Hi Rao !
Well some suadvice on interfacing !
Well I am not an expert , but just a
beginner like you.

U have 4 ports on '51 right?, Lets
say u use port 1 and 3 for 4x4 Key
board. and U use port 0 as U’r data
bus to connect to U’r A/D, D/A and
LCD. Use some Logic circuit to
decode the signal i.e. associate
some address to U’r A/D , D/A and
LCD.
That was for an 8 bit A/d or D/A. U
can adjust the Analog Signal for A/D
so as to have the Digital value of
the Peak analog Signal to be with in
8 bit , so that U can omit higher
Bits. Or use some 8 bit A/D and D/A.
If U want to use 12bit and 10 bit
one ..then use port 2 also, for rest
of the bits.
But first as “erik malund” and
“Peter Dannegger” said Do u’r
Groundwork ! and go step by step.
Bye.., Best of Luck !
Vignesh Baliga