From: Ethan Dicks (ethan.dicks_at_gmail.com)
Date: 2007-01-23 21:54:39
On 1/23/07, willi@allvantage.com <willi@allvantage.com> wrote:
> Hi!
>
> Quoting Ethan:
> <--- snip --->
> > To replace a single 6550 with a single 2114, you'd need to determine
> > what chip selects are high-true, what ones are low-true, and with a
> > series of gates (inverters, ORs, NANDs...), combine the chip selects
> > to a single chip select for the 2114.
> <--- snip --->
>
> I'n fairly certain I have a adaptor in my small keyboard PET that allowed me
> to subtitute 2114 chips for 6550s.
Willi,
It seems that I should have checked the PET schematics before replying
- what I said applies for generic circuits of the same age as the PET,
but in the specific case of the "static PET" board, all the chip
selects are tied high or low except one... the one that needs to go to
a replacement SRAM chip. I _have_ seen schemes as I described, on an
INS8073 "Tiny BASIC on a chip" system, where they inverted A10, A11,
A14 and A15 and used combinations of A10, /A10, A11, /A11, A14, /A14,
A15, and /A15 as various chip selects right at the RAM chips, with at
most one external gate to select RAM vs ROM space (the chip needs RAM
low and ROM, if present, at $8000). I just hadn't realized that the
PET used the now-standard technique of fully decoding the address bus
into selects of the appropriate size (1K in the case of the static
PET) and tied all other chip selects to true.
So in this case, check the schematics and just worry about the chip
select that is drawn as being attached to some logic, not to Vcc or
GND and keep in mind with other 1970s circuits, it might be done a
different way.
-ethan
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