COM20020ILJP datasheet(14/72 Pages) MICROCHIP

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

Datasheet

Revision 12-05-06

Page 14

SMSC COM20020I Rev D

DATASHEET

4.4

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 5.3 – High Speed CPU Bus Timing – Intel CPU Mode, pg.

20 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".

4.5

Extended Timeout Function

There are three timeouts associated with the COM20020ID 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.

4.5.1

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 COM20020ID 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 3.1 uses a value of 37.4

μS (15.5 + 15.5 + 6.4) to

determine if any node will respond.

4.5.2

Idle Time

The Idle Time is associated with a NETWORK RECONFIGURATION. Figure 3.1 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 it takes the COM20020ID 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 2 miles stated earlier. The logic levels on these bits control the maximum

distances over which the COM20020ID can operate by controlling the three timeout values described

above. For proper network operation, all COM20020ID'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 5 Mbps network, each byte takes exactly 11 clock intervals of 200ns

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