−5 V output was originally required because it was supplied on the ISA bus, but it became obsolete with the removal of the ISA bus in modern PCs and has been removed in later versions of the ATX standard.
Pin 20 (formerly −5 V, white wire) is absent in current power supplies; it was optional in ATX and ATX12V ver. 1.2, and deleted as of ver. 1.3.
幾年前我第一次買康舒的power時 也對於其power pin角少一支的問題email給康舒工程師過
那時工程師就回答我 那隻缺的pin腳是-5v 主要是供應ISA介面所需要的電源
後期ATX主機板根本就沒有廠商放上ISA介面 也就不需要那隻-5V的pin
你可以看到wiki的解釋 早期在atx 1.0 /1.2版的時候 -5v的接腳就是optional的(可做可不做)
ATX 1.3版後-5v的定義就完全被省略了
很多ATX電源供應器設計上就不再做出-5V的迴路了，因為根本沒有原件會用到，做出來浪費幹嘛
所以有些電源供應器就跟脆把那隻pin給省了(像康舒) 有些雖然有做出來 但也是空的
我想這跟電源供應器是幾回路沒有絕對關係才是
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