PM X500/X600 Power Supply and -5V?

[trag]

Do the X500/X600 and related clones use/need -5V on the power supply?

According to these two articles posted (but no longer available) on XLR8yourmac.com the answer is no. And given the readership at the time of publication, I assume they would have been corrected by several readers if they had been wrong back then.

Confusion arises, because Umax included -5V on the power supply connector for their clones. However, I'm pretty sure it isn't used by anything. But not certain...

If the X500/X600 don't need it, then the clones don't need it. They use the same chipsets...

This article specifically says it isn't used in the text amongst the diagrams at the bottom of the article.
http://www.xlr8yourmac.com/systems/ATX_PS_in_8600_9600/ATX_supply_in_8600_9600.html

This article simply doesn't list it anywhere in the wiring diagrams, but there's that question mark in the lower right corner of the short connector:
http://www.xlr8yourmac.com/systems/MacinPC_ATX_case/pages/PC-MAC_PS_pinouts.htm

 

[chelseayr]

@trag well the negative rail could be traced to either the pci slots and/or onboard audio circuit? just my thoughts on that

 

[trag]

@trag well the negative rail could be traced to either the pci slots and/or onboard audio circuit? just my thoughts on that

It could be in the audio circuitry. There's no -5V in the PCI pinout.

https://en.wikipedia.org/wiki/Peripheral_Component_Interconnect#Connector_pinout

 

[iJol]

@trag well the negative rail could be traced to either the pci slots and/or onboard audio circuit? just my thoughts on that

I think it could be the Audio circuit - I replaced the PSU of one of my S900's with an ATX power supply and I have issues with the sound, the S900 with the stock PSU works fine. I could try swapping them and check the audio. PCI is working fine.

 

[trag]

I replaced the PSU of one of my S900's with an ATX power supply and I have issues with the sound,

Late reply, because I didn't remember this until now....

The S900 does some oddball stuff with the sound jacks. If you unplugged the front audio jack for some reason (amongst that mess of wires and jumpers at the front/bottom of the logic board) then the rear jack will not work properly.

Just a possibility...

 

[Skate323k137]

From my experience with arcade circuitry, power supplies, etc., if anything is using -5v, audio is definitely the 1st guess.

 

[David Cook]

The mini-8/mini-9 Macintosh modem and printer serial ports output -5V (technically specified to be at least -3.6V) on the TxD- line, and expect that voltage on RxD-. Earlier Apple designs (like the LC III) get the -5V from the power supply. However, it appears that Apple must have used a newer serial chip to generate negative voltages in the X500 X600 series.

 

[trag]

The mini-8/mini-9 Macintosh modem and printer serial ports output -5V (technically specified to be at least -3.6V) on the TxD- line, and expect that voltage on RxD-. Earlier Apple designs (like the LC III) get the -5V from the power supply. However, it appears that Apple must have used a newer serial chip to generate negative voltages in the X500 X600 series.

Just to confirm I understand what you're saying. Then the X500/X600 aren't using the -5V for teh serial ports, because it's being generated elsewhere?

My devil's advocate to that is that the serial ports on the X500/X600 are supported by the AMD79C950 which supposedly contains an 85C30 cell. But I guess they could have modified it to supply the needed voltage.

But the Quadra AVs (840, 660) and the NuBus PowerMacs also used the 79C950 (CURIO) so presumably they would also lack a need for -5V. Unless it's something like a serial buffer chip providing the voltage.