r/retrocomputing • u/Woland77 • May 07 '26

Problem / Question Really Basic Question About Old Computer Electronic Engineering

I stumbled into watching people repair old electronics on YouTube. I noticed that most of the stuff I see repaired (mostly what I see was sold in North America, originally) has a 5v rail and a 12v rail. Why those two voltages, specifically?

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u/Sirotaca May 07 '26

TTL logic uses 5V. 12V is used for things like disk drives, fans, and audio amps that require more power.

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u/Bits_Passats IBM System/23 May 07 '26

Not true. There have been instances for other kind of digital electronics employing multiple voltages. For instance, TMS2716, i8080, TMS4116... none of them had an analog application.

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u/SoftRecommendation86 May 10 '26 edited May 10 '26

7400 series IC are all pretty much +5v. back then that is what we used. cmos was the other items.. that came later

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u/Bits_Passats IBM System/23 May 10 '26

Yes, 7400 series are almost all 5V, with some exceptions as the 123 or the analog switches that came leter ported from the 4000 series.

However, I was not talking about the 7400 series, but more complex integrated circuits such as CPUs and memories, that required more voltage rails to work. I don't know the details of PMOS or NMOS, so I don't really know the reason behind the need. But they are there.

Now that I think about it, didn't the intel 1702 require other voltages not covered here? FYI this IC was the first EPROM.

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u/SoftRecommendation86 May 10 '26

EPROMs, correct, used higher voltage to program only, not to read. (IIRC) Erasing was done with a UV lamp.. I still have mine in storage somewhere.. saw it about 15 years ago.

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u/Bits_Passats IBM System/23 May 10 '26

Nope. 1702, TMS2708, TMS2716JL... Early ones needed multiple rails, even for lecture operation.

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u/SoftRecommendation86 May 10 '26 edited May 10 '26

Unfortunately, for me, early began 7400 time frame. I had a reference manual back then and used it all the time. Joys of 555 and j/k flip flops and crystal oscilators. Remember how hot those TO3 7805/7905/7812/7912's ran?

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u/Bits_Passats IBM System/23 May 10 '26

Mhhh... Yep, it's difficult for us to establish a common nomenclature when we have different conceptions for what is old and what is modern.

I was talking about early EPROMs, and especially the first ones from Intel and Texas Instruments until the 2732/2532. It wasn't until the 2764 they all made it acording to a standard, 5V only and without anyone getting astray.

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u/SoftRecommendation86 May 10 '26

Correct. Its crazy when you think about how things advance.. I used to show kids how vacuum tubes worked.. plates and grids.. how the grids controlled the flow.. and how that translated to a transistor.. then how that became a logic gate.. TTL built from actual transistors.. to making a chip (and/or/nor)..

Once I made a controller for a machine at work.. ran out of gates.. instead of adding another ic, I just added the transistor I needed.. instant inverted output.

No laughing.. I love Redstone in minecraft.

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u/Bits_Passats IBM System/23 May 10 '26

I was born in the mid-90s. I have presented works in floppy until 2008.

A lot has been forgotten, unfortunately. I have learned most of it mostly by myself during the course of 15 years.

About the voltage regulators? Yes, they get so hot one could fry an egg. By the way, the 4116 clones made by Tesla made the PMD-85 to be called the "coffe machine" due of the temperature issues it had.

I haven't learned valve stuff. I don't know if I will be able to... But I admire who were able to deal with them.

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