IBM PC Power Suply Madness

In the olden days, memory was a big deal. It was expensive and complex to manage in hardware. The IBM PC did not use the latest available memory technology when it was introduced; instead, it used 4116 chips from about 1974.

1974! I hear you say. Yes, the original IBM motherboard had the capacity to be shipped with 16Kb installed on the motherboard (expandable to 64Kb), using exactly the same memory chips used in the TRS-80 Model 1 from 1974. The original motherboard could support 64K on the motherboard, and could also support three 64K expansion memory cards, providing 256Kb in total. Remember that in that era, 256Kb was a big deal . the motherboard also had a cassette interface to allow data to be recorded onto tape.

Because the IBM PC used 4116 (16 Kbit x 1 bit) dynamic memory devices, a -5v power rail was required to provide a bias current for the memory chips. A suitable power supply was specified that provided +5v, -5v, +12v and -12v for the system to cater for the memory requirements.

Here is where it gets silly - We stopped using the -5v rail after 4164 chips (64kb x 1 bit) were common, but the brain dead, thoughtless, engineering involved in duplicating the IBM PC didn.t notice. Modern ATX power supplies still provide a -5v supply rail for these obsolete memory devices. But what about analogue cards you may say? In those cases, they use a -5v regulator off the -12v rail.

All of those components being installed into boards, to supply a function that is not required. All because the original design was .conservative., as IBM wanted to get to market quickly!

What a waste!

[/Electronics] permanent link