SN75C189A: VIT+ and VIT-

Zach Bruce

Part Number: SN75C189A

Can you explain the difference between Vit+ (Positive-going input threshold voltage) and Vit- (negative going input threshold voltage)?

For this example, lets say I'm using a positive only voltage on the input (0V to +12V).

Does Vit+ define the tripping point going from 0V to +12V and +12V back to 0V?

Does Vit+ define the tripping point from going from 0V to +12V and Vit- define the tripping point from +12V to 0V?

I have read the other article https://e2e.ti.com/support/interface/f/138/t/615939 which was very useful in understanding how to adjust the tripping point but it wasn't clear on what exactly Vit+ and Vit- are.

Thank you.

over 4 years ago

0 Hao L over 4 years ago

TI__Mastermind 25705 points

Hi,

I think your second comment is how the threshold are defined. "Vit+ define the tripping point from going from 0V to +12V and Vit- define the tripping point from +12V to 0V." Actually since it's a single input, you can use the same input to sweep the voltage up and down.

Regards,

Hao

0 Zach Bruce over 4 years ago in reply to Hao L

Prodigy 80 points

Thank you for your answer. Are you confident in your answer? The reason I was doubting the second answer was because the part is rated to go to negative voltages (-25VDC). I was thinking that Vit- might define the trip point when you have a negative voltage applied to the input.

Thanks,

Zach

0 Hao L over 4 years ago in reply to Zach Bruce

TI__Mastermind 25705 points

Zach,

Yes. RS-232 signaling is from -15V to 15V, while the threshold of RX is -3V to 3V.

www.ti.com/.../slla544.pdf

However IC implementation follows the CMOS logic somehow.

Regards,

Hao

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SN75C189A: VIT+ and VIT-