It's becoming quite frequent to me having a new digital subsystem which I want to test in isolation before trying to make a prototype circuit. I've been using the digital analyzer by Saleae for inputs, which is quite handy, but I'm wondering if I could find or build its counterpart. Ideally it would be a device to plug in an USB port, would have many digital outputs (let's say 20 or more) and would come with software that would enable me to do a playback from a prerecorded file, or generate the output with a program or script.

Does anyone know about such a product, or have guidelines to build on my own?