COM20019IP datasheet(42/81 Pages) SMSC

42 / 81 page

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the message. The SID in Address 0 is used by

the receiving node to reply to the transmitting

node.

The COM20019 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

not

BROADCAST,

the

COM20019

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

COM20019 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

network

reconfiguration.

COM20019IP датащи(PDF) 42 Page - SMSC Corporation

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