Digital Input Current Limits

[sevincom]

Hello,

After reading all that manuals and specifications, I decided to ask a general question.

In the datasheets, user guides that I read, generally there are two warnings for DIO:

-Do not connect digital outputs to circuits which drives current above limitations,

-Do not drive the line with voltages outside its operating range.

Talking in general, these tell me that I need to pay attention to current when dealing with outputs and to voltage when dealing with inputs. So I have that question, can I wire a power supply to digital inputs directly without exceeding its' normal operating range and without any protecting circuit? Actually in my feelings, this is not possible. But why some product documents clearly mention the internal impedance of inputs while others does not clearly indicate those? How can I decide if I can connect a signal directly to an input (for example USB-6525 indicates a current limiting circuit, but I can not see a clear explanation in datasheet of USB-6251)? 

[johnsold · Accepted solution]

As long as the input voltages are within the specified limits, no damage will be done to the DAQ device. Logic devices often have two non-overlapping input ranges, one for Input Low and one for Input High. If the input voltage is between those ranges the performance of the device may be unpredictable. Also, check your power supply to make sure that it does not overshoot when turned on or off as that could exceed the DAQ limits.

Lynn

[crossrulz]

When looking at Digital Inputs, you only care about the impedance and current limit because the source of that voltage and current needs to be able to handle it.  This should not be an issue in 99% of the cases I have seen.  As Lynn said, just make sure your power supply is outputting a voltage that is in the range the Digital Input likes and the current limit is a least a couple of mA and you should be just fine.

[sevincom]

Is this fine because all DIO devices have designed with impedances which do not let a big current flow when it is in the voltage range? Since I can not see this kind of information on datasheets, I want to be sure about that, even with low-cost devices like 6501, 6009 etc.

Thank you both.

[johnsold]

Yes.

Lynn