Re: 6809 in a C64

Den Wed, 23 May 2018 15:22:44 +0200 skrev Konrad B
<konrad0x42@gmail.com>:
> 2018-05-23 14:35 GMT+02:00 Steve Gray <sjgray@rogers.com>:
> > Very cool! FYI, the Commodore 64 BTX cartridges also contain a
> > 6809, but i haven't seen schematics for them anywhere. They also
> > have their own video chip. Other than that, there's really not much
> > info out there about them.

> "BTX Decoder Modul II", the one in REU-like case, has the 6803 MPU and
> is interfaced to the "main CPU" with the 68HC34 dual port RAM.

Why were it made that way?

Did the German BTX have some fancy features that the British
(Viewdata?) and Swedish Teledata never had?

Handic/Datatronic, the Commodore importer in Sweden up to 85, made a
software-only solution for the terminal emulation. The cart only
contained an eprom, easily pirateable (or so I've heard...), and the
hardware were an AM7910/7911 (can't remember which) on a board in a box
that plugged in to the user port. Four "radio buttons" push buttons to
select 75/1200 with and without some kind of equalization, and 300/300
originate and answer (according to the European CCITT standard,
although you surely could rewire the modem to use the Bell standard for
300 baud if you wanted to dial cross the ocean). There were also two
momentary push buttons labeled data and telephone (but in Swedish), and
a LED. You dialed using your ordinary phone and when you heard the tone
of the modem in the other end, you pushed and held "data" for a while,
and hoped that the LED would still be lit when you released the button.
As long as the modem IC detected the other ends carrier the LED would
keep lighting and and the modem would be connected. The C64 provided
power to the modem and the phone wire were soldered onto the board
directly.

Anyways the "TeleData 64" cartridge for Videotex emulation seemed to
emulate everything you wanted.

Not sure how it was done though. Maybe they used hires mode to be able
to freely select foreground and background colors? With 1200 baud
receiving data rate I guess that the 6510 had enough time to both
produce text in hires mode and receive data via the usual user port
RS232 bit bang method. (Sending must have been trivial as it was 75
baud).

With 1200 baud you receive one bit every 13th scan lines, and with 75
baud you send a bit each 208th scan line. That equals to one byte sent
every 6-7th frame and one byte recieved every 130th scan line, which
equals to about 2.4 bytes per frame. There should be plenty of time to
do all processing although you'd probably need a (ring) buffer to not
drop characters after a screen clear or scroll operation.

-- 
(\_/) Copy the bunny to your mails to help
(O.o) him achieve world domination.
(> <) Come join the dark side.
/_|_\ We have cookies.