Dear Mahmood;
I think you mis understood. The problem is the keypad will be an input and output device. The data line between the mcu and keypad must be bi-directional, and the clock line must be controlled by the host mcu. The keypad circuitary that is built with discrete logic must understand weather the data line is an output or input for itself at the time. The problem can be summirized as: How can I use the data line bi-directional? Using a cpld will increase the cost and also programming cost/effort. And again my problem is not isolation or noise or communication prtocol. I know how to deal with these problems.
Thanks.
Ertan