Hi Meilan,
There are a couple more considerations we would need before I would be comfortable recommending a card to you. In response to your requirements:
1. Just about any card will support TTL signals
2. There are several of our Multifunction DAQ cards and DIO cards that will have at least 19 DIO ports - this leads to the question, do you need these to be correlated (hardware timed) or can they be non correlated (software timed)? How fast do you need to acquire?
3. What do you mean by bidirectional? Do you mean 16 lines that can do digital input and output, or analog lines that can do input or output? Or something completly different? Just about all of our DIO boards have lines that you can read and write from, with limitations. We don't have bidirectional analog IO.
4. This will vary from card to card - what dimenstions would be acceptable?
The quickest way to find a card that suits your application would be to go to www.ni.com/contact and select "Call me now" or just call in. This will get you to a Technical Representative that specializes in finding the right product for your application. They are great at making sure you have everything you need to be successful and ensuring that you take everything into account before you purchase. Also, I always end up running through this site - http://ni.com/dataacquisition/ when I'm trying to find the correct card. It allows you to search by constraints and then you can run a few cards by the Technical Representative to see which one would fit best.
Hope this helps- feel free to post back if you have more questions or if I can clarify anything for you.
Cheers,
Andrew S.
National Instruments
