the number of bytes (data
bytes) in the message information, or for long
packets, the value 0, indicating that it is indeed
a long packet.
(COUNT) would contain the value 512-N, where
N represents the number of bytes (data bytes) in
the message information. The SID in Address 0
is used by the receiving node to reply to the
transmitting node. The COM20020-5 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.
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.
configured
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
The format of the buffer is shown in
(SID);
Address 0 contains the Source
for
In the latter case, Address 3
the
Address
Command
1
If the device is
contains
Chaining
the
33
Transmit from Page fnn" command, which
resets the TA and TMA bits to logic "0". If the
message
COM20020-5 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.
1.
available at the destination node, in which case
it responds with an ACKnowledgement. At this
point, the COM20020-5 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".
successfully, TMA will not be set. A successful
transmission occurs when the receiving node
responds to the packet with an ACK.
unsuccessful transmission occurs when the
receiving node does not respond to the packet.
2. The second possibility is if the destination
node responds to the Free Buffer Enquiry with a
Negative AcKnowledgement.
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.
transmitting node would continue issuing a Free
Buffer
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
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.
If no limit of FBE-NAK sequences existed, the
The first possibility is if a free buffer is
Enquiry,
is
If the packet was not transmitted
via
not
the
even
a
"Disable
BROADCAST,
though
A NAK occurs
Transmitter"
it
would
the
An