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SN65ALS176: is it necessary to connect all the pins to test the function

Brian Li
Genius 13850 points
Part Number: SN65ALS176

Hi team,

My customer is testing our 485 receiver function.

They only connect the VCC to 5V and the GND to ground, other pins all floating as below:

And they find when they test the A and B, they find A - B=3.8V.

Could you please kindly help is it correct to have this 3.8V with only the VCC and GND connected ?

And is it necessary to connect all the pins to test the function?

Thanks,

Brian

  • 0
    TI__Guru 52462 points

    Hi Brian,

    If they want to test receiver they cannot leave everything floating except VCC and GND. Their results are meaningless from their first test and should be disregarded. 

    /RE must be driven low (to enable the receiver)

    DE must be driven low (to disable driver - you won't get correct readings for receiver tests if DE is high)

    The "D" pin input can be left floating if DE is low - if DE is high this pin cannot be left floating. 

    R can be left floating - but if you are testing differential receiver - the data output will be on "R" so you may be probing this pin to measure. 

    You can input the signal on A/B. 

    As a general rule logic inputs on RS-485 transceiver can never be left floating unless the datasheet explicitly states they have default levels (high or low) integrated into the device (usually through an integrated bias resistor (either pull-up or pull-down depending on device and pin)). 

    Please let me know if you have any other questions!


    Best,

    Parker Dodson