68K ISA project

On Tue, Jul 13, 2010 at 3:30 AM, Ben <bfranchuk at jetnet.ab.ca> wrote: Close - DTACK Grounded. DTACK is DaTa ACknowledge, a signal pin that allows for async memory operations... different memory mapped I/O chips or RAM or ROM, etc., can each have different access times, and when their subsystem "knows" that data contents are valid, that subsystem grounds DTACK to tell the processor to continue with the memory cycle (this is in contrast to 1970s 8-bit microprocessors which had fixed-length memory cycles, so you had to have memory and I/O pretty much all the same speed or resort to stall cycles. The trick about grounding DTACK is that when the right phase of a machine cycle comes up, it sees DTACK (/DTACK, really) asserted and keeps on going - as long as your memory and I/O are fast enough, the machine runs at full speed with a non-complex memory control circuit (ISTR the COMBOARD design (c. 1982) uses two bi-polar PALs and maybe one or two TTL parts as well to generate /DTACK at the "right" time, earlier for DRAM, later for EPROM, etc). The author of DTACK Grounded was rather fanatical about the issue (you don't have to read very much to pick up his opinions). If you mean paper copies vs a real chip, maybe not, but for PDFs, the second google hit reveals all... http://www.easy68k.com/paulrsm/dg/dg.htm In my own pile of parts, I have tubes of real Synertek and Motorola-sourced 68000L8 chips, tubes of 68000P8s, and a smattering of oddballs like 68000L12s, 68000Z8s, and maybe a 68000FN or two, but no loose 68020s. OTOH, I do have two ancient Mac 68020 boards that I bought long, long ago (back when one might care to stuff an accelerator into a machine vs buying a faster machine) but never got working. ISTR they are stuffed with MC68020RC16s or something close. I never did much with the '020 in the Amiga world; I jumped right from the straight 68000 (with the occasional 68010 "upgrade") to the 68030 (A1000 -> A3000) At one point, a 68000 motherboard with ISA slots would have fascinated me, but then I started making GG2 Bus+ cards, so any Amiga I run into that has slots (A2000, A3000, A4000...) can have ISA cards. Ultimately, I did very little with ISA parallel ports, a bit with ISA serial cards, enhanced an IDE driver for 1995-era drives, and did a lot with network interfaces. I always wanted to fiddle with unusual cards, but the lack of DMA on the GG2 Bus+ curtailed some options. Something that would still fascinate me is something I saw once - a 68000 CPU (not peripheral card - I have those) for Qbus. More so now than 20 years ago it'd be a curiosity, but that's the sort of machine I'd want to try to bring up from bare metal (but it'd be better with an on-board SCSI controller since real Qbus SCSI controllers are desired by many, including myself, for MicroVAXen). -ethan