KBK update for Sunday, 3rd May

There is a particular keycap family that has for years resisted identification. This family was used on most BBC Microcomputer keyboards (those made by Wong’s and PED, but not those from the SMK-made keyboards). An almost identical type can also be found on at least one SAGEM keyboard, seemingly from a TX-20. I concluded some years ago that the BBC Micro keycaps always seemed to be Comptec, but Signature Plastics never recognised them. Nonetheless, they have distinctly Comptec characteristics, that do not fit with any other manufacturer’s product range. This includes the internal step that is present in each of SA, DSA and DCS.

Recently I have dug into this again. As Acorn has closed down, all of their records are gone. Chris Turner, Acorn’s chief engineer at the time, reports that the BBC Micro keyboards arrived from the manufacturers fully-assembled, and it seems that Acorn did not choose the keycap manufacturer. I had already encountered an excerpt from a DRAM Electronics Ltd catalogue—listing BBC Micro spares—where one of the keycap manufacturers was given as “Comtec”, which I took to be Comptec spelt incorrectly somewhere (perhaps even by Acorn themselves). DSA family is structurally very similar, with the same thin walls and the same low internal step, and as a DIN-compliant family it likely goes back to the early-to-mid 1980s. This means that the reduced-plastic design seen with modern Signature Plastic keycaps could be as old as the 1980s. The BBC Micro keycaps tend to have writing on the inside where Comptec placed it, although unlike DSA and some old DCS, it seems to simply give the mould cavity numbers or some other internal code. (Comptec had more than one factory and more than one mould design per family, so these characteristics vary.)

For the mystery keycaps, we can rule out SA, as they are clearly not SA family: SA uses much thicker plastic and a much larger internal step. They are also not any of Comptec’s DIN-compliant families (DSS, DSA and DCS) due to their height. The Comptec-like BBC Micro keycaps are all stepped profile, and I remembered that there was one more family that has long since disappeared: SS. Since SS was sculptured, a row from this could be used to make a stepped profile. Thus, these mystery keycaps are likely to be Comptec SS family.

A keycap profile demonstration props topic at Deskthority from 2015 contains an unattributed chart of DSS, SA and SS. The middle row profile from SS is precisely the same shape as the BBC Micro keycaps. That row is 0.410″ tall. Signature Plastics did not state precisely where on the keycap that height is measured, but assuming it’s the centre over the base, the BBC Micro keycaps are 0.419″ tall at that point, which is very close. This chart seems to verify that the Wong’s and PED keycaps on the BBC Micro were Comptec SS family.

The SAGEM keycaps are flat (“all rows”) profile and the same height as shown for SA on the same chart, which suggests that SS also had a flat profile option, similar to SA but made using the revised reduced-plastic moulding.

Having suggested this theory to both Signature Plastics and a former Comptec employee with whom I was corresponding, neither have responded, but at this point it seems likely.

(While gathering more data on these keycaps I also talked to Professor Steve Furber about the implementation of the BBC Micro computer keyboard, which I have since documented in more detail on the encoding and output page.)

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