TriMod 2.0 Adapter – “IEC”, IEEE-488 and Parallel Interface for the 1541


As I had already written in this article, the CPLD version of my TriMod adapter had never been intended to function as an aftermarket solution for the 1541.

I had only constructed this prototype as a feasibility study and for the development of the firmware. Therefore, I did not care about the design, which served more practical purposes during development.

The final result has been integrated into my 1541 motherboard for a long time now, so this adapter became obsolete and was not pushed further, at least not in this form.

Until now…

What is that anyway?

As a brief explanation of what this adapter does in the first place:

It provides one or two additional interfaces for the 1541. On the one hand a parallel interface for SpeddDOS is provided in addition to the serial interface.

This is of course nothing special and it can be done easier.

But additionally the 1541 can be switched over, so that this becomes a Commodore 2031LP floppy drive.

The special thing about this floppy drive was that it had an IEEE-488 interface instead of the serial one. And with this the drive could be connected to the “big” business computers from Commodore.

So with this adapter you have the choice at the “push of a button” which drive is needed at the moment.

TriMod 2.0

Some asked me if I could not publish the adapter. Since the original adapter is simply too big…

What you can’t see in this picture is that it’s also built very tall, with the help of some DIL sockets inserted into each other.

Since the TriMod adapter has a total of 3 interfaces, 2 of which are parallel, some I/O lines are needed. Therefore at least one CPLD is needed in the TQFP-100 package, which alone needs some space and unfortunately doesn’t find room between a 40pin IC socket.

Additionally the two drivers for the IEEE-488 (GPIB) bus are needed, as well as the connectors for the two parallel interfaces.

This needs quite a bit of space. I replaced the DIP switches by simple jumpers. This saves space and you can also solder a cable to it and lead it outside.

And then I replaced the IDC connectors with Micromatch connectors. These also save a lot of space, but can be used with the normal ribbon cable, which is also used for the IDC connectors.

Here’s what came out of it:

TriMod 2.0 Adapter

The board has been reduced to the most necessary, and fits now without extra adapter socket in the IC slot UC3 of the 1541.

Installed in the 1541

Here you can also see as an example that you can continue to use the previously used cable, it really only instead of the IDC, a micromatch connector used, or crimped onto the ribbon cable.

Top view

What is still needed, of course, is a kernal adapter that converts from the original 2364 PROM, to a corresponding (E)EPROM, and contains the various versions of the kernel.

On the one hand the kernel for the IEEE-488 mode is needed and on the other hand either the original Commodore kernal, or SpeedDOS. If you don’t want to use the parallel cable for the floppy speeder, you can of course also use JiffyDOS for example.

Pin assignment

Here you can see the assignment of the individual pins. On the one hand there is the interface for flashing the CPLD, which is normally only needed once, or for firmware updates.

On the top left you can see the connectors for the external kernel. In the mode “CBM-Bus” (the normal serial connection, incl. parallel cable) there is the possibility to select a secondary kernel. For example Original and SpeedDOS.

In the “GPIB” mode (IEEE-488) only one kernel is provided, because I don’t know of an alternative kernel. Should something arise there, this can be extended quickly in the firmware.


If you would like to build such an adapter, I have uploaded everything you need on my GitHub account:

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