Wouldn't this test be better?

Set the port to all one's. Set the DPTR to all zero's (at least the lsb), write external zero (1st dpl=0 emits, then the data = 0 emits). Then sit in a loop watching the port pull high. If the 10K resistors are really pulled up to +5 this should happen quickly.


org 0
jmp test

org 40H

test:
mov p0,#0FFH
mov dptr,#0
clr a

test_lp:
movx @dptr,a
mov r2,#0
djnz r2,$
jmp test_lp

regards,
p