"as i understand, you might thinking that i wanna to connect 555 timer to the emulator board. "

No. You don't understand what I am currently thinking after reading your mail ;-)

If at all you are going to use the 555 as the clock source, its output will just have to be connected to the ADC clock pin. Nowhere else.

"the 8255 in my board is connected to the address bus, i dont think so i,m able to set or reset that P0!! i,m i right. "

Unfortunatley wrong. Why worry about P0 ? The 8255 has multiplied your I/O pin count and added to that you have some pins of P1 and P3 available also. Don't tell me you have committed ALL of them and you dont have some 5 pins for the ADC.

"i think you mean that jumping through the program??? "

No dear. You are referring to code jumping due to a bug. My "jumping" is different - it refers to the error in the ADC output due to a poor DC ground.

"what i,m thinking is that, i would like to use just like this circuit."

As I told, that circuit will help you learn how an ADC works. It will not help you read three channels.

Anyway Enjoy !!

Raghu