TriMod CBM Adapter

IEEE-488 floppies for the CBM world are unfortunately very rare and very expensive to get. Why not just take a cheap VC1541 and expand it with an IEEE-488 port, but without losing the serial IEC port. With the TriMod CBM adapter the floppy can do both. It can be connected e.g. to a C64 via the serial connection, or to a CBM 8032 via the IEEE-488 port.

Commodore had on the one hand the professional computer systems for industry, economy and above all research. In order to meet the requirements there, the parallel IEEE-488 interface, which was very widespread at that time, was used. This of course had its price. When the homecomputer market was added later on, it came down to the most economical construction possible. For this purpose the serial IEC bus was created. The corresponding floppy disk drive VC1541 was sold worldwide in large quantities and is still very well used today at reasonable prices.

If you own a CBM computer at home, you will want to buy a floppy disk drive sooner or later. But today the CBM floppy disk drives are almost only available at prices that often exceed the original price at that time.

Now even Commodore had the problem to produce the "big" CBM devices in sufficient quantities. Also these drives were already at that time awarded with a proud price. So Commodore released the drive 2031, which was basically a VC1541 with some modifications. But this drive had an IEEE-488 connector instead of the IEC socket.

If you look at the schematics of the 2031 and the 1540, respectively 1541, there are only a few differences. So it was obvious to simply adapt this. Exact details I had already described in the article Converting Commodore VC1541 to 2031 (switchable IEEE-488 / IEC adapter)

In this article I would like to go into more detail about the rebuild and installation. At the moment I have the board in a SMD version. So the adapter needs little space and can be installed very easily. Since a VC1541 basically makes two different floppies, of course two different kernels have to be used. First the original kernel of the VC1541, where an alternative kernel such as JiffyDOS can be used, and of course the kernel, which also turns a 1541 into a 2031 on the software side.


Kernel Adapter

So that the whole remains switchable also here, still a so-called Kernel adapter with switching possibility is needed. This is nothing special, but the standard adapter from 2364 PROMs to 27C128 EPROMs. Personally I am using the 2364 adapter from RETRO Innovations. But in the end you can also use any other adapter that can be switched.

If required, I can also offer the appropriate adapters or the Gerber files. Please leave a comment about this.

TriMod CBM Adapter Kernel Adapter

On the picture above you can see such an adapter. You can also see the green cable, which is needed for switching. I used the DuPont connectors, which are also used for breadboard experiments. Of course this connection can also be soldered.



The assembly is actually quite simple. Also the number of components is quite manageable.

TriMod CBM Adapter PCB


First the capacitors should be soldered on. They are all 100nF capacitors in 0603 format. After that you can continue very good with the 5 resistors. These are already much bigger with the 0805 format.

Followed by the two diodes the worst is actually already done. The ICs are relatively large for SMD and can be soldered very easily. The best way is to place an IC on the board and fix it with light pressure of a pair of tweezers from above. Danah you first solder a little leg at one corner and then the diagonally opposite one. So the chip is fixed and can now be soldered very easily.

Now solder the IC one after the other. A special order is not to be considered thereby.

If you have too many problems with SMD or don't dare to do so, feel free to contact me. 

Now that all SMD components are soldered, the next step is a pin header, which makes contact to the board of the VC1541. In order to gain some distance and the board does not rest directly on the adjacent parts, the pin header should not be completely plugged in and soldered.

For this purpose I built a quite simple auxiliary construction with the help of a breadboard and a few remaining pin strips.

TriMod CBM Adapter Hilfskonstruktion zum löten

Now you simply put the circuit board on it.

TriMod CBM Adapter Platine auf Kontakte legen

View from the side:

TriMod CBM Adapter seitliche Ansicht Kontaktabstand

The picture shows that the contacts are not completely plugged through, but only a small piece. This is completely sufficient to maintain the necessary distance to avoid contact with nearby components later. Please do not be surprised on the picture that the SMD components are not on it yet. Of course you should solder the pin header only when all other SMD components are already soldered. This picture should only show how the contacts of the pin headers look like if they are not completely plugged through.

However, only the upper pin header shown on the picture may be soldered in. If the pin header is already soldered to the edge of the board, the 40-pin socket can no longer be soldered!

When the pin header is soldered, the 40 pin IC socket is used. Then solder in the second pin header, again with the distance shown above.

The next step is to solder in the socket strip for the IEEE-488 connector. Also here the angled pin header may only be plugged in so far that it is flush with the board. If the pin header is soldered in, it should look like this afterwards:

TriMod CBM Adapter Kontakte zum löten

If the pin header is completely plugged in and soldered, you will later have problems with the female connector of the IEEE-488 cable. The other two connectors should also be soldered in the same way. After that the DIP switch has to be connected. Once this is done, all solder connections should be checked visually to see if there are any solder bridges.

Thus the adapter would be finished.

TriMod CBM Adapter seitliche Ansicht zum löten

On the picture above he can see that on the one hand the pin strips are not completely plugged in, but also the angled pin strip for the IEEE-488 is flush with the PCB at the bottom and thus comes a little further up. So the socket with the ribbon cable fits later without any problems.

TriMod CBM Adapter Platine von oben bestückt

On the left side you can see the IEEE-488 connector. This can either be led out with a ribbon cable or connected to a corresponding socket in the housing. 

The two lower pin headers are for the connection to the kernel adapter and the switch itself. 

  • Contact 1 = GND
  • Contact 2 = RESET
  • Contact 3 = Kernel switching
  • Contact 4 & 5 = switching between IEC and IEEE-488

A standard switch is connected to contacts 4 & 5.

When the two contacts are connected, IEEE-488 mode is selected. Contact 3 is used to switch the kernel as well. This contact is connected to the kernel adapter board mentioned above.

Contact 1 and 2 are currently not required. 


Installation in the VC1541

The TriMod CBM adapter is inserted into the socket UC3 of the VC1541. The 6522 chip is carefully levered out with a screwdriver.

VC1541 Platine Ansicht UC3

 Afterwards you insert this chip into the TriMod CBM adapter and then carefully insert it into the socket UC3. 

TriMod CBM Adapter Ansicht seitlich eingebaut

You can also see the connection cable to the Kernel Adapter. The DIP switch is set to Device ID 8.

The Device ID configurations look like this:

Device ID   SW1   SW2
#8 ON ON
#10 ON OFF

In general it should be noted that the DIP switch on the TriMod CBM adapter board only plays a role for the Device ID in IEEE-488 operation. In the "normal" VC1541 mode, as via the IEC bus, the original solder jumpers on the VC1541 board are still responsible.


Bill of material

The parts list for the TriMod CBM Adapter looks like this:

  • C1 - C7 = 100nf Capacitors, SMD 0603

  • D1 - D2 = 1N4148 Diodes, MiniMELF or SOD-123

  • CN1, JP1 = Pinheader single row angled, RM2,54

  • J1 - J2 = Pinheader 1x20pin, RM2,54

  • J3 = Pinheader double row angled 2x12pin, RM2,54

  • R1 - R3, R5 = 3k3 Resistors, SMD 0805 

  • SW1 = DIP Switch, 2x

  • R4 = 1k Resistor, SMD 0805

  • U1 - U2 = 4066, SOIC-14

  • U3 = 74LS86, SOIC-14

  • U4 = IC Sockel, 40pol.

  • U5 = SN75160, SO-20

  • U6 = SN75161, SO-20

  • U7 = 7406, SOIC-14

The cost for all parts is about 8-10 Euro, depending on where you buy the parts. The IEEE-488 transceivers are the cheapest from Asia. I can recommend UTsource, which can deliver these two components very fast and reliable, very cheap and in large quantities. Otherwise Kessler Electronic is also an extremely reliable supplier, which I can recommend without reservation, at least for the German area.

If you are interested in kits or finished modules, please leave a comment. If there are some, I would organize something accordingly. But also with questions, suggestions or of course also criticism I would be pleased always about a nice comment.

The Gerber files, anyway the IEEE-488 kernel firmware is available for free download in the download area for registered users.



Since I know that not everyone shares my affinity for SMD, I'm in the process of designing a circuit board that does entirely without SMD. Additionally I integrated the kernel adapter directly there. Due to the size of this board, this was the practical continuation to include it.

3D Grafik vom TriMod CBM Adapter

When I receive the board from China, I will also write a small article about it. I'm also working on version 2.0, which has some more features. But I don't want to tell you anything yet. I will then always show between by a little anteasern and pictures.



Who has problems with the assembly, or other questions, can ask these gladly in the comment area, and/or in the forum. If there is a larger number of interested parties, I can also offer a collective order of kits or ready assembled boards. If you are interested, just write a comment, with a reference to a collective order and whether you prefer a kit or a completely assembled board.


I still have some circuit boards left. This can be requested for a small contribution towards expenses: PCBs


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