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Comment by mct | Hacker News Reader

> A “crash” is an unscheduled system reboot or halt. There is about one crash every other day; about two-thirds of them are caused by hardware-related difficulties such as power dips and inexplicable processor interrupts to random locations.

Youch. Was the PDP-11 really so unreliable?

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The bigger ones definitely required decent periodic maintenance. Typically you'd contract DEC or a third party to come out semi-annually to vacuum things out, apply field fixes (yes, run wires on boards and the backplane), and run hardware diagnostics.

There were a lot of wire-wrap connections on those things. If they had been done right you were good. Done wrong, and your system was haunted until someone found the loose wire. We had an 11/45 with a flaky Unibus bay that could be "fixed" with a little percussive ablation. Finally got DEC to get serious about the problem, and they spent several days tracking down a bad socket.

I don't miss wire wrap. At all.

We have it good these days. Rack a bunch of servers, run them hard for years with only DIMM replacements or maybe the odd SSD, and recycle them once the bathtub failures start edging up. I don't want to think about the comparable compute power; my wristwatch runs rings around that 11/45. We live in the future.

Here's a wire wrapped PDP10 backplane. You'd see something similar inside a PDP8 and the earlier PDP11s.

https://upload.wikimedia.org/wikipedia/commons/f/f7/PDP-10_1...

Can you imagine trying to fault-find this with just a schematic, a meter, and a scope?

At least one RSTS/E release amusingly came with patch notes that included wire wrap instructions. I distinctly remember a great deal of grumbling when the patch caused issues and the downgrade included undoing the wire wrap patches. If you thought sharing your screen and coding or debugging was high pressure, imagine pulling out one of the drawers holding your computer's guts and dealing with this while a room full of people hovered over your shoulder. https://upload.wikimedia.org/wikipedia/commons/a/ae/PDP-11-3...

I guess it's a matter of perspective. By the standards of the decade before that, a computer which could run for 2 - 3 days reliably without hardware glitches would have been remarkably reliable. For such a machine to sell for under $100,000 would have been stunning.

The reliability we think of with modern computers is, I think, mostly a consequence of very high integration. Few solder joints to go wrong.

The PDP-6 predates the integrated circuits of the PDP-11 models, and had a board as part of the ALU path that almost always blew at least one of the 36 boards (one for each bit in the machine word) on a power cycle.

I wouldn't know, but a few years ago, I was rather impressed to see that some PDP-10 models featured a knob to manually adjust the supply voltage: https://upload.wikimedia.org/wikipedia/commons/7/71/Digital_...

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