[Coco] Multipak redesign/replacement / New CoCo...

[Gene Heskett]

On Monday 23 February 2015 09:11:31 pm RETRO Innovations wrote:
> On 2/23/2015 12:50 PM, Louis Ciotti wrote:
> > This may have been brought up already, but has anyone considered this
> > as a starting point:
> >
> > https://sites.google.com/site/tgmaxx/tandy-color-computer/expansion-inter
> >face
>
> I did.  I looked at this design, the 8 slot design mentioned in the
> thread, the original MPI schematic.  All of the designs are very similar:
>
> 1 bidir buffer for the data bus (u3)
> 3-4 unidir buffers for the address lines and some of the select lines
> (u1,u2,u4)
> latch for SW control (u6)
> buffer to read latch/switches (u7)
> 3 4->1 multiplexors (u9, u10,u20)
> some logic to trigger on ff7f (u17/u18/etc)
>
> I did not choose this design, since I feel it used too many TTL ICs to
> accomplish the goal. The design converts the selector switch into 1 of 4
> discrete selection signals, but the original MPI used a 2 bit binary
> selector, using a DP4T switch, which this design has as well.  Using 2
> bit binary in this design would eliminate u12/u13/u20, and u5/u4 appear
> to buffer too many signals (halt, nmi, slenb, you don't need to buffer
> them), and should be able to be reduced to 1 ls244.  A 2->4 decoder
> would be needed for the LED, so that adds an IC, but using 2 of them
> (which come in the TTL package) and 4 more LEDs one could track SCS and
> CTS mappings, which might be of interest)

Oh look, all those flashing leds must mean its working right? ;-)
Actually, that is one feature creep I'd pay for happily Jim.  Their value as a 
troubleshooting tool outweighs the cost IMO.

> As for the other designs, the 8 slot unit was gargantuan, and I wanted
> something I thought would be of interest, challenging enough to allow me
> to learn this platform, but not so large as to be a debug nightmare.
> The MPI design looked fine, but I felt I could minimize TTL count by
> using a CPLD, and still provide the same functionality.

I am sure it could, so all I'll mention is the additive delays can be a 
nuisance.  Check those carefully.
>
> Jim
>
>
>
>
> --
> RETRO Innovations, Contemporary Gear for Classic Systems
> www.go4retro.com
> store.go4retro.com

Cheers, Gene Heskett
-- 
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