NI 6501 Digital Input

[fcal]

Hi everyone,

I'm using a NI USB-6501 and I'm trying to understand how to read the inputs.

I've used this card to generate outputs using the example vi Write Dig Chan, it works great.

Now I'm trying to use the Read Dig Chan example vi to read an input voltage. But it seems that by default the card reads 5V (a logical 1) as input on every pins, even though nothing is connected to them. I tried to connect the output to the input, using a relay, to see if it detects a change in the input whether we send some voltage or not but it didn't change anything. It still reads 5V no matter what.

Can anyone help me figure out how to be able to read an input? Has this problem already happened to someone else?

Thanks in advance.

Frederic

[Dennis_Knutson]

Of course it will read a logic 1 with nothing connected.

I have no idea what you are doing with the relay. If you directly connect an output to an input and program the output for a logic 0, you will read a logic 0 at the input.

[crossrulz · Accepted solution]

If you dig through the data sheet, you will see that there is a 4.7kOhm pull up on all of the digital inputs.  So with the inputs floating you will get a high (logic 1).

As Dennis already stated, wire your digital output directly to your digital input.  Then whatever you set the output you will read on the input.  I don't know what you are doing with a relay.

[fcal]

Okay, I will try this without the relay.

Thanks for the insights.