My problem is not isolation or noise or communication prtocol.
usually you will discover it is noise problem after you finish your design and it sometimes works and some times doesn't. Don;t forget a 20 meter cable is a good antenna for noise.
Rather it is: How should I construct the keypad with discret logic elements like 74xx? And this solution is reliable and logical?
You have 2 senarios :
1. connect the keypad as 4x3 matrix
2. use each switch independantly.
both methods above you need to de-bounce the switches by either using switch debounce ic(s) like MC14490 etc ( search for debounce on the forum) or by building your debounce circuit using flip flop and R and C and few gates ( I would recommend this for educational purposes only).
For the matrix solution you need to make a counter and decoder and keep rotating it on each row and read the column(very missy) god save the microcontrollers.
search google for exact circuit.
mahmood