Connecting USB6343 onboard 5v to AI 4 5 6 7 through 4 switches and AI GND results in floating signals and noise

[artaxerxes]

Hey,

I am connecting USB6343's onboard 5v source to AI 4 5 6 7 through 4 switches (and AI GND).  This results in floating signals and sinusoidal noise when i close the switches and not the expected 0v / 5v.

I merely want to report on or off for each switch connected to the AI terminals, what very obvious step am I missing?

[Henrik_Volkers]

A 10k-100k pull down resistor from AIx to AIGND 🙂

Have a look in the manual about voltage measurement and input bias resistors.

And if you look into the manual at page 6-23 you see how to do that with a DI  , here with a pull up resisitor (same value range) and a switch to GND.

[taka87]

I'd like to do the same thing (reading a switch on a USB-6343) and had the same idea as Henrik_Volkers, but am confused, as the manual also says (page 3-4):

Caution Never connect the +5 V power terminals to analog or digital ground or to
any other voltage source on the X Series device or any other device. Doing so can
damage the device and the computer. NI is not liable for damage resulting from such
a connection.

In my low understanding of electronics this is a contradiction to page 6-23, where it shows that a +5V power terminals is connected to DGND, as soon as the switch closes. Would somebody be so kind to explain what I'm getting wrong?

[crossrulz]

Look at the diagram a little closer on 6-23.  There is a resistor in series with the +5V before the switch.  This resistor limits the current going into the digital input, preventing any damage.

[taka87]

Thank you crossrulz! I didn't remember that's the sign for resistor (sorry, I'm a real beginner with all this).

I hope it's okay to add another more or less stupid question? Is my assumption correct, that I need a resistor with min.20 kΩ, as on page 8 of the specifications of the USB-6343 it says:

Digital I/O Characteristics
I_IH input hight current (V_IN=5V).....................250µA

?

[crossrulz]

That 20k is the minimum built-in pull down resistor in the daq board.  You need to make sure there are at least 2.2V going into the pin of the daq board to ensure it can read a high level.  So I would use something more like a 10k resistor to act as the 5V series resistor (about 3.3V will be going into the DAQ based on 5V, 10k series, and 20k pull down).

[taka87]

I'd like to ask further questions, but being new to the forum, I'm not fully familiar with the customs, so please tell me if I should open a new topic or if it's okay, to bundle questions (if related).

The question are: how to extend the described wiring to multiple switches (I need 10)? I attached my assumption as "Wiring 1.png".

And I guess "Wiring 2" wouldn't work as it'd be how the topic-opener artaxerxes did it?