COM20020 データシートの表示（PDF） - SMSC -> Microchip

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COM20020

COM20020 3.3V ULANC Universal Local Area Network Controller with 2K x 8 On-Board RAM

SMSC -> Microchip 

is defined as no line activity and a logic "1" is

defined as a negative pulse of 100nS duration. A

transmission starts with an ALERT BURST

consisting of 6 unit intervals of mark (logic "1").

Eight bit data characters are then sent, with each

character preceded by 2 unit intervals of mark and

one unit interval of space. Five types of

transmission can be performed as described

below:

Invitations To Transmit

An Invitation To Transmit is used to pass the

token from one node to another and is sent by the

following sequence:

# An ALERT BURST

# An EOT (End Of Transmission: ASCII code

04H)

# Two (repeated) DID (Destination ID)

characters

ALERT

EOT

DID

DID

BURST

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

ENQ

DID

DID

BURST

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

11

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