NI-6351 card - How create trigger on negative voltage [ 0; -5V] ?

[did.rider38]

Hi everybody

I've just created a digital output trigger from an external source with a positive value [0;5V]. It works very well (see picture, in yellow the external signal and in blue the triggered signal). Now I'd like to get the same answer, but with a negative external signal [0; -5V]. Do you have any ideas? Because I'm trying to integrate the "Start Analog Edge" function instead of "Start Digital Edge", but it's not compatible with the rest of the program chain ....

Best regards

[ZYOng]

The digital TTL is always 0 to 5V. From NI 6351 Device Specifications,

Either you use the analog edge trigger, or you use an external circuit to invert the voltage.

[Bob_Schor]

Think of an analog signal as a "real" (or "float") quantity, and a digital signal as a Boolean (on = 5V, off = 0V).

To trigger on a negative (0V, -5V) signal, do you want to trigger on the rising edge (from -5V to 0) or the falling edge (from 0 to -5V)?  For the former, add 5V to you trigger line (to convert it to a TTL signal, triggering on the rising edge), and for the latter, simply invert the signal.  A simple op-amp circuit should handle either of these cases.

Bob Schor

[did.rider38]

thanks for your help

I see I can do

It's for un falling edge from 0 to -5V

[did.rider38]

Thanks ZYOng

I think this idea is good .... for the analog trigger, are you talking about APFI 0?
I tried but something doesn't work .... Did you see vi with my message? Can you help me modify it?

[did.rider38 · Accepted solution]

Hi,

I've found the solution!

I use a digital trigger to process an analog signal ....
I use AFFI0 and "start digital edge" function