RE: IEEE-488 adapter for 1541

Ah, OK. Somehow, I thought from your first message that the 6522 wasn’t need on the 1541-C and I did not see how that could work 😊

Thanks!

Jeff_Birt (Hey Birt!)

-----Original Message-----
From: Francesco Messineo <francesco.messineo_at_gmail.com> 
Sent: Saturday, April 6, 2019 12:48 PM
To: cbm-hackers_at_musoftware.de
Subject: Re: IEEE-488 adapter for 1541

Hi Jeffrey,
I've never went further than the schematic phase on my project, but the only difference (for this particular case) between a 1541 and 1541C is that the -C has a single 16kbytes ROM instead of the 2 x 8kbytes ROMs of the old 1541. It can however run the same firmware as all the memory mappings are the same. The original 1541C ROM has an additional CS2 line tied to +5V on pin 27, so to install a dual firmware, you can simply use a 27256 EPROM with pin 27 left outside the socket and connect it  to any of the power rails via a 4k7 resistor, then connect a switch to the same pin that goes to the other supply rail. So you can select either the lower 16K half or the upper 16K half of the 27256 (its pin 27 is A14). On one half you can burn the original 1541C (or any other 1541's version) firmware and on the other half, the complete 2031 firmware.
The daughterboard going into the 6522 socket is the same for all the
1541 versions.
It's just easier because you don't need an adapter to substitute the original 2364 ROMs of the 1541. Also 1541C tend to be cheaper (the same apply to the 1541-II, but this one has less space inside the
case) than the 1541 and often they have a mechanic with a track-0 sensor (that you enable by cutting JP3, but it needs the right firmware to be effective, and also I've seen some 1541C without the sensor installed) that it's really good to avoid the infamous track-0 sound when a formatting starts or in case of read errors.
As I said, I abandoned the conversion project because I'd rather have a write compatible unit with the 3040 that I'm now sharing between my PETs.