What is the -5 volt rail used for

[source]

This may be a stupid question but I have seen a number of YouTube videos of people remaining psu's for various amigas and I was just wondering what the -5volt rail is used for. Some amigas have it others don't.

[giantclam]

Iirc the A2000 used -5vdc to supply slots + video out

[hooverphonique]

I think it is mainly because ISA slots provide -5V, thus big box amigas with ISA slots need this rail.

[Daedalus]

-5V is part of the Zorro-II and Zorro-III specification (as well as ISA), and is supplied to the video slot and RGB port on big-box Amigas, though I've no idea if any cards or devices actually use it. There's nothing on the Amiga motherboard itself that actually requires it.

[Calabazam]

Some ISA sound cards do need -5V. It is something to know if using bridgeboards.

[pandy71]

If you have -12V then you can get easily -5V - so -5V presence is probably highly dependent from usage context.

[Promilus]

-5V was originally introduced in machines which had at least some logic based on DTL. It was generally only used for analogue parts beyond that and afterwards -5V became unused since there's -12V as well and generally better suitable for analog chips.

[pandy71]

Originally Posted by Promilus

-5V was originally introduced in machines which had at least some logic based on DTL. It was generally only used for analogue parts beyond that and afterwards -5V became unused since there's -12V as well and generally better suitable for analog chips.

Or ECL where -5V is same primary voltage as +5V on TTL logic, -5V was also used to bias older MOS technologies (later such bias was created on IC itself). But generally if circuit required -5V it could easily create it from -12V more common.