Ideas for a simple, but somewhat extendable computer bus

[alan at alanlee.org]

If you need simple arbitration, there is always this:

https://www.retrotronics.org/arbiter/

-Alan

On 2017-11-17 20:11, Jim Brain via cctalk wrote:
> 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