Wim Lewis wrote:
> Over on a web forum [1] someone asked about some of the older logic
> chips that required multiple supplies and it's been bothering me that I
> can't find out the answer.

> Some NMOS chips like the 8080 needed an extra negative (-5) and more-
> positive (+12) supply to function. But pretty soon there were chips like
> the Z80, which ran from a single supply, but was still described as "NMOS".
> And Wikipedia says that there was a Soviet clone of the 8080 (the
> KR580VM80) that used the extra supplies but could (?) operate properly
> with the -5v supply at 0v and the +12v supply at +5v.

> So what were the extra rails for? Textbook illustrations of NMOS gates
> always seem to show just one supply. What did the internal schematic
> of the 8080's logic gates look like vs. the Z80's? Why were later NMOS
> chips able to dispense with the extra rails?

> My best guess is that manufacturing allowed closer control of the
> transistors' Vt or other parameters, and so they could use smaller
> biasing voltages and still have acceptable margin. But that's really
> just a guess on my part. Surely someone in this group knows for sure!

>   [1] http://forum.sparkfun.com/viewtopic.php?t=11874