Select Boot disk

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Cudgel
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Select Boot disk

Post by Cudgel » Fri May 22, 2020 11:02 am

I have a dual disk drive connected to my BBC model B and I've swapped one of the 5.25 drives for a Gotek. Currently the Gotek is drive 0 and the 5.25 is drive 1. Can anyone tell me how I can wire up a switch to swap which drive is the boot drive? I'm sure I've seen this somewhere but can't find it now :(
My electronic skills are very basic. They tend to be of the "if it ain't broke, break it" variety so simple instructions would be appreciated.

Kazzie
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Re: Select Boot disk

Post by Kazzie » Fri May 22, 2020 1:25 pm

In the cable that goes from the Beeb to the floppy drives, there are two wires that carry drive activation signals for each of drives 0 and 1. (The cables are wired up according to the original Shugart standard, see the second table here.)

Each of the drives will be configured with a jumper to respond to one of these signals. One will be jumpered for drive 0, the other for drive 1. For this reason, pulling the data cable out of each drive and swapping them around won't change anything.

If you wanted to (semi-)permanently swap the drives over, the simplest method would be to remove your dual drive's outer case, locate these jumpers, and repostion the so drive A responds to Drive Select 1 instead of 0, and vice versa.

If you want to be able to switch back and forth frequently / quickly, you'll have to make some modifications to your floppy drive cable. The approach that first comes to my mind is to wire in a double-pole, double-throw switch (one lever operates two electrical switches, and they can each conduct in two different positions, i.e. an on-on switch).
Here are two examples of such a switch:
Image
Image

The two wires we want to divert to the switch are wire 10 (Drive Select 0) and wire 12 (Drive Select 1). Wire 1 is the one with a red stripe in ribbon cables.

Having carefully cut only these two wires, the Beeb end will need to be soldered to the two centre terminals of the switch. The drive end of the wires will need to be soldered to the corresponding terminals adjacent (say above) the centre terminals. When the switch is closed in this direction, the drives will respond to DS0 and DS1 as they currently do.

The clever bit is to fit two additional wires that go diagonally from top-left to bottom-right terminals on the switch, and top-right to bottom-left. When the switch is closed in the other direction, this crossing over will mean a signal from the Beeb on wire DS0 will show up on the drives' DS1 wire. The net result of this is that the drives will appear to swap numbers.

Here's a quick ASCII diagram of how the switch ought to be wired up:

Code: Select all

                    _____
                   |     |
Drive pin 10 -> -/-|-X X-|-\- <- Drive pin 12
                /  |     |  \
Beeb pin 10 -> -(--|-X X-|--)- <- Beeb pin 12
                |  |     |  |
                | /|-X X-|\ |
                | ||_____|| |
                | |       | |
                | \-------)-/
                \---------/
    
And an image that I found on the interweb that shows the same kind of wiring:
Image

If either (or neither) of these approaches sounds good, and you'd like more guidance, just ask.
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
Acorn System 1 home-made replica

Cudgel
Posts: 4
Joined: Sat Feb 10, 2018 7:59 am
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Re: Select Boot disk

Post by Cudgel » Sat May 23, 2020 1:22 pm

Brilliant. Just the detail I was looking for.
I will give it a try.

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