Is it possible to drive TTL inputs from 3.3v microcontroller
The datasheets should tell you the minimum voltage required to register as a digital high for your receiver, and the minimum voltage guaranteed at the output for a high from your sender.  Just make sure they're within each other's limits.


  A TTL input signal is defined as "low"
  when between 0 V and 0.8 V with
  respect to the ground terminal, and
  "high" when between 2.2 V and 5 V
  (precise logic levels vary slightly
  between sub-types). TTL outputs are
  typically restricted to narrower
  limits of between 0 V and 0.4 V for a
  "low" and between 2.6 V and 5 V for a
  "high", providing 0.4V of noise
  immunity.


http://en.wikipedia.org/wiki/Transistor-transistor_logic#Interfacing_problems