In my point of view, pull-up resistors for DS2408 are always needed. I did test on Friday, before I connected them(9 10k resistors), P0~P7 never become high. As long as I connected them as this picture show, I can controll P0~P7 for high or low.
The second photo shows the number of voltage after R/2R DAC and the after Opa.
1111 1111 4.80V -->1.93V
1111 0111 2.98V -->1.21V