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I just finished reading a story about an IBM 360 Model 20 rescue, and some of the pictures caught my eye.

Specifically this one:

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Searching online, led me to several more pictures, all with obviously different configurations. It seems that this panel is exposed in a way that makes changing the wiring "easy".

enter image description here

What is the purpose of these wiring panels? Did changing the wiring somehow configure the operation and features of the computer?

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    That poor IBM 360, lost in a decrepit warehouse. The article now appears at The Register _Wanted: Big iron geeks to help restore IBM 360 mainframe rescued from defunct German factory by other big iron geeks: Why random plans made in pub shouldn't always be acted upon . "They chanced upon a listing for an IBM 360 Model 20 or, as the German ad put it, "a relatively rare plant Puma Computer IBM 2020 probably from the 1980s in red". Yeah, there is a photo of a Puma apparel decal on the machine.
    – David Tonhofer
    May 21, 2019 at 9:42
  • This article generated some buzz on slashdot. I don't think this is authoritative enough to post as an answer, but a user there mentions "IBM was big on [wire-wrapping] as a way to customize hardware." slashdot.org/comments.pl?sid=13991796&cid=58620992
    – tolos
    May 21, 2019 at 13:18
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    The two answers below are correct - this is just the way the machine was built (or patched by the manufacturer) but these are not user serviceable parts. However, prior to programmable computers there were unit record machines with plugboards - the user in those cases did "program" the machines by wiring up plugboards (for some limited definition of "programming"). Different plugboards would represent different "applications" and you'd have a stack in the corner which you'd swap in and out for payroll tasks, acct's payable tasks, etc.
    – davidbak
    Oct 6, 2019 at 16:55

2 Answers 2

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What is the purpose of the yellow wired panels

It's the backplane, simply the wiring of the machine.

on the IBM 360 Model 20?

Not just there, but next to every mainframe was made that way. Depending on planned (and ordered) production run some would get printed boards, but usually all wiring was done as wire-wrap. The -20 was sold in quite high numbers and above fotos show a somewhat late model. But even here not all routing was done on a PCB, but as wirewrap.

Being trained on this technology in the late 70s, I still remember countless hours spend in debugging and adding of patches.

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  • So the wiring really wasn't meant to be changed at all? Are the differences between the photos just due to how the wires were routed on that particular unit, or possibly different production runs with different component layouts?
    – Bradley Uffner
    May 20, 2019 at 14:00
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    @BradleyUffner I don't think there are any significant differences between the wiring in the two photos. In the first photo you are looking down on the back plane from above. In the second we are looking at it from an angle between the side and straight on.
    – JeremyP
    May 20, 2019 at 14:28
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    The wiring wasn’t supposed to be changed by end-users, but it was common for service technicians to patch it (often with different-coloured wires).
    – Stephen Kitt
    May 20, 2019 at 14:41
  • The biggest difference in the wiring I see is what would be the top-left panel. In the top picture, the main "bus" of wires goes all the way up, and makes a hard left turn. In the 2nd picture, the "bus" splits just above the topmost "bridge" connection between panels, forming a Y shape. The top picture also contains several diagonally run wires that are not present on bottom picture.
    – Bradley Uffner
    May 20, 2019 at 15:08
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    There are several sources for differences. Starting with patches to increase stability (sometimes you will even find resistors or capacitors added in backplane wiring) made already in production, as well as later by service personal, over patches to add functionality (usually just a few wires, as more often than not additional hardware was build in but not activated) all the way to upgrades from one model to another (as far as the basic setup allowed). Not to mention changes to add more memory, beyond what a certain model originally supported. so yes, much changes happened, but never by users.
    – Raffzahn
    May 20, 2019 at 15:53
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Couldn't find much on the purpose but physically those appear to be terminals that are used for wire-wrapping. You see this application used in all manner of early electronic equipment and its used to interconnect various circuits.

IBM Service technicians (customer engineers or CEs) would be the ones to actually change these (not customer serviceable).

I'm speculating but apart from circuit interconnects this could have been used as a crude form of "microcode" changes / adjustments or feature additions.

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