SMSC COM20020 manual Data Packets, Negative Acknowledgements

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 BURST

SOH

SID

DID

DID

COUNT

data 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	ACK
BURST

Negative Acknowledgements

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	NAK
BURST

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