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COM20020ILJP3V Datasheet(PDF) 10 Page - SMSC Corporation |
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COM20020ILJP3V Datasheet(HTML) 10 Page - SMSC Corporation |
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10 / 72 page  10 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. 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 COM20020 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. 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 COM20020 to start sending a message in response to a received message) which is approximately 6.4 "S. 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 "S translates to a distance of about 2 miles. The flow chart in Figure 1 uses a value of 37.4 "S (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 "S. This 41 "S is equal to the Response Time of 37.4 "S plus the time it takes the COM20020 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 2 miles stated earlier. The logic levels on these bits control the maximum distances over which the COM20020 can operate by controlling the three timeout values described above. For proper network operation, all COM20020's connected to the same network must have the same Response Time, Idle Time, and Reconfiguration Time. 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 each. As a result, one byte is transmitted every 2.2 "S and the time to transmit a message can be precisely determined. The line idles in a spacing (logic "0") condition. A logic "0" |
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