S100 was one of the first to see people using different cpus as they could still use their old boards (sometime with mods) or they could "bend" the cpu to fit the bus adaquately. <You may indeed be onto something here, Allison, but the ISA is no less <general in its inherent qualities than the S-100, and I'd submit that a <major case for the S-100's popularity for non-8080 applications was the <ready commmmercial availability of numerous desirable functions at <reasonable prices. That's how the ISA occurred to me. Indeed, the key was commonly available generic functions. ISA8/16 fits the bill fairly well. Infact myself and another are working on a ISA16 motherboard with a z280 insted of intel. Why, keyboard, serial, video, floppy and hard disk interfaces are all done and cheap to free. We do not need a raft of interrupts, so thats not a big problem. It's not a perfect match but general enough if we bend a rule or two it will be fine. After all we want to run z280s at full bore speed for software development and the specifics of the platform are relitively unimportant other than we'd like the two of them to be the same. In 1989 S100 would have been a choice for the same reasons. <Aside from that, a general purpose not processor-biased architecture would <provide a few control signals, e.g. IORD, IOWR, MEMRD, MEMWR, maybe a coupl <of clocks, probably one fairly fast one suitable as a dot clock for a vide <circuit, and one slower, suitable for bus transaction timing, a few <interrupt and DMA support signals, and a couple of dozen address lines. <Parity and maybe a "tilt" line would be handy, as well as a wait signal. <Most busses have these signals in one form or another. Sounds like ISA. ;) If memory is kept off the bus ISA is just fine for most 8/16 bit cpus I can think of. The only provision would be the cpus that may DMA impossible or very painful but even then DMA is not a required capability for some smaller designs. <The key element for generalized development, though, is whether or not you <can afford to buy the functions you don't want to build right away. Don't <you agree? Bingo! or for many reasons don't care to build. A 16450/16550 serial is uninteresting but, serial is one of those must haves in a system like mabe a modem. Others make little sense to fabricate when even new they are under $50! Allison