THVD1400: Output High and Low

lu tan

Part Number: THVD1400

Tool/software:

What is the VOH and VOL range for the THVD1400DR on the RS422 side? I know it is just RS422 standard, but I am having hard time finding the voltage logic range. I hope this is a simple question.

11 months ago

0 Clemens Ladisch 11 months ago

Guru 319150 points

It is assumed that the outputs are properly terminated, resulting in a load of about 60 Ω or 100 Ω. The datasheet specifies the output voltages with |V_OD| and V_OC(SS).

0 Ethan Sempsrott 11 months ago

TI__Genius 17152 points

Hi Lu,

The driver outputs according to the VOD spec which I have highlighted below. RL=100 is for the RS-422 spec and RL=60 (or 54) is the RS-485 spec. The receiver V_IT is the voltage threshold for a valid signal (also highlighted), where in between V_IT+ and V_IT- is undefined. The receiver spec is the same for either RS-485 or RS-422.

-Ethan

0 lu tan 11 months ago in reply to Ethan Sempsrott

Prodigy 160 points

Hi Ethan,

The info you provided may be the answer I seek. I would like to provide a bit more info. From 7.4 section of datasheet:

1. When D is high, the differential output voltage defined as VOD = VA – VB is positive.

2. When D is low, the output states reverse, B turns high, A becomes low, and VOD is negative.

I am looking for the specification of "positive" and "negative" in the above statements.

+1 Clemens Ladisch 11 months ago in reply to lu tan

Guru 319150 points

Assuming typical values with a 5 V supply, when D is high, then A = 3.75 V, B = 1.25 V; when D is low, then A = 1.25 V, B = 3.75 V.

0 Ethan Sempsrott 11 months ago in reply to lu tan

TI__Genius 17152 points

Hi Lu,

The values Clemens provides will be generally accurate in normal scenarios. What matters most is the voltage differential. In the case of a ground shift, the voltages might be A=7V and B=5.5V, but the receiver output would still be VOD=VA-VB which is a logic high signal.

Best,

Ethan

0 lu tan 11 months ago in reply to Clemens Ladisch

Prodigy 160 points

Hi Clemens,

Can you point me to where these values are shown in the datasheet?

0 Ethan Sempsrott 11 months ago in reply to lu tan

TI__Genius 17152 points

Lu,

Those values are not in the datasheet, and most RS-485 datasheets do not explicitly list those values.

Please reference our RS-485 guide, particularly Figure 2-3: https://www.ti.com/lit/wp/slla545/slla545.pdf

From this, VA and VB can be estimated with VOS and VD (also labeled VOD in the datasheet).

Best,

Ethan

0 Clemens Ladisch 11 months ago in reply to lu tan

Guru 319150 points

These values are computed as V_OC(SS) ± ½|V_OD|.

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THVD1400: Output High and Low