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Output of SN65HVD3082E to square wave

James Johns
Prodigy 10 points

Hello,

  This is my first time posting on these forums, so I hope I'm not asking anything incorrectly.  I need to trigger a camera which accepts a 5 V differential signal, and recommends a SN65HVD3082E to provide it.  The user manual is linked here.   I have a very simple circuit that is taking an input trigger of 20-50 ns at 200KHz, using decadic and binary counters to convert  that to a 5V square wave at a  frequency that the camera can use (2 KHz).  I've verified that the square wave looks fine on a scope. 

I then send that square wave to the driver input pin (pin 4) of a SN65HVD3082E .  I have pin 3 (Driver enabled) held high by the 5V power supply.  Pin 8 (Vcc) is high, and Pin 5 is grounded.  I then attached 2 BNCs to pins 7 & 6 ( A & B) and ran each of those to a scope.  Instead of seeing a 5V square wave on A and a logical inverted wave on B, I see a 0 to 1 V  square wave on B (of the right polarity) and a 4.75 V -  5 V really messy looking wave on A.  I'd be very very grateful if anyone had any thoughts on what might be going wrong.

Thanks so much,
James  )

  • 0
    TI__Mastermind 20820 points
    Hi James,

    Thank you for posting your question to the forum! For your first post you did better than most experienced people by giving me lots of information about your setup, your problem, and what you have tried. Usually I get, "my design doesn't work, your part is broken...."

    How do you have the scope inputs configured? Do you have the bus line, A and B, terminated? If so how do have them terminated?

    I recommend that you place a 60 ohm load between A and B, which is typical for RS-485, and use high impedance probes to look at the signal or set the scope inputs to high impedance from 50 ohms to ground. If the driver is not properly terminated the output will look funny like you described above. Please let me know if this helps and let me know if you have anymore questions.

    You can check the RS-485 design guide for more detailed information about RS-485:
    www.ti.com/.../slla272b.pdf