- Old motherboard (specifically in my case a Intel Socket 478 motherboard)
- Asus P4C800-E
So you have a trusty old 478 system that you cant bare to part with, but the PSU has just blow out on it and you need a new one. You know that a good PSU is worth the investment, you get a brand name, a high wattage value and something with a high efficiency value.
You get home, you wire it up and… nothing.
The machine turns on, it may post, it may just hold up on a black screen before the POST. The latter is what happened with my Asus P4C800-E Deluxe when I came to replace the PSU with a £70 Antec TruePower PT-550. It would not start, however if I swapped back a cheap PSU that I borrowed as a stop gap, it would POST fine.
Another 15 mile one way trip back to the store with the Antec and we stood around scratching our heads over this one for a few minutes.
It turns out that all (decent) modern PSU’s have a design change which makes them incompatible with boards that require -5v – and probably quite a few others.
The Asus P4C800-E Deluxe requires a -5v rail on the ATX in order to get going. In every brand name PSU box we opened in the store – including the one I was returning – the -5v rain pin was missing from the 20-pin ATX connector.
Net result: the system cannot boot.
Why are PSU’s shipping without this?
Some post-event research leads me to understand that with the release of the ATX12V specification version 1.2 in January 2002, the -5v rail became optional on the ATX specification. With revision 1.3 in April 2003 and the 2.x revision the -5v rail was banned altogether. Look at any modern PSU and you’ll find that pin 20 in the ATX connector is blanked out.
So if you are looking for a decent PSU for a Intel 478 system, you may have a problem. In the end I was forced to get one of those cheap PSU’s that case manufacturers supply with cases – which conveniently has a -5v rail but is unremarkable in every other way aside from probably its inefficiency.