I'm busy working on a project that uses TTL logic As I recall from a discussion elsewhere, the Pi (all of them) runs on 3.3 Volt logic and as such the GPIO connections require specialty methods to translate from one to the other.
(Author Don Lancaster describes a method in one of his books on how to interface 5 Volt logic, that is in his discussion, both CMOS at 5 Volts, and TTL (which always runs at 5 Volts) to CMOS logic at 3 Volts. Oddly enough the parts to do it were then originally made by RCA, they are now available from TI, and the part number is CD4041UB.)
Would such a method do that for the PI? In a nod to the announcement of 10/07/2015 regarding the new Scratch programming language I've decided that instead of using one of the other platforms here, it be running on the Pi and using (new) Scratch for the language.
Incidentally my first hardware project involved Mono and a library written especially for the job.