TriMod CBM Adapter – One Commodore Floppy for two worlds

Introduction

Commodore had on the one hand the professional computer systems for industry, economy and above all research. To meet the requirements there, the parallel IEEE-488 interface, which was very widespread at that time, was used. Of course, this had its price. Later, when the home computer market was added, it was important to have the cheapest possible construction. Therefore the serial IEC bus was created. The corresponding disk drive VC1541 sold worldwide in large quantities and is still very good to get used at low prices.

If you have a CBM computer at home, you will surely 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.

Now even Commodore had the problem to produce the “big” CBM devices in sufficient numbers. Also these drives had a high price even then. So Commodore released the 2031 drive, which was basically a VC1541 with some modifications. But this drive had an IEEE-488 connector instead of the IEC connector.

If you now look at the schematics of the 2031 and the 1540, or 1541 differ only in very few details. So it was obvious to simply adapt this. I already described the details in the article Commodore VC1541 to 2031 (switchable IEEE-488 / IEC adapter).

In this article I want to go into more detail about the retrofitting and installation. At the moment I have provided the board in a SMD version. So the adapter needs little space and can be installed very easily. Since basically two different floppies are made from one VC1541, of course two different kernels have to be used. One is the original kernel of the VC1541, but you can also use an alternative kernel like JiffyDOS, and of course the kernel that turns a 1541 into a 2031.

Kernel Adapter

To keep the whole thing switchable, a so called kernel adapter with switching capability is needed. This is nothing special, but the usual adapters from 2364 PROMs to 27C128 EPROMs. Personally I use the 2364 adapter from RETRO Innovations. But in the end you can use any other adapter which can be switched.

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

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.

Assembly

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

First the capacitors should be soldered on. They are all 100nF capacitors in 0603 format. After that you can continue very well 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 is to put an IC on the board and fix it with light pressure of tweezers from above. Then solder one leg to one corner and then the diagonally opposite one. Now the chip is fixed and can be soldered easily.

Now solder the IC one after the other. There is no special order to follow.

Who has too much problems with SMD or does not dare to do it, can contact me.

When all SMD components are soldered, the next step is to connect the pin header to the PCB of the VC1541. To gain some distance and to avoid that the board is directly on the neighboring parts, the pin header should not be inserted and soldered completely.

For this I built a quite simple auxiliary construction with the help of a breadboard and some leftover pin headers.

On top of it you simply put the board.

View from the side:

On the picture you can see that the contacts are now not completely through, but only a small piece. This is perfectly sufficient to get the necessary distance to avoid contact later with nearby components. Please don’t be surprised on the picture that the SMD components are not yet on it. 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 when they are not completely plugged through.

But only the upper pin header shown in the picture may be soldered. If the pin header is soldered to the board edge now, the 40pin socket cannot be soldered afterwards!

When the pin header is soldered, the 40 pin IC socket is soldered. And after that the second pin header is soldered in, again with the spacing shown above.

Next, the female header for the IEEE-488 connection is soldered in place. Again, the angled header should only be inserted so far that it is flush with the board. When the header is soldered in, it should look like this:

If the header is completely inserted and soldered, you will have problems with the female connector of the IEEE-488 cable later. The other two pin headers should be soldered as well. After that the DIP switch has to be soldered. If this is done, all solder connections should be checked again by visual inspection, whether no solder bridges have been created.

Now the adapter is ready.

On the picture above you can see that the pin headers are not completely inserted, but also that the angled pin header for the IEEE-488 is flush with the board and thus comes a bit further up. So later the socket with the ribbon cable fits without problems.

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 once for the connection to the kernel adapter and the switch itself.

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


A standard switch is connected to contact 4 & 5. Connecting the two contacts switches to IEEE-488 mode. Contact 3 is used to switch the kernel then as well. This contact is connected to the Kernel Adapter board mentioned above.

Contact 1 and 2 are not needed at this time.

Installation in the VC1541

The TriMod CBM adapter is inserted into the socket UC3 of the VC1541. The 6522 chip has to be removed carefully with a screwdriver.

Then insert this chip into the TriMod CBM adapter and insert it again carefully into the socket UC3.

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
9 OFF ON
10 ON OFF
11 OFF 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 mode. In “normal” VC1541 mode, as over the IEC bus, the original solder jumpers on the VC1541 board are still responsible.

Parts list

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 = pin header single row angled, RM2,54
  • J1 – J2 = male header 1x20pin, RM2,54
  • J3 = pin header double row angled 2x12pin, RM2,54
  • R1 – R3, R5 = 3k3 resistor, SMD 0805
  • SW1 = DIP switch, double
  • R4 = 1k resistor, SMD 0805
  • U1 – U2 = 4066, SOIC-14
  • U3 = 74LS86, SOIC-14
  • U4 = IC socket, 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 cheapest from Asia. I can recommend UTsource, who 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 there is interest in kits or even finished modules, you can best leave a comment. If there are some, I would organize something accordingly. But also with questions, suggestions or naturally also criticism I would be pleased always about a nice comment.

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

Outlook

Since I know that not everyone shares my affinity for SMD, I’m currently designing a board that does not use SMD at all. Additionally I integrated the kernel adapter directly. Because of the size of this board, this was the practical continuation to include it.

When the board arrives from China, I will 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 reveal anything yet. I will then always between a little anteasern and show pictures.

Update (2021-03-30) / Download

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