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