COM20019IP datasheet(11/81 Pages) SMSC

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Response Time

The Response Time determines the maximum

propagation delay allowed between any two

nodes, and should be chosen to be larger than

the round trip propagation delay between the two

furthest nodes on the network plus the maximum

turn around time (the time it takes a particular

COM20019 to start sending a message in

response to a received message) which is

approximately

101.6

The

round

trip

propagation delay is a function of the transmission

media and network topology. For a typical system

using RG62 coax in a baseband system, a one

way cable propagation delay of 248

S translates

to a distance of about 32 miles. The flow chart in

Figure 1 uses a value of 597.6

S (248 + 248 +

101.6) to determine if any node will respond.

Idle Time

The Idle Time is associated with a NETWORK

RECONFIGURATION.

Figure 1 illustrates that

during a NETWORK RECONFIGURATION one

node will continually transmit INVITATIONS TO

TRANSMIT until it encounters an active node. All

other nodes on the network must distinguish

between this operation and an entirely idle line.

During NETWORK RECONFIGURATION, activity

will appear on the line every 656

S. This 656

S is equal to the Response Time of 597.6

plus the time it takes the COM20019 to start

retransmitting another message (usually another

INVITATION TO TRANSMIT).

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

miles

stated

earlier.

The

logic levels on these bits control the maximum

distances over which the COM20019 can operate

by controlling the three timeout values described

above.

For

proper

network

operation,

all

COM20019's connected to the same network

must have the same Response Time, Idle Time,

and Reconfiguration Time.

LINE PROTOCOL

The

ARCNET

line

protocol

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

S each. As a result, one

byte is transmitted every 35.2

S 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

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

BURST

EOT

DID

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