COM20020-5P Ver la hoja de datos (PDF) - SMSC -> Microchip

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COM20020-5P

ULANC Universal Local Area Network Controller with 2K x 8 On-Board RAM

SMSC -> Microchip 

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:

X16 + X15 + X2 + 1.

ALERT

SOH SID DID DID COUNT

data

BURST

data CRC CRC

Acknowledgements (ACK)

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 (NAK)

A Negative Acknowledgement is used as a

negative response to FREE BUFFER

ENQUIRIES and is sent by the following

sequence:

• An ALERT BURST

• A NAK (Negative Acknowledgement--ASCII

code 15H) character

ALERT

BURST

NAK

11

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