What is a digital output (DO) good for?

[Camell]

Actually I wanted to use digital outputs to switch a 24VDC circuit (to switch on/off other devices etc.).

But the output current of the digital outputs is so low that I even can't switch an optocoupler (opto isolator).

Therefore I'm wondering what you use the digital outputs for if you can't use them to switch anything?

Of course, I can create a circuit with MOSFET or transistors to switch 24VDC on/off with the 5V TTL signal of the digital output. But I guess most of you don't do that 🙂

And of course I know that I can buy NI relay modules/cards. Actually I have many digital outputs available and don't want to buy new modules/cards.

[JB]

TTL output --> any kind of TTL compatible equipment (integrated circuits...)

We have realized a printed circuit board to interface the TTL DIOs of NI's boards to 24V devices.

By the use of one 74HCT244 and optocouplers you can easily interface 8 TTL DOs.

Anyway, galvanic isolation is indispensable or at least recommended in many cases.

As you probably know, NI also sells some boards with 24V compatible DIOs.

[Camell]

I have to add the information that I use a cDAQ chassis with DIO modules. These modules source only 2mA at 4,4V (100uA at 4,75V).

Common cards like NI 6251 source 24mA. With 24mA you can use common opto couplers. But with <=2mA you can do almost nothing !?!?

[Camell · Accepted solution]

Now I can test and actually I get 10mA @ 5V out of an digital output of NI 9401 (DIO), using the digital output to switch an opto-coupler.

It seems like the information in the data sheet is supposed to mean something else ...