interfacing digital devices like DA's with optocoupler's is sometimes a little bit tricky if you want to realise high data throughput. in order to save current, you normally have to make a compromise between the rise-time (fast with lowest value of pullup resistor, where optocoupler transistor output doesnt saturate) and power dissipation (higher with lower values of pullup resistor). At the time, where you want to latch the information to the DA, all input signal have to have their TTL-logic level (LOW<0.8V , HIGH > 2.5V for example) in order to reach errorfree transmission.
In other words, when you are not using a schmitttrigger stage between the optocoupler output and the logic-input, you have to wait a longer time until the right logic levels are reached and you can't latch as fast, as using the schmitt-triggers.
greetings, tiwag