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THVD1426: Output drive question

Whitney Ballard
Prodigy 90 points
Part Number: THVD1426


Tool/software:

This part is a  half duplex RS485  transceiver.   When the Driver Data input ( D pin)  is low,  output A  is driven low and output B is driven high.   

When the D input goes high,  that data sheet says that output A will be driven high and B will be driven low BUT only for a certain period of time specified by  tdevice-auto-dir  (.4 us  to 1.45 us for the THVD1426 part)  after which the A and B outputs go high impedance. 

My question - after the time period of  tdevice-auto-dir  expires,   how do the   A and B outputs maintain their voltage levels?    Suppose we wish to be driving A high and B low for longer than 1.45 us ?   After 1.45us goes by,  the outputs may float to the wrong value.   

Thank you 

Whitney Ballard

Whitney.ballard@collins.com 

  • 0
    TI__Guru* 77381 points
    Whitney Ballard said:
    how do the   A and B outputs maintain their voltage levels?

    So there are two ways you can do this. One is to do nothing but make sure the devices on the bus have an internal fail safe where the Vth+ is negative (below GND) since an undriven bus should ideally float to the same voltage and Va-b becomes 0.

    The second is to place external fail safe resistors on the bus to bias the bus when there is no driver active. I would recommend this approach and would try to target around 200mV of idle voltage (RS485 standard for max Vth+).

    EDIT: if you have both internal fail safe receivers AND use external fail safe biasing resistors, then you will get additional noise margin which is the best approach.

    -Bobby