i make a program in z80 PIO using conditional , but the result is different

Question

Program to make output port B = FFH, if and only if input port A = 01H (another input then output is 00)

Here my code:

       LD     A, 4FH
       OUT    (82H), A
       LD     A, 0FH
       OUT    (83H), A
LOOP:   IN     A,(80H)
        CP     01H
        JR     NZ,S1
        LD     A,00H
S1:     LD     A,FFH
        OUT    (81H),A
        JP     LOOP

the problem is when i give input another than 01, the output still FF

Answer 1

I'm a bit rusty since I haven't used Z80 assembler in 35 years, but it seems to me like you'll LD A,FFH before output every loop.

Consider adding a label (S2) and jumping to that after having loaded A with 00H - and haven't you swapped the JR NZ logic? JR NZ jumps if CP 01H doesn't set Z.

        LD     A, 4FH
        OUT    (82H), A
        LD     A, 0FH
        OUT    (83H), A

LOOP:   IN     A,(80H)
        CP     01H
        JR     NZ,S1      ; jump to S1 if 01 was not read
        LD     A,FFH      ; we got 01, load FF
        JR     S2         ; ... and jump to S2

S1:     LD     A,00H      ; will only be reached if 01 was not read

S2:     OUT    (81H),A
        JP     LOOP       ; could probably be  JR LOOP