Commodore floppy disks with an IEEE-488 interface have unfortunately become very rare in the meantime and are sometimes sold at top prices. At that time Commodore brought the model 2031 onto the market. It was basically a 1541 with modified electronics. The differences between the two models is extremely small.  At that time, this already led some resourceful hobbyists to take a close look at the circuit diagram of the 2031 and to make a corresponding adapter. 


UPDATE: This article is obsolete. The current entry can be found here: TriMod CBM Adapter - One Commodore Floppy for two worlds

There are many different variants of this adapter, but they are all based on the same principle or the original circuit diagrams of Commodore. I wanted to rebuild this adapter again.

I could find one of these hobbyists in one of the countless Retro forums. He simply made a small adapter based on the original Commodore schematics of the 2031, which made the missing link from a 1541 to the 2031. What was interesting about this simple Commodore replica, however, was that a hermaphrodite was created here. This adapter was able to switch between IEEE-488 and IEC at the push of a button. Thus the functionality of the 1541 was preserved, but it was possible to switch to the 2031 at the push of a button.

A few years ago, parts of these plans were published on the Internet so that they could be reproduced. Unfortunately, these flyers were not very legible, and also many remarks were not quite clear. But the biggest problem was that there were contradictory versions.

A contact with this hobbyist could be established for a short time, but was rather counterproductive. A basis to decipher the old documents could unfortunately not be found. So I decided to solve the puzzle from single fragments myself. But since the real performance of Commodore was basically achieved, I could also take the actual IEEE-488 adapter from the schematics of the Commodore 2031 floppy. 

After some experiments I managed to create the adapter from the whole single parts and the original Commodore circuit diagram. 


Finally, the main circuit, the IEEE-44 part, consists of the two GPIB drivers SN75160 and SN75161 and the driver 7406. The circuit part is the same as it was invented by Commodore. This reconstruction was simple, because the former author had simply copied this from the original circuit diagrams. The author's only achievement was to integrate the analog switches CD4066 into the corresponding signal lines. Basically nothing special, but already an interesting idea.

The first circuit board was quickly made.

Extensive tests have shown that Commodore did a good job back then. But does the switch also work? So that the 1541 can function as 2031 at all, of course the kernel of the 2031 is still needed. In the Internet one becomes fast f√ľndig. To have the whole thing switchable, you need a standard kernel adapter. On a 27C128 EPROM you burn once the kernel of the 1541 and directly behind it the 2031. Between the adapter board and the kernel board you have to create the connection to switch the kernel.

And the following tests have shown that the switching works. 

Now the whole thing had to get even smaller. So I decided to use SMD components.


The parts list is as follows:


C1-C7 Capacitor 100nF SMD 0603
D1-D2 Diode 1N4148 Mini-Melf
R1, R2, R5 Resistor 3k3 SMD 0805
R3 Resistor10k SMD 0805
R4 Resistor1k SMD 0805
SW1 DIP Switch x2 THT
U1-U2 CD4066 SOIC-14
U3 74LS86 SOIC-14
U5 SN75160B SOIC-20
U6 SN75161B SOIC-20
U7 SN7406DR SOIC-14


Furthermore you need a 40pin IC socket, from which you can remove the bars and pin headers.

The kernel adapter board I didn't go further here, because any switchable version can be used.


This article is now outdated! The article to this current project can be found under TriMod CBM Adapter - One Commodore Floppy for two worlds.


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RuudBaltissen's Avatar
RuudBaltissen replied the topic: #15 1 month 1 week ago
AFAIK only the ROM can be downloaded. I would be more interested in the schematic. Is a bigger size available, please?
Thank you!

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