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COM20019I-DZD Datasheet(PDF) 11 Page - SMSC Corporation |
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COM20019I-DZD Datasheet(HTML) 11 Page - SMSC Corporation |
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11 / 65 page  Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM SMSC COM20019I Page 11 Rev. 03-07-06 DATASHEET Chapter 4 PROTOCOL DESCRIPTION 4.1 NETWORK PROTOCOL Communication on the network is based on a token passing protocol. Establishment of the network configuration and management of the network protocol are handled entirely by the COM20019I's internal microcoded sequencer. A processor or intelligent peripheral transmits data by simply loading a data packet and its destination ID into the COM20019I's internal RAM buffer, and issuing a command to enable the transmitter. When the COM20019I next receives the token, it verifies that the receiving node is ready by first transmitting a FREE BUFFER ENQUIRY message. If the receiving node transmits an ACKnowledge message, the data packet is transmitted followed by a 16-bit CRC. If the receiving node cannot accept the packet (typically its receiver is inhibited), it transmits a Negative AcKnowledge message and the transmitter passes the token. Once it has been established that the receiving node can accept the packet and transmission is complete, the receiving node verifies the packet. If the packet is received successfully, the receiving node transmits an ACKnowledge message (or nothing if it is not received successfully) allowing the transmitter to set the appropriate status bits to indicate successful or unsuccessful delivery of the packet. An interrupt mask permits the COM20019I to generate an interrupt to the processor when selected status bits become true. Figure 3.1 - COM20019I OPERATION is a flow chart illustrating the internal operation of the COM20019I connected to a 20 MHz crystal oscillator. 4.2 DATA RATES The COM20019I is capable of supporting data rates from 156.25 Kbps to 312.5 Kbps. The following protocol description assumes a 312.5 Kbps data rate. For slower data rates, an internal clock divider scales down the clock frequency. Thus all timeout values are scaled as shown in the following table: Example: IDLE LINE Timeout @ 312.5 Kbps = 656 μs. IDLE LINE Timeout for 156.2 Kbps is 656 μs * 2 = 1.3 ms INTERNAL CLOCK FREQUENCY CLOCK PRESCALER DATA RATE TIMEOUT SCALING FACTOR (MULTIPLY BY) 20 MHz Div. by 64 Div. by 128 312.5 Kbps 156.25 Kbps 1 2 4.3 NETWORK RECONFIGURATION A significant advantage of the COM20019I 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 COM20019I is turned on (creating a new active node on the network), or if the COM20019I has not received an INVITATION TO TRANSMIT for 6.72S, or if a software reset occurs, the COM20019I 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 COM20019I senses an idle line for greater than 656 μS, which occurs only when the token Is lost, each COM20019I starts an internal timeout equal to 1.168mS times the quantity 255 minus its own ID. The COM20019I 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 |
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