COM20019I-HT datasheet(13/65 Pages) SMSC

13 / 65 page

Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM

SMSC COM20019I

Page 13

Rev. 03-07-06

DATASHEET

This 656

μS is equal to the Response Time of 597.6 μS plus the time it takes the COM20019I to start

retransmitting another message (usually another INVITATION TO TRANSMIT).

4.5.3

Reconfiguration Time

If any node does not receive the token within the Reconfiguration Time, the node will initiate a NETWORK

RECONFIGURATION. The ET2 and ET1 bits of the Configuration Register allow the network to operate

over longer distances than the 32 miles stated earlier. Thelogic levels on these bits control the maximum

distances over which the COM20019I can operate by controlling the three timeout values described above.

For proper network operation, all COM20019I's connected to the same network must have the same

Response Time, Idle Time, and Reconfiguration Time.

4.6

LINE PROTOCOL

The ARCNET line protocol is considered isochronous because each byte is preceded by a start interval

and ended with a stop interval. Unlike asynchronous protocols, there is a constant amount of time

separating each data byte. On a 312.5 Kbps network, each byte takes exactly 11 clock intervals of 3.2

μS

each. As a result, one byte is transmitted every 35.2

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 1.6

S 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:

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