COM20051 View Datasheet(PDF) - SMSC -> Microchip

Part Name

Description

Manufacturer

COM20051

Integrated Microcontroller and Network Interface with Watchdog Timer and A/D Converter Interface

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 PAC (Data Packet--ASCII code 01H)

• An SID (Source ID) character

• Two (repeated) DID (Destination ID)

characters

• A single COUNT character which is the

2s complement of the number of data

bytes to follow if a short packet is sent,

or

00Hex 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

PAC SID DID DID COUNT data

BURST

data CRC 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 (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

18

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