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SN75976A: Some questions about SN75976A testing...

Peter Tseng
Genius 5930 points
Part Number: SN75976A

Dear Sirs,

My customer wants to design control board with SN75976A to replace existing end customer control board.

The configuration shows as below.

1. Any external pull high/down necessary for input and output channel?

2.  customer found the input "1A" cannot back to logic low level and will keep 1~2V DC level. Once add pull down resistor on 1A, then will back to low level. Is there any modification for such issue?

3. compared with end customer existing control board and SN75976A board, the output signal DC level of SN75976A of 1B+ and 1B- will lower than around 1~1.3V. Is it normal or not?

4. testing with above configuration, 

a. signal DC level from TP2 is 0 +/- 2.5V

b. replace existing Rx board with SN75976A board, the signal DC level of TP2 is 1~5V

c. if replace existing control board with SN75976A board, the signal DC level of TP2 is 1~3V

what is the reason to have the DC level on the diff. signal?

Please help to provide your comments.

Thanks.

  • 0
    TI__Intellectual 2885 points

    Hi Peter,

    1. The device does not require external pull up/down resistors, they are already inside the device. If you are using an open-drain output you will need 620 Ω resistor for pull ups to drive high logic levels.

    2. I'm not sure what you are asking. It looks as if the pull down resistor resolved the issue.

    3. Are you measuring the voltage differential of 1B? What are you expecting to see when you got the 1~1.3 V? Were you measuring a high, low, or default state.

    4. Are you placing the SN75976A on both ends? What exactly are you seeing at has those DC signal levels.

  • 0 in reply to Matt Mendez
    Genius 5930 points

    Dear Matt,

    Please refer to attached waveform.

    waveform1: get from TP2 with connection as above, the signal level is 0 +/- 2.5V. connect SN75976A instead existing Rx board, the signal level at TP2 will become 2.5 +/- 2.5V.

    waveform2: get from TP1 and use SN75976A as control board, the signal level is +2V ~ -1V. connect with original control board, the TP1 signal level will be 0 +/- 1.5V. 

    How to get same signal level of TP1 and TP2 with SN75976A?

    Thanks.

  • 0 in reply to Peter Tseng
    TI__Intellectual 2885 points
    Hello Peter,

    Thank you for the waveform. I have a few questions. Were these taken from the SN75976A receiving or transmitting?

    One possible explanation for the variance is any termination or pull up/down resistors on the bus. A simple test you can try is to measure the outputs directly (not the differential). This can tell us if the irregularities are a result of a single line.

    Also, what part are you using for your existing control/RX board? I'd like to look at them see if I can find any information.

    From each test, you used the SN75976A part with your existing device. Have you tested what happens when you replace both with the new part?