COM20020D datasheet(10/72 Pages) SMSC

10 / 72 page

INVITATION TO TRANSMIT.

Since this node

does not respond, the previous node times out

and

transmits

another

INVITATION

TRANSMIT to an incremented ID and eventually a

response will be received.

The

NETWORK

RECONFIGURATION

time

depends on the number of nodes in the network,

the propagation delay between nodes, and the

highest ID number on the network, but is typically

within the range of 12 to 30.5 mS.

BROADCAST MESSAGES

Broadcasting gives a particular node the ability to

transmit a data packet to all nodes on the network

simultaneously.

ID zero is reserved for this

feature and no node on the network can be

assigned ID zero. To broadcast a message, the

transmitting node's processor simply loads the

RAM buffer with the data packet and sets the DID

equal to zero. Figure 4 illustrates the position of

each byte in the packet with the DID residing at

address 0X01 or 1 Hex of the current page

selected in the "Enable Transmit from Page fnn"

command. Each individual node has the ability to

ignore broadcast messages by setting the most

significant bit of the "Enable Receive to Page fnn"

command to a logic "0".

EXTENDED TIMEOUT FUNCTION

There are three timeouts associated with the

COM20020D operation.

The values of these

timeouts are controlled by bits 3 and 4 of the

Configuration Register and bit 5 of the Setup 1

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

COM20020D to start sending a message in

response to a received message) which is

approximately 6.4

µS. 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 15.5

µS translates to a

distance of about 2 miles. The flow chart in Figure

1 uses a value of 37.4

µS (15.5 + 15.5 + 6.4) 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 41

µS. This 41 µS is

equal to the Response Time of 37.4

µS plus the

time

takes

the

COM20020D

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

2 miles stated earlier. The logic levels on these

bits control the maximum distances over which the

COM20020D can operate by controlling the three

timeout values described above. For proper

network operation, all COM20020D'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 5

Mbps network, each byte takes exactly 11 clock

intervals of 200ns 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"

COM20020D Scheda tecnica(PDF) 10 Page - SMSC Corporation

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