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COM20022

10Mbps ARCNET controller with 2Kx8 on-board RAM

SMSC -> Microchip 

INVITATION TO TRANSMIT. Since this node

does not respond, the previous node times out

and transmits another INVITATION TO

TRANSMIT to an incremented ID and eventually a

response will be received.

RG62 coax in a baseband system, a one way

cable propagation delay of 7.75 S translates to a

distance of about 1 mile. The flow chart in Figure

1 uses a value of 18.7 S (7.75 + 7.75 + 3.2) to

determine if any node will respond.

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 6 to 15.3 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 8 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 (see Table 6) to a logic "0".

EXTENDED TIMEOUT FUNCTION

There are three timeouts associated with the

COM20022 operation. The values of these

timeouts are controlled by bits 3 and 4 of the

Configuration Register and bit 5 of the Setup 1

Register.

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 20.5 S. This 20.5

S is equal to the Response Time of 18.7 S

plus the time it takes the COM20022 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

1 mile stated earlier. The logic levels on these

bits control the maximum distances over which

the COM20022 can operate by controlling the

three timeout values described above. For proper

network operation, all COM20022's connected to

the same network must have the same Response

Time, Idle Time, and Reconfiguration Time.

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

COM20022 to start sending a message in

response to a received message) which is

approximately 3.2 S. The round trip propagation

delay is a function of the transmission media and

network topology. For a typical system using

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

10 Mbps network, each byte takes exactly 11

clock intervals of 100ns each. As a result, one

byte is transmitted every 1.1 S and the time to

11

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