PROTOCOL DESCRIPTION
NETWORK PROTOCOL
Communication on the network is based on a
token passing protocol. Establishment of the
network configuration and management of the
network protocol are handled entirely by the
COM90C66's internal microcoded sequencer. A
processor or intelligent peripheral transmits data
by simply loading a data packet and its
destination ID into the RAM buffer and issuing a
command to enable the transmitter. When the
COM90C66 next receives the token, it verifies
that the receiving node is ready by first
transmitting a FREE BUFFER ENQUIRY
message. If the receiving node transmits an
ACKnowledge message, the data packet is
transmitted followed by a 16-bit CRC. If the
receiving node cannot accept the packet
(typically its receiver is inhibited), it transmits a
Negative AcKnowledgement message and the
transmitter passes the token. Once it has been
established that the receiving node can accept
the packet and transmission is complete, the
receiving node will verify the packet. If the
packet is received successfully, the receiving
node transmits an ACKnowledge message (or
nothing if it is received unsuccessfully) allowing
the transmitter to set the appropriate status bits
to indicate successful or unsuccessful delivery of
the packet.
An interrupt mask permits the
COM90C66 to generate an interrupt to the
processor when selected status bits become true.
Figure 1 is a flow chart illustrating the internal
operation of the COM90C66.
NETWORK RECONFIGURATION
A significant advantage of the COM90C66 is its
ability to adapt to changes on the network.
Whenever a new node is activated or
deactivated, a NETWORK RECONFIGURATION
is performed. When a new COM90C66 is turned
on (creating a new active node on the network),
or if the COM90C66 has not received an
INVITATION TO TRANSMIT for 840 mS, or if a
software reset occurs, the device causes a
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NETWORK RECONFIGURATION by sending a
RECONFIGURE BURST consisting of eight
marks and one space repeated 765 times. The
purpose of this burst is to terminate all activity on
the network. Since this burst is longer than any
other type of transmission, the burst will interfere
with the next INVITATION TO TRANSMIT,
destroy the token and keep any other node from
assuming control of the line.
When any COM90C66 senses an idle line for
greater than 82 µS, which will only occur when
the token is lost, each COM90C66 starts an
internal timeout equal to 146 µS times the
quantity 255 minus its own ID. It also sets the
internally-stored NID (next ID representing the
next possible ID node) equal to its own ID. If the
timeout expires with no line activity, the
COM90C66 starts sending INVITATION TO
TRANSMIT with the Destination ID (DID) equal to
the currently-stored NID. Within a given network,
only one COM90C66 will timeout (the one with
the highest ID number).
After sending the
INVITATION TO TRANSMIT, the COM90C66
waits for activity on the line. If there is no activity
for 74.7µS, the COM90C66 increments the NID
value and transmits another INVITATION TO
TRANSMIT using the NID equal to the DID. If
activity appears before the 74.7µS timeout
expires, the COM90C66 releases control of the
line. During NETWORK RECONFIGURATION,
INVITATIONS TO TRANSMIT will be sent to all
256 possible IDs. Each COM90C66 on the
network will finally have saved a NID value equal
to the ID of the COM90C66 that it released
control to. At this point, control is passed directly
from one node to the next with no wasted
INVITATIONS TO TRANSMIT being sent to IDs
not on the network until the next NETWORK
RECONFIGURATION occurs. When a node is
powered off, the previous node will attempt to
pass it the token by issuing an INVITATION TO
TRANSMIT. Since this node will not respond, the
previous node will timeout and transmit another
INVITATION TO TRANSMIT to an incremented
ID and eventually a response will be received.
SMSC [ SMSC CORPORATION ]
