22 Mar
2000
22 Mar
'00
6:52 a.m.
On Mar 22, 14:52, Philip.Belben(a)powertech.co.uk wrote:
Expansion box.
The old PETs don't AFAIK have the internal expansion connector - they
have an
edge connector sticking out at one side. Very useful
for toggling RESET
with a
pair of tweezers!
Yes, I know :-) Mine has the edge connector, not the expansion block.
That aside, on this edge connector are brought out
most of the block
select
lines from the 74154 I mentioned in my previous post.
In particular,
brought
out are lines 1 to 7, 9, A and B.
It takes only three four-input AND gates to re-encode any eight of these
you
choose into three upper address lines for a 62256.
Seems like the hard way to do it...
My suggestion is: disable the select to the upper 4K
of memory within
your PET,
and encode lines 1 to 7 and 9. This will give you 32K
of main memory,
several
spare 6550s, and 4K of RAM above the screen.
Essentially, you will have
a 4K
PET fully expanded.
I was actually thinking of replacing the 74154 with a socket, to get access
to all the signals I'd want -- I could enable (or not) all the RAM that
way, and use the internal ROMs. It would only require one 22V10; and I
would prefer to use several ROMs (probably 27256s) for the alternate
BASICs. I'd take the data lines from the edge connector, probably. If you
look at the PET circuit, you'll see the data lines to the edge connector
are buffered, and the control to that buffer is hardwired (via AND gates
and an inverter) to several of the SEL block selects from the 74154;
similarly, the RAM buffers are hardwired to SEL0 and (via an OR, a NAND,
and a link) SEL1. So you can't put replacement RAM on the expansion
connector without doing something about that 74154... Plus, the SEL lines
relevant to the ROMs aren't present on the edge connector, but they are on
the 74154.
--
Pete Peter Turnbull
Dept. of Computer Science
University of York