Teaching kids about computers...

Jules Richardson julesrichardsonuk at yahoo.co.uk
Thu Nov 29 16:11:37 CST 2007

Tony Duell wrote:
>> You're right about the PSUs though - plus I think that for small SMPSUs (i.e. 
>> without lots of protection circuitry on the 'hot' side) there's some trick to 
>> making them work from 110V anyway (i.e. the '110V operation' stuff can be 
>> added after the fact if needed).
> Most small SPMSUs start out by rectifying and smoothing the mains to give 
> about 340V DC (in Eurrope). If you have such a supply, you can replace 
> the input stage with a voltage doubler circuit and run it off 115V -- the 
> output of that will be about 340V DC, so the rest of the supply will 
> neither know nor care.

Yep - I was poking around at the Acorn ACW PSU recently [1] and it struck me 
that it should be hard to modify pretty much any 'simple' SMPSU to run off US 
'mains', even if it was notionally 220V (-ish) only. You're right about 
'auxiliary' stuff run direct from the AC input of course.

[1] It was a model made by VMS - *not* the same as the one in your hand-drawn 
ACW schematics. I actually found two ACWs with the same supply, suggesting I 
didn't just happen to have someone's post-production replacement. I ended up 
drawing out the schematics, which I should probably mail a copy of to you 
sometime just for completeness' sake...

>> never prodded a 6809 myself (just 6502 and Z80), but I guess I could stand to 
>> learn too :-)
> You'll love it. It's waht the 6502 should ahve been. 16 bit index 
> registers, 16 bit stack pointer -- no make that 2 16 bit stack pointers, 
> progam counter relative addressign modes and long branches for 
> position-independant code, 'zero page' in any 256 byte page of memory, 2 
> accumulators that can be combindd to make a 16 bit one, the most 
> orthogonal instruction set of any 8-bitter, etc, etc, etc.

Sounds good. I got on quite well with the Z80, which was the first CPU I ever 
did any assembly with, and the 6502 always seemed a bit clunky in comparison 
(not that I remember the Z80 being particularly elegant or anything!)

> I always wish the Beeb had used that as its CPU. Acorn clearly knew about 
> it (they'd made a 6809-bsed System CPU board), but for some odd reason 
> they didn't use it in the Beeb.

You know, one day when I get time (yeah, right), I'd quite like to build one 
of the 6809 System boards. I've got the board manual/schematic, and I believe 
I've got a copy of the firmware ROM somewhere. I've also got at least one 
Control Universal System-bus 6809 board and want to compare one to the Acorn 
schematic sometime - as Acorn licensed the System technology to CU, it'll be 
interesting to see if the CU board is essentially the same as the Acorn one.

I'm not sure [1] when the 6809 CPU came out? Acorn's System 1 was circa 
1978/79, so maybe that pre-dates the 6809 by a while - and it was just for 
legacy reasons that Acorn stuck with the 6502 for the beeb after using it for 
the System and Atom machines? (somewhere I've even got an Acorn System bus 
expansion module for a BBC micro, so it'll take System-era boards)

[1] Google seems to suggest 1979 - perhaps too late (or back then, too 
expensive) for use over the 6502...



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