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SN75LBC180: Oscillation waveform for connect with SN751178

Guru 19555 points
Part Number: SN75LBC180
Other Parts Discussed in Thread: SN751178

When SN75LBC180 connect with SN751178 for below circuit, waveform was oscillation, please let me know about three points below;

①When SN751178 Tx set disable, waveform was occur large oscillation,

 What is the reason? (For example; Impedance matching, SN751178 spec, etc)

②(I think that receiver difference level is enough)

 Is oscillation point possible to occur redundant data?

③Is there any idea for reduce oscillation, please let me know.

Best regards,

Satoshi

  • Satoshi,

    Thanks for reaching out to us. Can you let me know when SN751178 is disabled (I assume it's the DE pin pulled low), what's the state of SN75LBC180? Before SN751178 transmits data and after SN75LBC180 transmits data, the bus voltage are all pulled to the middle (3V?). I think we might need to figure out first why the bus voltage drops when SN751178 is disabled. Is it possible that you give more time before turning SN75LBC180 on? I would like to see where the bus voltage would settle.

    Regards,
    Hao
  • Hao-san

    Thank you for reply,

    I Updated your question, please see below;

    ①When SN751178 is disabled (pulled low), SN75LBC180 state is DE: Low and RE: Low.

     ※RE is always Low (Pull down)

    ②Customer can't set more time before turning SN75LBC180 on.

     Because RS485 communication is controlled by special driver IC for Arcnet, this IC can't change turning time. 

    ③Latest waveform is attached below.

    ・SN751178 waveform

    ・SN75LBC180 waveform

    If there need additional information, please let me know.

    Best regards,

    Satoshi

  • Satoshi-san,

    Thanks for your information. The behavior should be related to the shutdown mechanism of the devices. I'll order some samples and do some measurement to confirm the speculation.

    Regards,

    Hao

  • Hao-san

    Thank you for reply,
    I understood.
    I looking forward your any update.

    Best regards,
    Satoshi
  • Satoshi-san,

    I did some investigation in the lab. Below are the waveforms I captured, which basically duplicate your results. 

    The reason of the voltage shooting up then going down is due to the characteristic of the driver. When the device is disabled, usually the P side and N side of the driver have some timing mismatch. If the N side is turned off first, the P side will pull the voltage up. After both sides are turned off, the output pins are in high impedance mode. Where the signal settles depends on the load of the bus. But somehow SN751178 still discharges <100uA leakage to ground after it is disabled for about 2us. This small leakage is negligible if the bias is strong and it's common mode. Therefore this current should not impact the communication. Please let me know if you have any concerns about this disable timing behavior.

    Regards,

    Hao