Ideas for a simple, but somewhat extendable computer bus

[Jim Brain]

I'm currently working on a single board computer system, designing from 
scratch partially as an education experience, and also as something that 
might be of interest to others.

I've laid out the first version of the SBC, and I realize it would cost 
nothing to add an edge connector on the PCB, allowing expansion 
options.  As well, assuming the design has any merit, I can see creating 
one of these SBcs for each family (8080/Z80, 65XX, 68XX, and maybe even 
16 bit options like TMS9900, 68K, etc.)

However, as the design is not *for* any purpose, and I've never designed 
a bus that could be shared among multiple CPUs, I am wondering what bus 
layout would satisfy the following criteria:

  * At least enough to support a traditional 8 bit CPU (A0-15,D0-7,
    RESET, READ/WRITE,CLOCK,INTERRUPTS) with potentially a few more
    address bits (A16-23)
  * Minimal number of bus signals to support multi-processors and
    peripheral cards, but not so few that usefulness is severely crippled
  * Easy to implement (minimize need for logic that serves to solely
    handle the bus)
  * (If 16 bit data bus is part of the design): Easy for 8 and 16 bit
    CPUs and peripherals to share the bus (Maybe this means 16 bit units
    need to be constrained to 8 bit, not sure)
  * Works out to a size that I can buy edge connectors cheaply (62 pin
    .100" connectors are looking like my cheap option at present)

I looked at home computer busses (Atari, Apple, Commodore, Tandy, TI) 
for a bit of inspiration, but they all seem overly simplistic (not 
horrible, but hate to just punt on the idea).  I also looked at the ISA 
bus and the S-100 bus, but they are a bit overwhelming to me (I can grok 
all the signals, but ensuring they are all responsive seems like it will 
drive more logic be on the PCB jsut to handle the bus, and I am trying 
to keep costs very minimal).

Thus,

Is there a bus (or a fraction of a bus standard) that I should consider 
to accommodate the above?  Anyone else interested in this idea and in a 
collaborative mood?

Jim

-- 
Jim Brain
brain at jbrain.com
www.jbrain.com