COM20020_00 Datasheet, PDF (51/82 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

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 COM20020 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 COM20020 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 COM20020
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
COM20020 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 COM20020 sets the RI bit to logic "1"
to signal the microcontroller that the reception is
complete.
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