Dear Duncan.
I solved my problem previously reported. 
>Please correct me if you are wrong.
>The C11 uses to generate the signals DIR the CI LTV847, Higth Density
>Photocoupler and to generate the signals STEP the MT25310. 
>The LTV is much slower, your time varies between 18 us, and you used one
>10Kohm RL, so it was slower still. 
>See Figure 10 ( Response Time vs. Load Resistance) Data Sheet. 
>The MT25310 with the same RL is 0.3 - 1 us. 
>The ideal would be for my system that all output were quick with 25310. 
>But could solve my problem replacing all the RL to 1.2 Kohm, significantly
>improving the response and thereby obtaining the desired result. 
>I did tests and the system worked perfect. 
>Please in the next C11 that I will buy, make this exchange. 
>Output D / A, only worked properly when put in the time of STEP 5us, but
>when I changed the RL functioned normally, without the need to change the
>time of STEP. 
>Thank you.
Prendice.