COM20019IP datasheet(12/81 Pages) SMSC

12 / 81 page

DISCONTINUED DATASHEET

Free Buffer Enquiries

A Free Buffer Enquiry is used to ask another node

if it is able to accept a packet of data. It is sent by

the following sequence:

An ALERT BURST

An ENQ (ENQuiry: ASCII code 85H)

Two (repeated) DID (Destination ID) characters

ALERT

BURST

ENQ

DID

Data Packets

A Data Packet consists of the actual data being

sent to another node. It is sent by the following

sequence:

•

An ALERT BURST

•

An SOH (Start Of Header--ASCII code 01H)

•

An SID (Source ID) character

•

Two

(repeated)

DID

(Destination

ID)

characters

•

A single COUNT character which is the 2's

complement of the number of data bytes to

follow if a short packet is sent, or 00H

followed by a COUNT character if a long

packet is sent.

•

N data bytes where COUNT = 256-N (or 512-

N for a long packet)

•

Two

CRC

(Cyclic

Redundancy

Check)

characters.

The CRC polynomial used is:

16 + X15 + X2 + 1.

ALERT

BURST

SOH

SID

DID

COUNT

data

CRC

Acknowledgements

An Acknowledgement is used to acknowledge

reception of a packet or as an affirmative

response to FREE BUFFER ENQUIRIES and is

sent by the following sequence:

•

An ALERT BURST

•

An ACK (ACKnowledgement--ASCII code

86H) character

ALERT BURST

ACK

Negative Acknowledgements

A Negative Acknowledgement is used as a

negative

response

FREE

BUFFER

ENQUIRIES

and

sent

the

following

sequence:

•

An ALERT BURST

•

A NAK (Negative Acknowledgement--ASCII

code 15H) character

ALERT BURST

NAK

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