Gerd,
i am not aware that TTL specifies detailed timing information. The main reason: TTL was introduced as "standard" in 1961. In the mean time, all our semicondictors are way faster than back in that time. So if TTL would specify timing in detail, there would be the requirement for different versions of TTL (just like USB 2.0 vs. USB 3.0).
I don't know about TTL 2.0 (or higher), so i doubt timing specs to include hard numbers.
NI specified TTL signals for older controller chips (compare here) but i am not aware of any updated document for STC3 chips which are used for X-series devices.
Norbert