Re: cbmlink -c serial and 6551

From: Nicolas Welte (
Date: 2003-03-14 14:26:29

Marko Mäkelä wrote:
>>Now I wonder what is the fastest speed the 6551 can run in the Plus/4 and 
> The SwiftLink overclocks the 6551 to double speed by using a higher-frequency
> crystal.  I don't know, maybe it could be overclocked further by changing
> the crystal, but there must be some limitation.  76,800 bps (with quadruple
> crystal frequency) doesn't sound familiar, and neither does 153,600.  You'd
> have to configure the other end of the RS-232 line to use the non-standard
> speed as well, or you'd need an odd-frequency crystal.

There's a reason I asked this, because I remember that the Swiftlink goes up
to 115k with its double speed crystal. After a short look into the 6551
register description this must be done by selecting the "external 16x clock
source" via the baud rate register. With the single speed crystal in the 264
and CBM-II series it should still be possible to go to 57k, but 38k isn't

The Swiftlink lacks the 57k rate, and this is why the Turbo232 has an extra
register added that allows selection of 57k and 230k (!) rates. CMD did this
with a GAL, so the logic can not be reverse engineered easily, but it should
be simple enough to clone it because the register mapping is well known (e.g.
Novaterm supports it).

> I find it somewhat annoying that the CBM II has an on-board cassette
> connector but no ROM support for it.  Thus, the machine cannot be
> bootstrapped via the C2N232; the software has to be transferred to disk
> first.

A while ago I ordered copies of some articles on the Supertape speeder from
c't magazine (via the library, it's cheaper than directly from c't). I have
copies of the implementations for CBM and CBM-II machines, and the latter is
a ROM patch :) If someone's interested in scanned copies of those articles, I
can upload them somewhere. I'm too lazy to type in the programs.

The implementation list of Supertape is quite impressive: ZX81, VIC-20, C64
(this is the most well known version), ZX Spectrum, Apple II, Eurocom II (an
6809 system), CP/M (not specified), Oric-1 and Atmos, Colour Genie, CBM
3/4/8000, Sharp PC1500, TRS-80, Sharp MZ 700, Amstrad/Schneider CPC 464, and
the C264 series. c't has a search machine to find articles at,
but then these old articles have to be ordered on paper.


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