COM90C66 datasheet(30/76 Pages) SMSC

30 / 76 page

represents the message length. The SID in

Address 0 is used by the receiving node to reply

to the transmitting node. The COM90C66 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 one 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.

packets of these lengths must be sent, the user

must add dummy bytes to the packet (00's) in

order to make the packet fit into a long packet.

Once

the

buffer

written

into,

the

microprocessor 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. Typically, the conclusion

of the transmit command, which is flagged when

TA becomes a logic "1", generates an interrupt.

If the device is configured for the Command

Chaining operation, please see the Command

Chaining section of this document for further

detail on the software.

Once the TA bit becomes a logic "1", the

microprocessor issues the ENABLE TRANSMIT

FROM PAGE nn command, which resets the TA

and TMA bits to logic "0". If the message is not a

BROADCAST,

the

COM90C66

automatically

issues a FREE BUFFER ENQUIRY to the

destination node in order to send the message.

At this point, one of three possibilities may occur:

The first possibility is if a free buffer is available

at the designation note, in which case it responds

with an ACKnowledgement.

At this point, the

COM90C66 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 is 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 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 try to retransmit a FREE BUFFER

ENQUIRY. If a NAK is again received, the token

is again passed onto the next node. A software

timeout on the TA bit is used to prevent this from

happening endlessly. That is, if no software

timeout existed, the transmitting node would

keep trying to issue a Free Buffer Enquiry, even

though it keeps receiving NAKs as a response.

The software timeout on the TA bit is used to tell

the microprocessor to disable the transmitter via

the DISABLE TRANSMITTER command, which

will cause 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".

The third possibility which may occur after a

FREE BUFFER ENQUIRY is issued is if no

response is received from the destination node.

In this case, the TA bit is set to a logic "1" in

preparation for the next time the token is

received, while the TMA bit remains at a logic

"0". The user should determine whether the node

will try to retransmit the next time it receives the

token.

COM90C66 Datasheet(PDF) 30 Page - SMSC Corporation

COM90C66 Datasheet(HTML) 30 Page - SMSC Corporation

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