Re: 1551! Re: 9V AC needed for C64? (C128 and serial bus q)

From: Ruud Baltissen (
Date: 1998-09-06 21:29:57

Hallo Richard,

> Wow. You don't waste any time :-) ZX80s are incredibly, ridiculously,
> utterly, hopelessly, ludicrously rare. Needless to say, I have one (but it
> doesn't work). Anything else?

I have the SCH as GIF, I'll send it to you personally. So you can repair it. If 
you don't want to, I'm still very interested!!! :-)

> Argh! Butchery of sacred Commodore machines! Well okay, so it's only a
> C64, but even so... I suppose I have to look up the specs on 7406. I
> always thought open collector devices could sink quite a lot of current,
> but just how much remains to be seen.

Use a good desoldertool I don't call Butchery. But it is more work then just 
cutting the wire of the resistor (and not the wire on the board!!!)
> So, the question is: how fast can the 1551 be driven? I had a brief look
> inside the drive (hmm, quite similar to the 1541C board) and the interface
> cartridge, and I can tell you that there are 16 (sixteen!) wires between
> the cartridge and the drive. So what are those mysterious coloured wires,
> and how can they help us? I figure it ought to be possible to get proper 8
> bit parallel transfers, rather than the standard 3 bit (why?!?) protocol.

You didn't mention opening the little box at the end of the cable. There you 
find a 6523(?), a 28-pins version of the 6525, but with less I/O-pins.

Why the 3 bit protocol? The story is that 1) they ran out of IEEE-cables and 2) 
they needed a cheaper system (an original cable costs about $50). The original 
idea was to use the serial I/O-port of the 6522 but they found out too late 
that there was a bug in the 6522. So they had to invent another protocol.
The bug was removed from the 6526 and that's why the original idea could be 
used again with the C128/1571-combination. With soldering some wires, the trick 
can be used with the C64 as well.

Does anybody know what that bug exactly is? I know nothing of it then that an 
interrupt could disturb the transmission in some way.

Groetjes, Ruud

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