Serial interfaces (was Re: Any former Psion 5 owners out there?)

[Tony Duell]

> TBH, I don't know. I'm sure you're right, though. I improvised a
> solution involving a couple of instances of DRIVER.SYS and DRIVPARM or
> something like that to persuade DOS that a 1.4MB drive was actually a
> 1.2MB drive, I think.

In which case you may have onl;y 15 valid sectors on each track (1.2M 
format) not 18 (1.44M format). This could end up similar to the strange 
disk image that I received...

> > Wel, there's this PC for a start. It's a much-hacked PC/AT (8MHz version)
> > using the origianl motherboard and many of the original expansion cards.
> > The main changes are a 486 kldugeboard in the 286 socket (so I can run
> > linux), an earle IDE controller linked to a now-ancient drive, a 1.44M
> > 3.5" floppy (as well as the origianl 1.2M one of course), a patch to the
> > ROM BIOS to modify the drive parameter tables (2 extra EPROMs and some
> > address deocnding logic fitted ot the mainboard, kludgewires all over
> > theplace), and various other piggy-back chip mods to correct timing
> > problems, add a drive-in-use LED, and so on.
> 
> Wow! *Impressed*
> 
> A decade and a half ago I was considering trying to do something much
> like that with an original IBM PC if I could find one, using a 386sx

The problem with using the PC cabinet is that it has only 5 expansion 
slot cutouts, and almost no ther motherboard will fit. On the otehr hand, 
an XT chassis will take the XT-286 motherboard.

> board bodged into an XT-286. The idea being to try to make the best,
> fastest PC I could that was at heart an actual IBM PC, not a
> compatible.

Mine started out as a challenge. As is well-known, I am forever moaning 
about lack of real service data -- schematics and the like -- for 
computer equipmetn. So, about 15 years ago I was set a challenge -- to 
make a machine capable of running linux but which was 100% docuemtented 
(not necessarily 'open' -- the scheamtics could be copyrighted, etc, but 
they had to be avaialble). I had to be able to get full schematics and 
source listings of all software involved (that included the ROM BIOS 
needed to boot linux -- the linux sources took care of that requirement 
for the OS). 

I didn;t maanage it -- because I couldn't get a scheamtic of the hard 
disk. But that was the only part I couldn't get. The guy swho set the 
challenge acknowledged that I'd certainly met the spirit of the 
requirements :-).

> I'm impressed that you managed not only to get such a thing running,
> but to be using it still. For any kind of contemporary usage on the
> 21st-century Web, it must be incredibly slow.

It is slow. Very slow. And the video output is an origianl MDA text-only 
card and monitor (I have a CGA card in there too, X doesn't support that 
either, and I find text on the 5151 a lot easier to read than on any CGA 
monitor I've tried (I have the original 5153 and an NEC TTL RGB monitor 
that I modified to have RGBI inputs -- I did that before I amanged to 
obtain a 5153). So not much hope of accessing most web sites. But it runs 
gcc, it runs LaTeX, and things like that. 

To be honest, on the few occasions that I need to look at graphical web 
sites (mostly to read data sheets, technical manuials, etc) I go to an 
internet cafe. 

-tony