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COM20019IP Datasheet(PDF) 10 Page - SMSC Corporation |
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COM20019IP Datasheet(HTML) 10 Page - SMSC Corporation |
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10 / 81 page  10 DISCONTINUED DATASHEET NETWORK RECONFIGURATION A significant advantage of the COM20019 is its ability to adapt to changes on the network. Whenever a new node is activated or deactivated, a NETWORK RECONFIGURATION is performed. When a new COM20019 is turned on (creating a new active node on the network), or if the COM20019 has not received an INVITATION TO TRANSMIT for 6.72S, or if a software reset occurs, the COM20019 causes a NETWORK RECONFIGURATION by sending a RECONFIGURE BURST consisting of eight marks and one space repeated 765 times. The purpose of this burst is to terminate all activity on the network. Since this burst is longer than any other type of transmission, the burst will interfere with the next INVITATION TO TRANSMIT, destroy the token and keep any other node from assuming control of the line. When any COM20019 senses an idle line for greater than 656 S, which occurs only when the token Is lost, each COM20019 starts an internal timeout equal to 1.168mS times the quantity 255 minus its own ID. The COM20019 starts network reconfiguration by sending an invitation to transmit first to itself and then to all other nodes by decrementing the destination Node ID. If the timeout expires with no line activity, the COM20019 starts sending INVITATION TO TRANSMIT with the Destination ID (DID) equal to the currently stored NID. Within a given network, only one COM20019 will timeout (the one with the highest ID number). After sending the INVITATION TO TRANSMIT, the COM20019 waits for activity on the line. If there is no activity for 597.6 S, the COM20019 increments the NID value and transmits another INVITATION TO TRANSMIT using the NID equal to the DID. If activity appears before the 597.6 S timeout expires, the COM20019 releases control of the line. During NETWORK RECONFIGURATION, INVITATIONS TO TRANSMIT are sent to all NIDs (1-255). Each COM20019 on the network will finally have saved a NID value equal to the ID of the COM20019 that it released control to. At this point, control is passed directly from one node to the next with no wasted INVITATIONS TO TRANSMIT being sent to ID's not on the network, until the next NETWORK RECONFIGURATION occurs. When a node is powered off, the previous node attempts to pass the token to it by issuing an 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 192 to 488 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 COM20019 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. |
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