Example: "Digital - Conitinuous Output" How does it work?

[Hannes_kcd]

Hello ThiCop,

- My device passed the self-test (whether there was anything connected to my device or not) in MAX.

- When a DO is low and then I connect a resistor between the DO COM and the used DO, the DO falls down from 8.xx V to 0 V. When I disconnect the resistor, the DO becomes 8.xx V again.

Maybe I should place pull-down resistors? Although I didn't read this in the manual and I don't know this is a long-term solution...

I use the spring terminal version.

I'm pretty sure I've measured between the DO and the DO COM.

Tomorrow I'll do a new test to check everything, I don't have the device right now.

Thanks for your concern. 🙂

[ThiCop]

Hello Hannes,

Ok, that's already positive news and partially what I expected.

If you don't plan on having any input impedances from other devices, then a pull-down resistor might be a good idea.
Please also check that your resistor is "big enough" so that you don't try to draw more current than the module can provide.

Well, it is actually mentioned somewhere in the manual, but it could be that the term didn't ring a bell.. 🙂

Your digital outputs are sourcing so you do need some load to let your voltage go low.

Otherwise you'll be "using" the input impedance of your multimeter as load for your sourcing outputs.

This is some thing to be careful with.

Just for completeness I add a small document that explains the differences between sourcing and sinking:

http://digital.ni.com/public.nsf/allkb/A10122C63A7F5CFE86256B4C007491DD?OpenDocument

[Hannes_kcd]

Thanks for the document and the information!

So with the relays, it's not necessary to place pull-down resistors.

For my 3 other DO's which generate the sequence (directly to another circuit), this might be a problem. So I'll have to check the electrical scheme's of the installation 'underneath' (where the sequence goes to), looking for any pull-down resistors or impedances connected to COM?

[ThiCop · Accepted solution]

Hello Hannes,

I you have the relays, then you of course should test and measure it.

Relays (depending on which type you use) can sometimes be a bit "tricky" with concerning impedances etc..

It's always important to know the input impedance if you're connecting something to it.

This is not only with digital signals. For analog signals impedances are also very (if not more) important.

Please also note that the input impedance of a turned on device can be different from when it's turned off.

So keep this in mind when you're measuring impedances and converting them to expected voltage levels.

And of course the other side (output impedances) are also important to know for similar reasons.