CP/M on the Beeb

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Well, not exactly on the Beeb – more like via the Beeb.

Ever since I built my copy of Grant Searle’s Multicomp I’ve had a hankering to hook it up to my BBC Master. There’s really no good reason to do this. The Zolastar 2000 (as I inexplicably called it) has perfectly good keyboard and VGA interfaces. And yet…

Maybe it’s because it’s one way of ridding my desk of an unnecessary keyboard. Or maybe it saves me switching the TV/monitor constantly between VGA and TV modes. See what happens when you start rationalising? As if there’s anything rational about a vintage computing fetish.

The reason I hadn’t done this before was a lack of a +5V pin on the Zolastar’s serial port. Frankly, ‘port’ is a bit of a fancy name for a row of pins. But in any case, not one of these pins carried the 5V necessary to drive the MAX232-based converter board I’d be using. This board is required because the Zolastar’s serial port (there I go again) is TTL-level logic and the Beeb has a full-blown and magnificently anachronistic RS-423 port.

I’ve been here before. I put together a cable to link the Beeb with my Kim Uno. The Beeb’s serial cable ends at a board with a 9-pin D plug. Into this I plug the MAX232-equipped board with its DB-9 socket.

Having reworked the Zolastar’s serial port to provide a +5V pin, I hooked it all together, fired up Terminal on the Beeb and switched on the Zolastar to see … nothing.

I won’t bore you with all the gory details, so here’s the potted version for all the tl;dr types out there (you know who you are). While I was putting together the Zolastar, I changed the speed of the serial port from the default 115,200 baud to 9600, thinking that this would be more compatible with whatever I might throw at it. And everything worked and I was pleased. But at some point I started tinkering with the Multicomp and must have changed it back again. So anyway, after a full morning’s faffing about (which included emailing Grant Searle only to email him again a little while later to say “never mind”), I finally reflashed the Altera FPGA board again with the correct settings.

So again, I hooked it up to the Beeb and lo! There was the Zolastar’s prompt. I booted CP/M and all was well.

Did I say all?

Well, simple stuff is okay. Stuff like getting directory listings.

The Beeb’s keyboard seems to map okay, so long as you remember to do stuff like Ctrl-H instead of using the Delete key. So you can do stuff like play text adventures. Inevitably, I found myself in the world of Zork.

But Terminal’s ANSI emulation is, I suspect, not perfect. And anything that really uses console capabilities is tricky. Trying to run Wordstar or Turbo Pascal resulted in an unholy maelstrom of characters and beeping. Time for the Big Red Switch…

So I guess this means I have another project – to write a terminal that will handle all this stuff properly. I’ll add it to the list…

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