I dont know entirely. 

I do know that the HDI cable has a pin in the corner that tells the machine to go into SCSI disk mode, and it has to be supported by the ROM.

My guess is ROM. they probably omitted it in the ROM, so it doesnt work. All the PB1XX motherboards with the exception of the 100, and 190, appear to be the same, all except for the CPU cards which are slightly different between models especially the color models.