non-CP-M Z80 board

non-CP/M Z80 board Roy J. Tellason rtellason at verizon.net <mailto:cctalk%40classiccmp.org?Subject=non-CP/M%20Z80%20board&In-Reply-To=> Mon Jun 16 20:01:54 CDT 2008 * Previous message: Fixing a DEC Letterwriter (LA100) * Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] ________________________________ I'm thinking about building a z80 board, with varying amounts of "stuff" attached, with RAM and ROM that may be NMOS or CMOS, and some indeterminate number of Z80-family and compatible peripheral chips. My question is this: At what point do you _need_ to have address and data bus buffer chips? -- Member of the toughest, meanest, deadliest, most unrelenting -- and ablest -- form of life in this section of space, a critter that can be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters" - Information is more dangerous than cannon to a society ruled by lies. --James M Dakin -----REPLY----- Hi, I think the safe range for 74LSxxx style TTL is about 5-10 or so connections per output gate. You can compute the ratio of the various datasheet currents and get a more exact figure. However, here is a pretty good set of "rules of thumb" for circuit design: http://kurser.iha.dk/eit/dtm1/DTM1_html/Wakerly/udgave3/xc03.pdf See line 3.49 for the table of the various combinations. They say 74LSxxx on 74LSxxx can do up to 20 but I'd keep it lower to be safe. That should be enough for some static RAM, a ROM, a couple of peripherals, some simple glue logic and that's about it. You can stretch it a bit using CMOS chips or the more recent TTL families as long as you keep the PCB fairly compact. Doing anything more than the above or going off board with a bus requires buffers. You'll find there are few if any absolute rules on the subject but lots and lots of opinions. If you examine the IBM XT motherboard schematic, you'll see it uses many buffer chips -- primarily due to the DRAM, ISA bus interface, and decent sized PCB. On the WaveMate Bullet schematic, it has many buffers to for similar reasons. There is only so big you can make the PCB and only so much current can be sunk before the signals start to degrade. If you want to avoid buffers, think 1) small PCB, 2) small number of chips, and 3) shortest wiring connections as possible. Thanks and good luck with your project! Andrew Lynch