6809 single board computer (was: "New" PDP-8)

see below, plz. Dick ----- Original Message ----- From: "John Honniball" <coredump(a)gifford.co.uk> To: <classiccmp(a)classiccmp.org> Sent: Sunday, March 31, 2002 7:18 AM Subject: Re: 6809 single board computer (was: "New" PDP-8) bit could What amazes me is that I frequently run into oxymorons such as this one. People want a "simple" system, yet they want a fancy video interface and sound. Now, I'm convinced that at least 75% of the problems I've encountered (not on my own systems, since I always omit/remove/disable it) on PC's relate to sound. Sound consume huge amounts of bandwidth and, moreover system resources. The thing that made our computers of yesteryear simple and manageably small was the fact that the console functions were provided by a terminal. If you want to build that functionality, into your "simple" circuit, keep in mind that the board in a typical terminal was larger than the one on which the computer lived. Further, keep in mind that the video that simple computers e.g. the Commodore types, (not because they're poor, though they are, or simple, because they're not) were considerably less capable than many terminals, and certainly much less capable than what one is accustomed to seeing even in a cheap PC. I built a VERY capable system last week. One with 128KB+ of memory and considerable I/O. It does, of course, rely on a serial terminal, but it is supported by a wide range of FREE compilers, assemblers, interpreters, etc. The wire count was on the order of 200, and the entire design, wiring, checkout, and software installation took under three hours. Of course, the software's simple, since there's no mass-storage, and no file-system. Mass storage is provided by the host system to which it's tethered by means of a serial link. This system runs VERY fast, and is capable of even more speed, but, since it's small, that wouldn't interest game-freaks. Another thing. 600 wires is NOT a big, complicated circuit. 600 IC's might be, but 600 wires is a 2-hour wiring/checkout job. Once I get set up, even with my failing eyesight and aging memory, I still wire and continuity-check (for both missing and undesired connections) a 600-wire circuit in less than two hours. If you want a small and simple circuit, supported with a decent OS and a wide range of software, whip up a modern version of one of the classic CP/M systems. Use SRAM, extracted from an old '486 motherboard's cache, a 20 MHz Z80, (still avaiable at DigiKey) and substitute standard I/O peripherals for the overpriced, underpowered devices in the Z80 family. Use an 82C55 in place of a PIO, and an SCC2681 in place of a DART. Use a CPLD in place of the multi-IC video circuit, or, if you're scared of PLD's, use a 6845-based approach for the video display. You'll have a lot more trouble getting a decent keyboard and NTSC-compabible monitor than you will in building up a high-speed copy of, say, a Xerox 820 or a Ferguson BigBoard-II, particularly if you leave out the stuff you don't need. So long as you leave the mass-storage interface alone, you'll have little trouble generating a working system. Now, if you want a 6809-based system, you CAN build an even simpler one, but unless you like the garbage-quality video of the 6847 or similar chips, you're still going to be better off with a terminal. You can easily hit the 200-wire mark with a 6809-based system. What you'd do with it is another issue, of course.