[TheOtherHobbes]

First gen transistor computers often used standard functional units - gates, flip flops, and such - packaged into small modules with edge connectors and wired together with wire wrap on a backplane. Like this DEC PDP-8.

http://www.oldcomputers.arcula.co.uk/files/images/pdp8104.jp...

It's fairly easy to design a computer like this.

Later TTL/CMOS designs replaced the packaged modules with much smaller 74xx/40xx ICs.

You can make basic logic gates with just diodes and resistors, but you need transistors for inversion, buffering, and a usable flip flop.

That's probably the minimum level for useful computing/calculating. If civilisation has ended and you have no transistors you probably don't have the resources to make glass valves either, so that's going to be a problem.

Of course there's always clockwork...

[Koshkin]

A famous example of a modular design is IBM's Solid Logic Technology that was used in their System/360:

https://en.wikipedia.org/wiki/IBM_Solid_Logic_Technology

[pavlov]

These modules seem to be the primary influence on sci-fi movie computer design, starting with HAL in "2001".

When sci-fi writers need to create some plot tension around getting a computer either up and running or down and disarmed, the characters will inevitably be plugging/unplugging colorful modules at some point.

[dhosek]

We could use vacuum tubes instead of transistors.

(I googled this to make sure I wasn't misremembering what I read 40 years ago in an already outdated book at the library and I was suddenly filled with a sense memory of the smell of the interiors of old electric appliances loaded with tubes and dust.)

[kjs3]

Yes...there were a couple of generations of what we would recognize as vaguely 'modern' computers (say...roughly ENIAC to the IBM 704/709) built completely out of stuff that looked like this:

http://www.righto.com/2018/01/examining-1954-ibm-mainframes-...

[TheOtherHobbes]

Yes, that was the first all-electronic generation after the very earliest relay designs.

They were shockingly unreliable and incredibly expensive. Tubes have a very low mean time between failure, so any design that uses tubes exclusively can't work for more than short periods without breaking down - possibly minutes, maybe hours, probably not days, and absolutely not months or years.

And each failure means a cycle of fault finding, which can take hours or days in turn.

As a technology it sort of works in a prototype way - you can get some work done until you can't. But the unreliability means it's qualitatively different to a modern laptop or server farm.

The wonderful thing about integration on silicon is that it's the opposite - it's incredibly reliable, as long as you keep the thermals reasonable.

[kjs3]

Well...certainly true of the original tube computers (ENIAC was famously temperamental), but that module comes from an IBM 700-series, which was a production product. Tube machines from IBM, Burroughs, Univac, Bendix, Ferranti and many others were in no way mere prototypes with hundreds built. The tube based AN/FSQ-7 was for years the basis for the USAF SAGE air defense network.

Tube reliability improved radically over the 15-20 years tube computers were a thing; it had too. And just like you point out about silicon, reasonable thermal management became recognized as important to tube reliability and designs changed accordingly. MTBF was lower than a modern computer, but they certainly ran for days or weeks and more. And debugging was usually fairly quick as you ran some diags that pinpointed the module (not single tube) that failed and replaced the whole thing.

I have an acquaintance with a Bendix G15 that still runs. Admittedly, the G15 is much simpler than an IBM 700, but it's a nearly 65 year old tube machine.

[bashinator]

We could use telegraph relays instead of vacuum tubes - might be better reliability and repairability.

[Koshkin]

Except that mechanical contacts are the bane of all things electrical. Vacuum tubes are lightyears ahead of relays in this regard.

[HeyLaughingBoy]

Repairability, yes. Reliability, no.