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Need a bit of a history lesson...What was the -5v rail on the ISA bus used for?

salamontagne

Experienced Member
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Feb 27, 2010
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Harwinton,CT
After OCD'ing myself trying to find an new atx power supply that provided the -5v rail, I decided to try
and plug a later psu and see if it would work with my isa AWE32 card. Surprisingly, it worked just fine.

My question is, what boards (and from what era's) actually required the use of the -5v rail? I seem to remember some scsi boards needing it, but after that I'm drawing a blank. If i recall my electronics theory correctly, the biggest use of a negative rail is for noise suppression or to assist a class D push-pull amplifier.

So, to put it another way: Why the hell are the -5v and -12v rails needed in PC's?
 

per

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Western Norway
There's also some older FDC's that need one, a small handfull of soundcards, and misc cards using analog components.
 

mbbrutman

Associate Cat Herder
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I think the original 5150 used 4116 DRAM chips. Those require a -5v power input.
 

Chuck(G)

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The first 5150s used 4116 DRAM chips--as did some early RAM expansion boards. The chips needed +12, +5 and -5 to operate.

When the 5150 debuted, RS232 voltage levels were conventionally set a -12 and +12V. Look where the driver supply for an IBM async card gets its supply.
 
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