SMSC COM20020 manual Idle Time, Reconfiguration Time, Line Protocol, Invitations To Transmit

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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 82μS. This 82μS is equal to the Response Time of 74.7μS 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 4 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 2.5 Mbps network, each byte takes exactly 11 clock intervals of 400ns each. As a result, one byte is transmitted every 4.4μS and the time to transmit a message can be precisely determined. The line idles in a spacing (logic "0") condition. A logic "0" is defined as no line activity and a logic "1" is defined as a negative pulse of 200nS duration. A transmission starts with an ALERT BURST consisting of 6 unit intervals of mark (logic "1"). Eight bit data characters are then sent, with each character preceded by 2 unit intervals of mark and one

unit interval of space. Five types of transmission can be performed as described below:

Invitations To Transmit

An Invitation To Transmit is used to pass the token from one node to another and is sent by the following sequence:

∙An ALERT BURST

∙An EOT (End Of Transmission: ASCII code 04H)

∙Two (repeated) DID (Destination

ID) characters

ALERT

EOT

DID

DID

BURST

 

 

 

 

 

 

 

Free Buffer Enquiries

A Free Buffer Enquiry is used to ask another node if it is able to accept a packet of data. It is sent by the following sequence:

∙An ALERT BURST

∙An ENQ (ENQuiry: ASCII code 85H)

∙Two (repeated) DID (Destination ID) characters

ALERT BURST

ENQ

DID

DID

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Contents Features General DescriptionTable of Contents PIN Configuration Description of PIN Functions A1,A2/ALETransmission Media Interface DIP PIN Plcc PIN Name Symbol DescriptionMiscellaneous Instead, it must be connected to XTAL1 with COM20020 Operation Network Protocol Network ReconfigurationProtocol Description Data RatesBroadcast Messages Extended Timeout FunctionResponse Time Line Protocol Reconfiguration TimeIdle Time Invitations To TransmitAcknowledgements Data PacketsNegative Acknowledgements System Description MULTIPLEXED, 8051-LIKE BUS Interface with RS-485 Interface Figure C Backplane Configuration Traditional Hybrid InterfaceCOM20020 Network Using RS-485 Differential Transceivers Differential Driver Configuration Programmable Txen PolarityInternal Block Diagram Nominal Functional DescriptionAttenuation Cable Type Impedance AT 5MHZRead Register Summary Write Data Register Internal RegistersInterrupt Mask Register IMR Tentative ID RegisterNext ID Register Diagnostic Status RegisterCommand Register Status RegisterConfiguration Register Setup Register BIT BIT Name Symbol Description BIT BIT Name Symbol Description DupidData Command Description Address Pointer High Register Address Pointer Low RegisterConfiguration Register ResetCKP3 CKP2 CKP1 Sequential Access Operation Internal RAM Sequential Access MemoryAccess Speed Software InterfaceSelecting RAM Page Size Transmit SequenceRAM Buffer Packet Configuration Page Receive Sequence Command Chaining Transmit Command ChainingReceive Command Chaining Reset Details Internal Reset LogicImproved Diagnostics Initialization SequenceNormal Results Abnormal ResultsOscillator Parameter Symbol MIN TYP MAX Unit Comment Maximum Guaranteed RatingsDC Electrical Characteristics Operational DescriptionXTAL1, XTAL2 Input Capacitance Timing Diagrams Multiplexed BUS, 68XX-LIKE Control Signals Read CycleMultiplexed BUS, 80XX-LIKE Control Signals Read Cycle Multiplexed BUS, 68XX-LIKE Control Signals Write Cycle Multiplexed BUS, 80XX-LIKE Control Signals Write Cycle NON-MULTIPLEXED BUS, 80XX-LIKE Control Signals Read Cycle NON-MULTIPLEXED BUS, 68XX-LIKE Control Signals Read Cycle NON-MULTIPLEXED BUS, 80XX-LIKE Control Signals Write Cycle NON-MULTIPLEXED BUS, 68XX-LIKE Control Signals Write Cycle Normal Mode Transmit or Receive Timing These signals are to and from the hybridNTXEN NPULSE1 TTL Input Timing on XTAL1 PIN 28-PIN Plcc Package Dimensions 24-PIN DIP Package Dimensions Date SECTION/FIGURE/ENTRY Correction RevisedStandard Microsystems Corp
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