Reason I'm asking is just about every ATX power supply released in the past 6 years lacks -5V at the motherboard power connector. It was deleted because only the ISA bus used it and new systems no longer have those slots. I was looking into using newer power supplies on older boards, maybe even with an ATX to AT adapter as the older power supplies are getting harder to find. Will this pose a problem with commonly used ISA boards (sound cards, video cards, etc.)?
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Is the lack of a -5V rail a problem for ISA cards?
- Thread starter njroadfan
- Start date
Chuck(G)
25k Member
Not usually. Remember that if it really matters, you can always take a 7905 voltage regulator off the -12 line (which, the last time I checked, was still on most ATX power supplies).
The -5 was useful when the 5150 was using 4116 DRAMs, but that requirement pretty much vanished when the 5150 went to single-supply 4164 DRAMs.
There may be some cards out there, such as old RAM cards with 16K DRAMs that requires it, but otherwise, the need is nearly non-existent. I seem to recall that the current available on the -5V line was very limited.
The -5 was useful when the 5150 was using 4116 DRAMs, but that requirement pretty much vanished when the 5150 went to single-supply 4164 DRAMs.
There may be some cards out there, such as old RAM cards with 16K DRAMs that requires it, but otherwise, the need is nearly non-existent. I seem to recall that the current available on the -5V line was very limited.
eeguru
Veteran Member
I think a lot of older serial cards used it to make necessary RS-232 voltage swings before purpose built charge pump drivers became cheap and popular.
Chuck(G)
25k Member
Not on the PC--the PC has always had +12 and -12 supply rails. Trying to use the -5 supply on one of those MC1488 line drivers would likely have exceeded the current output capability of the -5 rail.
AFAIK, the -5 was used only for substrate bias--there may have been some IBM-specific RAM modules or hybrid chips that used it as well on the first version of the 5150. By the 5160, I don't think it was used anywhere.