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Comment by AnimalMuppet | Hacker News Reader

Wow, wire wrap. That could be a nightmare to debug, because the contact between the wrap and the post could be flaky. And if you had to change something, you often had to cut a wire, but you couldn't remove it, because there was too great a chance of disturbing something else in the rat's nest of wires. But that left obsolete wires with cut ends, and they could also on occasion make contact.

It's amazing that anything ever got working at all...

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IBM's computers of the 1960s had wirewrapped backplanes and they were reliable. (The Xerox Alto and Apollo Guidance Computer are two other systems with wirewrapped backplanes.) I don't think we've ever had a backplane problem with the IBM 1401 at the Computer History Museum. IBM did a lot of field changes to the wirewrapped backplane, either to fix bugs or add new features. They had a specific process to go through. You couldn't just cut a wire; you had to unwrap the wire and remove it. But there could be up to 3 layers of wirewrap on one pin, so in worst case you had to remove 2 wires before you could get to the wire you actually wanted to remove. After you replaced the wire you wanted to replace, you'd need to replace the wires you removed to get to that wire.

And hopefully the other ends of those two were top-most, so it could end there. If not, there could be even further collateral rework.

Represent that network of wires as a directed graph.

Find the biggest tree spanned within the graph. This will be the wire that, should you ever have to replace it, will have you cancel all your other plans for some foreseeable time…

Yes, I think it was a requirement that both ends of a wire were at the same level on the pin. So you might have an additional top-level wire but you wouldn't end up with a long dependency chain.

I should also mention that the original (non-field) wire wrapping was done with an automated machine, the Gardner-Denver Automatic Wire-Wrap Machine. It had X-Y carriages with wire-wrap tools and it wire-wrapped the backplane according to a deck of punch cards. Details: http://www.bitsavers.org/pdf/gardnerDenver/SM506753_Paramete...

Wirewrap is actually very reliable. From Wikipedia:

> Wire wrap construction can produce assemblies which are more reliable than printed circuits: connections are less prone to fail due to vibration or physical stresses on the base board, and the lack of solder precludes soldering faults such as corrosion, cold joints and dry joints. The connections themselves are firmer and have lower electrical resistance due to cold welding of the wire to the terminal post at the corners.

It was used on satellites until fairly recently.

Back in the 80s / 90s, I worked for a company that made custom compute boards for military applications. The boards had to withstand the vibration levels, temperature extremes and other stresses encountered in a main battle tank. They were all wire wrapped. Frequencies were low, though-about 25MHz max.

Yup, very reliable for low(ish) frequencies around 20MHz. I built 68010 and 68020 prototype systems using the DIL packages. Those were better for wire-wrap than the later pin grid versions of the 020 because you didn't get such a density of wires fanning out. A great text book we used circa 1990 was Alan Clements "Microprocessor Systems Design" (PWS Kent). It's an almost faultless book based on his practical systems lectures at Teeside Poly, full of schematics from which you can build working 68000 computers using pairs of 8 bit DRAM and EPROMS of the time.

This is interesting. I presume in the example shown they stuck to 1MHz as it would be more reliable than clocking to the kind of speed you mentioned, again presumably because the nature of a bunch of discrete components meant the switching of logic levels imposed a hard limit?

Another sibling mentioned the Amiga prototypes, I'm again presuming that the ~7MHz that that chipset ran at was well within the acceptable ranges for clock speed using discrete components?

<grabs book and dusts-off failing pages...> On page pp 142-3 is the minimal config schematic that shows "oscillator module" (IC4) but it isn't in the BOM. However pp 108 (fig 4.2) shows it as 8MHz. IIRC it was a MAX 6 pin 4MHz module I used, later I cranked it up to 10MHz on a 8MHz 68k uP and it still worked. Crazy overclocking!

Clements book is indeed excellent. I doubt I'd have been as successful homebrewing a 68k back in the day without it.

You had to follow a careful set of rules when wiring it up, but it worked-- for slow stuff. Anything faster than a few megahertz and the lack of proper transmission line traces will round off your edges, or give serious crosstalk if you drive them harder. We had to give up on wirewrap years before this.

I remember around 1996 we made a prototype that was a single board about 14x12 inches IIRC and after a certain point of adding things, the digital stuff started behaving erratically. When we looked at the 5V DC line it had started approaching a sine wave shape from 2V-5V, so we switched out a lot of discrete chips for an FPGA and started over to get our prototype working again. I only did the software and wonder if we could have got it working with just wirewrap cause this 68030 seems like a lot more stuff.

I don't recall seeing more than a handful of flaky WW connections ever from someone who knew what they were doing and using a decent wrapping tool. You saw them all the time from novices using cheap, crappy wrapping tools. That's undoubtedly where WW 'reputation' for unreliability comes from. I have stuff wrapped in the 80s that works just fine.

We had a roundness/excentricity measuring machine at an old workplace (truck engine plant). I saw it get a full service+upgrade once. Electronics was all wire wrap. This was in 2005. Apparently they made so few of these machines, and each was made to order with custumizations so wire wrap made sense. And the service guy said that wire wrap was way more reliable than solder.

> the contact between the wrap and the post could be flaky

Not if done correctly. A wire wrap joint is more reliable than a solder joint, and much much more reliable than the socket connection sitting above it.

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