SMSC COM20020 manual Differential Driver Configuration, Programmable Txen Polarity

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In typical applications, the serial backplane is terminated at both ends and a bias is provided by the external pull-up resistor.

The RXIN signal is directly connected to the cable via an internal Schmitt trigger. A negative pulse on this input indicates a logic "1". Lack of pulse indicates a logic "0". For typical single- ended backplane applications, RXIN is connected to nPULSE1 to make the serial backplane data line. A ground line (from the coax or twisted pair) should run in parallel with the signal. For applications requiring different treatment of the receive signal (like filtering or squelching), nPULSE1 and RXIN remain as

independent pins. External differential drivers/receivers for increased range and common mode noise rejection, for example, would require the signals to be independent of one another. When the device is in Backplane Mode, the clock provided by the nPULSE2 signal may be used for encoding the data into a different encoding scheme or other synchronous operations needed on the serial data stream.

Differential Driver Configuration

The Differential Driver Configuration is a special case of the Backplane Mode. It is a dc coupled configuration recommended for applications like car-area networks or other cost-sensitive applications which do not require direct compatibility with existing ARCNET nodes and do not require isolation.

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The Differential Driver Configuration cannot communicate directly with nodes utilizing the Traditional Hybrid Configuration. Like the Backplane Configuration, the Differential Driver Configuration does not isolate the node from the media.

The Differential Driver interface includes a RS485 Driver/Receiver to transfer the data between the cable and the COM20020. The nPULSE1 signal transmits the data, provided the Transmit Enable signal is active. The nPULSE1 signal issues a 200nS negative pulse to transmit a logic "1". Lack of pulse indicates a logic "0". The RXIN signal receives the data, the transmitter portion of the COM20020 is disabled during reset and the nPULSE1, nPULSE2 and nTXEN pins are inactive.

Programmable TXEN Polarity

To accommodate transceivers with active high ENABLE pins, the COM20020 contains a programmable TXEN output. To program the TXEN pin for an active high pulse, the nPULSE2 pin should be connected to ground. To retain the normal active low polarity, nPULSE2 should be left open. The polarity determination is made at power on reset and is valid only for Backplane Mode operation. The nPULSE2 pin should remain grounded at all times if an active high polarity is desired.

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Contents General Description FeaturesTable of Contents PIN Configuration A1,A2/ALE Description of PIN FunctionsMiscellaneous DIP PIN Plcc PIN Name Symbol DescriptionTransmission Media Interface Instead, it must be connected to XTAL1 with COM20020 Operation Protocol Description Network ReconfigurationNetwork Protocol Data RatesResponse Time Extended Timeout FunctionBroadcast Messages Idle Time Reconfiguration TimeLine Protocol Invitations To TransmitNegative Acknowledgements Data PacketsAcknowledgements System Description MULTIPLEXED, 8051-LIKE BUS Interface with RS-485 Interface Figure C Traditional Hybrid Interface Backplane ConfigurationCOM20020 Network Using RS-485 Differential Transceivers Programmable Txen Polarity Differential Driver ConfigurationInternal Block Diagram Attenuation Functional DescriptionNominal Cable Type Impedance AT 5MHZRead Register Summary Write Interrupt Mask Register IMR Internal RegistersData Register Tentative ID RegisterCommand Register Diagnostic Status RegisterNext ID Register Status RegisterConfiguration Register Setup Register BIT BIT Name Symbol Description Dupid BIT BIT Name Symbol DescriptionData Command Description Address Pointer Low Register Address Pointer High RegisterReset Configuration RegisterCKP3 CKP2 CKP1 Sequential Access Operation Access Speed Sequential Access MemoryInternal RAM Software InterfaceTransmit Sequence Selecting RAM Page SizeRAM Buffer Packet Configuration Page Receive Sequence Transmit Command Chaining Command ChainingReceive Command Chaining Internal Reset Logic Reset DetailsInitialization Sequence Improved DiagnosticsAbnormal Results Normal ResultsOscillator DC Electrical Characteristics Maximum Guaranteed RatingsParameter Symbol MIN TYP MAX Unit Comment Operational DescriptionXTAL1, XTAL2 Input Capacitance Multiplexed BUS, 68XX-LIKE Control Signals Read Cycle Timing DiagramsMultiplexed 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 These signals are to and from the hybrid Normal Mode Transmit or Receive TimingNTXEN NPULSE1 TTL Input Timing on XTAL1 PIN 28-PIN Plcc Package Dimensions 24-PIN DIP Package Dimensions SECTION/FIGURE/ENTRY Correction Revised DateStandard Microsystems Corp
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