Hi Chris,
There is one variation that I see with the "A" model, and I
presume the early "B".
Because they ran out of room for two EPROMS, and a battery, they
decided to use a single 27512 EPROM, which uses up the entire
address space of the M6808.
Well, don't despair! Yet...
Their description says:
The programable array logic device also generates the /OE and /WE
signals to the random-access memory (RAM). This RAM can be accessed
with addresses 8000 to 9FFF if either ROM SELECT or PAGE SELECT
signals are HI. In this mode the ROMS, U2160 and U2260 are not
accessible in this address range.
I'm not sure what this means. It could mean that we need to write
to a port that sets ROM SELECT, or PAGE SELECT HI, only they don't
identify any such port in the address map, or it could mean that the
signals get created when you try write or read 8000 to 9FFF.
Siggi warned me about this, but I wasn't on the same page as he was
at the time.
The signals ROM SELECT and PAGE SELECT come from U2310, a 74LS174.
U2310 is sampling the data bus, and gets clocked by "PORT1 CLK",
which comes from U2550, pin 12.
So, it looks likely that we have to set data bit 3 or 4 high and
write it to PORT1 CLK, which is identified on the address map as
being addresses 08C0-08FF.
As I understand the description, we should be able to run with
ROM SELECT and/or PAGE SELECT high all the time because we don't
need the bottom of the 27512 for our project... We gladly sacrifice
the 8K of code space.
Can anyone confirm my thinking?
-Chuck Harris
On Fri, 12 Apr 2024 18:52:12 -0500 "Chris Elmquist" <chrise@...>
wrote:
On Friday (04/12/2024 at 04:08PM -0700), Don via groups.io wrote:
The 27128 EPROM is 16K bytes and so does that mean it is fully
decoded from $C000 to $FFFF or ?? If so, then our program could
start at $C000 but this is another detail I need to confirm.....
If Don wants to send me his 256 word cal table, I'll make the
first version of the code just for him! :-)
Chris N?JCF
Thank you, Chuck for the detailed information and Chris for trying
to get my cal data back on to my scope.
You are welcome. It's an interesting exercise and should be handy
for a lot of folks, including myself, if I never get around to
replacing the battery :-)
Not sure if this matters, but the EPROM in my scope is a 27512 (not
27128).
Hmm. Interesting. I will take a look at the schematics but they are
most likely using only part of it since that's a 64K byte device which
would consume all of the 6800 address space if fully decoded. They do
use a bank switching scheme for the RAM but not for the executable
PROM code as far as I can tell.
I did not take a video of the cal data, but I wrote them down. I
double-checked to make sure that I wrote them down correctly. I
will make a text document or spreadsheet of the cal table and send
it to you. Should I put the file in the files section here or send
it to you by email?
You can send them by email and I will build them into the first
attempt of the program, simply for reference. Other folks will put
their own constants into the same place in the program, keeping only
the copy routine ahead of them but we can then show the constants in
spreadsheet form and also in hex form in the program so folks can see
where they go.
For my direct email, just use that callsign earlier in this message
@arrl.net, in case it's not revealed in the groups.io header for you.
But use the actual zero key on your keyboard rather than the Danish Oh
as shown.
Chris
