BBC Micro/linear keyswitches

[Daniel Beardsmore]

I'm still not clear on what linear keyswitches are, and what exactly remains unchanged throughout the stroke.

On a BBC Micro keyboard, you just keep pressing downwards with constant force; likewise, every coin you place onto a key just moves it down a bit further -- at no point does the key suddenly drop. Is this what linear means?

I've found that the actuation point is something like 3/5 of the way down; being (presumably )linear, I find that I can type without bottoming out almost effortlessly as the keys don't drop to the bottom the instant they actuate, so you can press as hard or light as you like. Since I've not used a BBC Micro regularly for around 15 years, I don't recall what it was like to type large amounts of text on, but I couldn't type as a kid anyway!

Keyboard with metal-plate-mounted keyswitches (detached keyboard is a spare); another BBC Model B was my first computer:




Keycaps appear to be double-shot, and somewhere around 0.85 mm thick, the same as my Tulip ATK and a FILCO, which dispels my fear about FILCO keys being flimsy, as 0.85 mm seems perfectly fine to me.

[ch_123]

You pretty much got it. In a linear switch, the required force to push the button increases consistently until you hit the bottom. In a tactile switch, it builds up suddenly until it hits a 'bump' point where the required force drops off.

Before the advent of the original IBM PC, pretty much everyone (except IBM with their beam springs) used linear switches. As they are more simple to implement that a tactile switch, they last longer and are cheaper (at least when compared with other mechanical switches). They aren't much to type on though.

[microsoft windows]

IBM's keyswitches last a long time no matter how much they're typed on as long as they're taken care of.

Now, those BBC Micro keyboards appear to have Cherry-compatible keycaps. Maybe I'll get one of those with a Cherry keyboard and make something out of it.

[keyb_gr]

They aren't much to type on though.

Fun, that is - although that depends quite a bit on the force displacement characteristic. Cherry MX blacks, with an initial force of 40 cN (grams) ramping up to 80 cN, are a bit on the heavy side but OK. MYs however, going from about 20 cN to 105 cN (95 cN + some nonlinearity), have that popular "dead octopus feel". Activation force and travel are pretty much the same for both. A linear response without any offset would probably feel really crappy.

[ch_123]

In the early 80s, BBC launched a scheme to provide a computer for every school in the country, or something along those lines. Acorn was the company chosen. The BBC Micro is one of the most famous English machines of the early 80s, although their more enduring legacy was the processor architecture they designed for the Micro's successor - the Acorn RISC Machine.

(I don't think the good folks at ARM use that one any more, but that's where it came from originally)

[Daniel Beardsmore]

That looks to me to be a old school Cherry.

Indeed. The only difference seems to be that on the Beeb, there's a small notch on one side of the keyswitch. One of my photos clearly shows that the shift lock keyswitch is installed 180° out compared to Z.

You pretty much got it. In a linear switch, the required force to push the button increases consistently until you hit the bottom. In a tactile switch, it builds up suddenly until it hits a 'bump' point where the required force drops off.

Rubber domes are tactile too, I presume (notwithstanding their terrible variation in behaviour)?

I am not sure that I have got it. The force required to depress a key on a BBC Micro doesn't increase. You just keep pushing at a constant force, like pushing an object across a table. It's not quite that linear, but the change in force is too subtle to figure out. The definition of linear suggests that if you strike the key, your finger would be decelerated as the key pushes back harder and harder, but this doesn't happen.

But yes, with the mechanical switches combined with the double-shot keycaps, these keyboards seem to be indestructible. (Even if the keycaps are only a FILCOish 0.85 mm ;-) Even the surface texture doesn't wear -- you never see a bald patch on a spacebar like with modern keyboards; the spacebar texture remains completely consistent as with all the other keys.

Not the best for typing on, unless you like stiffer keyboards, but it will last you a few lifetimes and then some!

[ch_123]

The force increase in most linear switches can be quite subtle. If it was a constant force switch, you'd probably bottom off the instant you started pressing it.

On the Master, which has very similar switches (same sliders, but a different surround) I'm pretty sure I can feel it getting stiffer as it goes along. And yeah, these keyboards REALLY sucked. Still not as bad as the DEC LK-201 though...

[Daniel Beardsmore]

The Master keyboard was different somehow, as the keyswitches tended to die. Really annoying.