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Operating System Switcher, BBC Model B+

1st April 2015

No, this isn't a Aprils Fool update - honest! :D

After reading up on a BBC model B+ being auctioned by RetroClinic, I noticed that it came with an OS switcher which allows switching between the older BBC Model B OS (v1.20) and the B+ OS (v2.00). This allows for maximum compatibility as, although most games and software are compatible, not all will work on a model B+.

So, on the off chance that RetroClinic might sell the switcher separately, I emailed and as luck would have it one was available for the the sum of £16.95 (including shipping). So I ordered one.
That was at roughly 11pm on Sunday just gone and I've now received the switcher today (Wednesday) - not bad at all.

So... what does it look like? As I predicted (and it's the most logical way of doing this), both OS's come packaged in a PLCC socketed PLD chip which is then sat on a PLCC to DIP converter board. A switch is then plugged in (provided with the kit) and this allows switching back and forth between the two OS's.

"But what about Basic?", I hear you cry... Well, maybe not. However, it is actually a valid question as the BBC model B came with Basic (version 1) in a separate ROM whilst the model B+ came with Basic (version 2) incorporated in the OS ROM.
Well, the PLD has not only both OS's, but has Basic 2 as well. This is available irrespective of the OS version selected.
Here's a picture of the kit:

ModelB+_OS_switcher
(click image to enlarge)

The kit also comes with good instructions, although to be fair if you've ever fitted a paged ROM into a BBC Micro before then you can do this without resorting to being told how.
First of all I removed the top plastic cover of the BBC in the usual way and located the paged ROM sockets. On the B+ these are situated in the top left corner of the mainboard (as opposed to the bottom right corner when dealing with a model B). I then had to locate the OS/Basic ROM and remove it. I used a DIL IC removal tool to do this.

Model B+ OS switcher, motherboard, paged rom sockets Model B+ OS switcher, motherboard, OS chip removed Model B+ OS switcher, IC removal tool
(click an image to enlarge)

Next, I fitted the switcher PCB and IC. Something to note is that an extra DIL socket has been added to the underside socket of switcher PCB which help elevate it so that it sits of the top of the paged ROM to the left, but doesn't hit it. Once the board was in place, I then plugged the switch cable in and hung that over the back of the case where the top/rear vents are as I don't wish to start cutting into my computer.

Model B+ OS switcher side view Model B+ OS switcher fitted Model B+ OS switcher fitted 2 Model B+ OS switcher fitted 3
(click an image to enlarge)

As you'd expect, testing was quite easy: turn the BBC B+ on and you get OS 2.00 (B+). Turn it off, use the OS switch, turn the B+ on again and you get OS 1.20 (B):

Model B+ OS switcher side view Model B+ OS switcher side view
(click an image to enlarge)


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