You may need to use a multimeter to check whether PSB is connected to R9. My guess is that PSB is connected to both R9 and R10, that the other side of R9 goes to +5V, and the other side of R10 goes to ground. So the display can be fixed in serial or parallel mode depending on which one is fitted, or can be selected via the PSB pin if neither is fitted. So if you want to use it in serial mode, you will have to unsolder R9.