COM20020ILJP datasheet(15/72 Pages) MICROCHIP

15 / 72 page

5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM

Datasheet

SMSC COM20020I Rev D

Page 15

Revision 12-05-06

DATASHEET

each.

As a result, one byte is transmitted every 2.2

μS and the time to transmit a message can be

precisely determined. The line idles in a spacing (logic "0") condition. A logic "0" 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:

4.6.1

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

BURST

EOT

DID

4.6.2

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

4.6.3

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: X

16 + X15 + X2 + 1.

ALERT

BURST

SOH

SID

DID

COUNT

data

CRC

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