SN74AHC244: About Level conversion the scea035a.pdf has the following description. What does it mean?

A transceiver can transmit data in both directions (one direction is selected with a DIR pin).

AHC inputs are overvoltage tolerant, AHC outputs are not. For a transceiver like the SN74AHC245, where all I/O pins have both input and output hardware, this means that the I/Os are not overvoltage tolerant.

However, the SN74AHC244 is not a transceiver, it is a buffer/driver with a single direction. Its inputs are overvoltage tolerant, and it can be used to convert from a higher voltage (e.g., 5 V) to its own V_CC (e.g., 3.3 V).

Input 5V PWM and the output end is directly connected to CPU IO.

Why is the output 3.9v?Has any effect on the chip?

Now the input terminal 13 pin  to the ground impedance of about 2K, normal should be several M ohms above.

What might cause this phenomenon？

Can you explain it with the figure below？

That figure shows the ESD protection circuits.

If the input has 2 kΩ, then the device is damaged (probably due to overvoltage, more than 7 V). Replace it.

Normally, the output of the chip is 3.9v PWM, is that correct? What causes this phenomenon? Does it affect the chip?

How do you ensure that the output end is VIH = VCC?

The chip is indeed broken. I want to find out why the chip failed.

The input is 5V PWM of the spin-coded chip output, which is almost impossible to overvoltage.

Normally, the output of the chip is 3.9v PWM, is that correct? What causes this phenomenon? Does it affect the chip?

The SN73AHV244 Datasheet has the following description. Please See NOTES 1.

If the failure is caused by overcurrent, how many ohms of input resistance to ensure the reliable operation of the chip is relatively safe.

Thanks for your help!

The input clamp current works only for negative input voltages. No current is allowed to flow for positive input voltages. The problem is likely to be caused not by overcurrent but by overvoltage.

According to the datasheet, the maximum output voltage is VCC.

Now the chip VCC= 3.3v and the output is 3.8v.

What is the cause of this phenomenon and what is the impact on the chip?

Thank you!

Apparently, the input isolation got damaged, and some of the 5 V input voltages affects V_CC.

The principle block diagram is as follows. In this application, there will be a sequence in the power supply.

I think LVT would be appropriate. Which chip do you recommend? Why is that?

You can use any family with overvoltage-tolerant inputs; both AHC and LVC would work.

A motor can generate huge noise spikes. If it affects the encoder in a way so that the encoder's output signals exceed the absolute maximum ratings of the buffer, then the buffer can get damaged. I suspect this is what happened. Consider adding overvoltage protection.

Power on, The chip in high-impedance state, input pins floating,

Is there any possibility of input pin failure?

The SCBA004.pdf has described below.

Will this cause the input pin fail?

Thanks for your patience!

If there is a longer time after power-on where the '244 inputs are not driven to a valid logic level, you can add weak pull-up or pull-down resistors.

But that problem cannot damage the chips in the way you've shown.

Sorry ， I  didn't describe it very clearly!

Input 5V PWM, output measured 3.8v is peek-to-peak value, actually it is 3.3v PWM. There's no problem with that. 

My question is, will the input pins be damaged when the input pins is floating in the high resistance state (OE=1)?

The OE pin affects only the output. The input pins should never float.

Hi,

Inputs should not be left floating. Yes, floating inputs can cause damage to the device. To better understand what you are trying to describe as the issue, could you please provide a scope shot? I feel like the discussion is jumping around a lot and its hard to focus on what the issue is you are having.