On 9/27/22 19:43, Brian K. White wrote:
And I'm now working on yet a different idea that dispenses with the
special $6 Eurocard connector and supplies both banks from a single
ram chip on a single pcb that just lays across both connectors and
uses only plain vertical male pins. That will be the simplest and
cheapest way to max out a 600.
I opened my 600 discovered the 2 ram connectors are closer together than
I remembered. They're exactly 1/2" apart, that's pin to same-pin or
center to center, so less than 1/2" gap in between!
And the pcb can't overhang and extend very far in the direction that
would be useful. (it could extend a long way in a direction that isn't
helpful to make a long skinny stick, but that wouldn't help much with
finding room for all the traces)
So I thought I was going to have to walk back such a grand claim, but I
think I actually did it!
I even managed to meet an arbitrary goal I had set myself of keeping the
pcb strictly to the limit of the connector footprints. For a while I had
to cheat slightly and expand the pcb dimensions just a bit to allow a
couple traces to run along the outside of the pins, and provide ground
pour paths, but I managed the get all the traces inside after all.
https://github.com/bkw777/TANDY_600_RAM
I just hope I got the schematic right. I think it's simple enough that
there's not much risk I missed something, but I still wouldn't mind
other eyeballs on that.
There's an image of the dual ram schematic at the bottom of the page.
Most of the lines I'm not worried about. The only things I'm unsure
about are all the stuff going through the 74148.
The messy looking symbol with all the illegible labels is basically a
virtual component for the bus, with two physical connectors and two
physical pins for most signal lines. Most of that you can ignore.
The address lines, data lines, /OE and /WE are all simple, no question
about those. They are all common to both connectors and there's no funky
logic to them, just run one copy of each to the sram.
The interesting bits are, the 6 active-low "/CS1" lines and the 2
active-high "CS2" lines.
The 6 /CS1A,B,C,D,E,F are each essentially /CE or /CS1 for a 32K bank of
4 8K chips on the original ram modules.
Each connector has 3 of them, and they are not common between the two
connectors. So there are 6 total bank-select lines, active-low. So I
named them /CS1A,B,C,D,E,F and run them directly to the inputs of a 74148.
And then each connector also has a CS2 line which is used to disable the
ram during power transition and while main power is off. I don't know if
they are ever individually lowered without both being lowered, but they
are two separate lines, not the same line going to both connectors, so I
run them both through a NOR to the enable of the 74148. So the 74148 and
the SRAM should be enabled if either CS2_B1 or CS2_B2 is high.
And that should be that. Seems simple enough there should be no problem,
I hope?
It's also wasting more than half of a 512K chip just to get 192K, but no
one seems to sell a 5v 256K chip, and I didn't want to deal with the
complication and extra parts to use a 128K plus a 64K, or level shifters
to use a 3.3v 256K part that is available. Maybe in the future I can
squeeze something on there eventually to get 2 switchable 192k banks out
of the 512.
--
bkw
