Control different DA output value!

I've two question about using Turbo C++ programming to control a hardware which is a 12-bit ADDA (analog-to-digital) card
1. I use a function "outport()" to output a byte to the DA output port like this:
outport(0x27F, 0x01);
outport(0x27E, 0xff);

where 0x27F and 0x27E is the I/O port address for output DA high byte and low byte data respectively.

I cannot adjust the output ampitude( voltage ). I have tried to put different value e.g 0x01, 0x0f, 0xff, ... to the output port, but the output ampitude is almost the same. How can I do to adjust the output value? How can I keep the output steady, like a DC output? Any code can be provided to explain?
For more information about the hardware, see attachement(128.zip) P. 11 -2 Digital to analog (D/A) procedure.

Thanks and regards

Can you show us the function you use to send out the signal for DA conversion? Have you selected to channel to be used? Are you sure you need to write 0xFF to 0x27E?

Perhaps you can ask the manufacturer for a demo C program.
http://www.decision.com.tw/

Another link with more info:
http://www.pciadda.com/