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Welcome to the PET/CBM 8296 40/80 Column Switcher Page! Commodore sold both 40-column and 80-column PET models. The 40-colum machines were for home use and the 80-column machines were more for business. The main difference between them were obviously the number of columns on the screen, the screen editor, and also the keyboard. The early PET models were dedicated 40 or 80 column machines but eventually, to reduce costs, they designed a Universal Dynamic PET (UDP) motherboard that could be configured to either 40 or 80 columns... but not both at the same time. Third-party add-on boards were eventually released that could do both.
Commodore also did not have an Editor ROM that could handle both 40 and 80 columns at the same time. The 40 and 80 column screen editors work differently and use different memory locations for critical functions. This is why the original PET switchers such as the Comspec 4080 board required the machine to switch ROMs and reset. My EditROM Project allows anyone to build their own Editor ROM by selecting which features they want, including 40/80 switching, built-in wedge, support for different keyboards, and more.
The 8296 and 8296D models were the last PET machines, and Commodore redesigned the motherboard to be smaller, use less components and have more RAM than the UDP board. This motherboard was installed in a brand-new "SK" (separated keyboard) case that were based on the CBM-II line. The motherboard could also be installed in the older PET cases. Although these were only sold as 80-column machines, the 40/80 configuration feature was retained, but was implemented differently, and so requires a new switcher board design. This is a project to design a simple to install PCB to do just that.
This single PCB will mount on the 40/80-column multiplexer chips as well as the JC configuration area. It has a 3-pin MODE header that can take an ON-OFF-ON switch or a single jumper that can select 80 or 40 column, or software control (pending!). When software control is selected, a single flyout wire must be connected to a pin on the CRTC controller. Installation requires de-soldering the two multiplexer chips and 6 configuration jumpers and replacing them with machine pin sockets. The two multiplexers can be installed on the switching board.
In order to work in both 40 and 80 columns you either need to install two compatible Editor ROMs, or use a custom EditorROM that handles 40/80 switching in software. Having two separate Editor ROMs is best for compatibility. To do this you will need to combine the two ROMs into one and connect the ROM Select line to the upper address of the ROM. The 40/80 switch will then select the mode when you power the machine on. You can also use a Multi-ROM adapter and manually switch it to the appropriate ROM. It is also possible to connect the ROM Select line in place of one of the DIP Switches.
A custom Editor ROM gives you flexibility at the expense of some compatibility. A custom ROM can add built-in wedge, keyboard reboot, custom power-on screen, Escape Codes, Colour, and other features.
NOTE: The ability to switch modes while the machine is on does NOT seem to work, unlike my 40/80 switcher for the earlier machines. I believe this is due to the way the 8296 machine accesses memory multiple times during one CPU clock cycle. I don't know if this issue can be fixed, so in the mean time the board is limited to switching modes WHILE THE POWER IS OFF!
The 8296 machine has the "B" keyboard in normal or DIN versions, and 50 or 60 Hz screen refresh. The original EditorROM will work exactly like normal in 80 column mode. In 40 column mode you will need to use a special EditorROM for the B keyboard. These can be generated with my EditorROM Project. You can use one of my Multi-ROM boards, and my Multi-ROM Builder utility to combine both ROMs (or more) into one Multi-ROM.
The 8296D is a special case. It uses a different monitor which requires different timing for the CRTC controller. You need to get the specific 8296D 40-column EditorROM, or build your own customized one with additional features if desired.
This board was specifically designed for the 8296 motherboard (ASSY#324645) and is NOT compatible with any other PET motherboard. The motherboard looks like this:
(click for bigger picture!)
Kicad schematics and gerbers can be found here.
The version 1.0 PCB is working with manual 40 and 80 column switching. This means you have to power down the machine to change 40/80 mode. Software 40/0 switching did not work. I believe this is a hardware issue. Perhaps faster multiplexer chips are needed. More testing is required. The design files and gerbers have been released on Github. Standard and custom Editor ROMs are available on my EditorROM github.
Page Created: Apr 6/2026. Last updated: May 9/2026, 3:00pm EST
Send comments or feedback to Steve Gray(sjgray@rogers.com)