To this they add various bits of custom hardware, to allow for I/O on the one chip.

These chess playing systems include custom hardware or run on supercomputers.

Or are they using their own custom hardware that's merely plugged into the phone (I didn't notice anything external in the video)?

No one's building big rooms full of custom chess-playing hardware.

The machine used custom hardware to drive the overall I/O process.

One obvious approach is custom hardware but this can be expensive.

To complete it, he had to design custom hardware and development tools.

You have to do - people would require custom hardware.

The drawback of this system is that it requires custom hardware which can add significant cost to the aircraft.

The system involved both custom software and custom hardware.