Using a centre-off switch gives a positive/guaranteed make-break-make characteristic to avoid connecting pins together whilst moving between 40<>80 track.
It also gives a definite period where nothing is connected.
Either of those functions may be needed to switch the drive mode without having to turn power off to the disc drive.
The downside of this is that the centre position selects an "undefined" mode where the drive will spin, but the head doesn't move, because the switch only controls double/single stepping of the head.
It may be that the original design used a slide switch, which is more prone to make before break (where you have all three pins of a changeover switch connected when you move it), then the physical switch type was changed, but the centre off position was retained.
Another reason could be ergonomics. The case needs to be labelled 40 and 80.
You would normally do this above and below the switch.
A standard toggle switch moves from a centre position to "up" or "down", which doesn't match the "above and below" labelling.
And on-off-on three position toggle switch moves from "up" (say 80), through centre to "down" which
So you end up with a control whose action (selecting berween two options) matches it's labelling.
Compare this with a switch to write protect the drive. This could easily use a two pole switch with one position labelled "Write protect" where you only need to label one option (enable or disable), and the other (implied) option ,where the switch lever is in the middle, doesn't need labelling.
It also gives a definite period where nothing is connected.
Either of those functions may be needed to switch the drive mode without having to turn power off to the disc drive.
The downside of this is that the centre position selects an "undefined" mode where the drive will spin, but the head doesn't move, because the switch only controls double/single stepping of the head.
It may be that the original design used a slide switch, which is more prone to make before break (where you have all three pins of a changeover switch connected when you move it), then the physical switch type was changed, but the centre off position was retained.
Another reason could be ergonomics. The case needs to be labelled 40 and 80.
You would normally do this above and below the switch.
A standard toggle switch moves from a centre position to "up" or "down", which doesn't match the "above and below" labelling.
And on-off-on three position toggle switch moves from "up" (say 80), through centre to "down" which
So you end up with a control whose action (selecting berween two options) matches it's labelling.
Compare this with a switch to write protect the drive. This could easily use a two pole switch with one position labelled "Write protect" where you only need to label one option (enable or disable), and the other (implied) option ,where the switch lever is in the middle, doesn't need labelling.
Statistics: Posted by johnkenyon — Fri Jun 14, 2024 6:16 am