Hi,
I need help understanding the concept of Pull-Ups and Pull-Down Registers. Can nyone suggest a good link for the same. (that explains what they are and how to use em). I need it for the following design of ours that use 89C51 of Atmel or Philips. The design is as follows...

1. 3 Chips (controllers) connected to one master chip.
2. ADC0800 (2 nos) to one controller
3. DAC0800 to another controller
4. 8255 to another controller (and the 8255) accepts Digital Inputs Only.
So, for which controllers am I supposed to use Pull-Up/Pull Down resistors? If required, I can send you the detailed drawing of my system. Yeah, I know so many controllers are not required and you can simply extend the bus and address lines, but this just happens to be the case.
Regards,
Sriharsha