hi,

8051--->Optocouplers--->ULN2803

Which optocouplers are used?

You know, optocouplers have "Swithing Time". Well, many vendors indicate this time short, let say, 1...5us. But it depends on "Load Resistance" very much. Let me show a little example.
Input resistance of ULN2803 is about 5k. With such value, response time of ordinary optocoupler`s is about 100us or even more. In theory it means that such schematic goes totally useless at 10kHz. In real, the limit of input frequence even lower.
How to fix it? Well:
- use fast optocouplers (for example, 6N135, 6N136);
- put power-down resistors between inputs of ULN and ground. It decreases load resistance and so switching time as well.

Regards,
Oleg