Generating Step ladder analog output

Hi

i would like to generate a step staircase function that goes from -15 to +15 V using NiDAQ.

But this needs to be triggered externally ie for each trigger pulse I want to generate one voltage -15 and the next pulse generates -14 and so in.

currently I am inputting a vector [-15:15] and defining a digital trigger port. So at a single trigger I am able to generate the full ramp and it somehow ignores the rest of the trigger.

While that is okay I really want to generate a step at every trigger.

How can I do that?

Trigger rate is 1 kHz

I was thinking of using a counter ie each trigger the counter steps up. But how do I go from a digital counter to an analog voltage out?

any help is appreciated .

I am using a USB NiDAQ device like 6621

Thanks