5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
Datasheet
number of information bytes in the message. The SID in Address 0 is used by the receiving node to reply
to the transmitting node. The COM20020ID 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 expressible 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. 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 COM20020ID 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 COM20020ID 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
COM20020ID 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 COM20020ID 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.
SMSC COM20020I Rev D
Page 43
Revision 12-05-06
DATASHEET
SMSC [ SMSC CORPORATION ]
