Åtta Bitar

Hardware project

© 1994,2000 Peter Karlsson. English translation © 2000 Peter Karlsson. Published in Åtta Bitar issue 5/1994.

Commodore C128D is a very nice design, with both the computer and the drive in the same box, with a common power supply. The only bad thing is that it is not possible to change the device number of the drive, which makes it impossible to connect an old model 1541, or load programs that require a 1541 as device 8, or have a C1581 as device 8.

By Peter Karlsson

This, however, can be changed (why else would I have written this article..?). NB! If you are not used to soldering and handling electronics, you should not attempt this, as it could damage the computer. I disclaim all responsibility of what you do, and remember to turn the computer off before you do anything + that it is best if you yourself is connected to ground. OK, let's open the box. Right underneath the upper left corner of the drive board, there is a IC named 65C22. Two of the pins on the upper left part of this IC (the third and fourth counted from the uppermost side) is connected to two soldering tags, which in turn are connected to the C114 capacitator (which is connected to ground). These tags are right below and to the right of U114.

It is best to first connect the switches, before cutting the tags. Doing that, the chance for static electricity is minimized.


The switches should be connected in such a way that they always let the current flow past a resistor at 4.7 kohm, but if you switch it over it short-circuits the resistor (see figure). This makes sure that the current always flows past here, and that you can turn the switch while the computer is turned on. The switches are, however, only read at reset and when the computer is turned on. I have installed my switches in the lower front (which makes them easily accessible without disturbing the design).

One side of each switch should be connected to each soldering tag, while the other should be earthed, i.e connected to the C114 capacitator. When you have connected the two switches, you should cut the soldering tags. Now re-assemble the computer and - voila - you can now switch the device number.

You can select the device number as follows:

Device number 8:Both short-circuited (as before).
Device number 9:Switch 1 via resistor, switch 2 short-circuited.
Device number 10:Switch 1 short-circuited, switch 2 via resistor.
Device number 11:Both switches via resistors.

These settings corresponds to the dip switch settings on the 1541-II and 1571.

Needed components:

Cost ca SEK 70

[Detail of C128DCR circuit board]

This article in Svenska (Swedish).

Return to the article index