P82B96: About the waveform during communication

Ryu Yamashita

Part Number: P82B96

Hi team,

During communication, a phenomenon like the attached waveform occurred.
The potential of Sx was floating and something like a pulse waveform was observed in Rx/Tx.
The circuit is as shown on p17 of the datasheet, with Sx and Sy directly connected to the MUC.
The resistor R2 is 2.2kΩ and the capacitor C2 is connected to 47pF.
Can you think of any reason why this would happen?

Sincerely.
Ryu

over 2 years ago

+1 BOBBY over 2 years ago

TI__Mastermind 49370 points

Hi Ryu,

What you're seeing on the Sx/Sy side is the static voltage offset. When a low is received from the Rx/Ry side, the Sx/Sy side will sit at around ~900mV (VoL spec in datasheet).

When you drive the line low on Sx/Sy and release, the Sx/Sy will hang around the VoL level as you can see in the step in the Yellow waveform. This is what we call the static voltage offset which is how the device can tell which side the low is coming from and maintains support of bidirectional signals. (otherwise the signal would just get stuck low since the buffer has both an input connected to it's output when driving low and see's its own Low on the input)

There is an app note that discusses this in the link below:

https://www.ti.com/lit/an/scpa054/scpa054.pdf?ts=1645747679846&ref_url=https%253A%252F%252Fwww.google.com%252F

-Bobby

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P82B96: About the waveform during communication