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Welcome to the Comspec 40/80 Switcher Project Page! Commodore's first models were 40-column machines that played mostly games. Later they released 80-column "Business" machines. In order to cut down on costs they designed the "Universal Dynamic PET" motherboard that was used in both types. These motherboards were able to be configured for either 40 or 80 columns... but NOT at the same time. Soon after, add-on boards were released to allow you to choose the mode via a simple switch. One such board was made by Comspec, a Canadian Commodore dealer in Toronto. This is a project to clone the Comspec 40/80-S2 board. I own a working board currently installed in a CBM4032.
Once the board is cloned I would like to enhance it to allow multiple Editor ROM's and also to allow switching via software without needing to re-boot the machine. I already have a Mult-EditorROM board, so this will combine it with the 40/80 switching.
The board works by mounting on the Universal Dynamic PET motherboard's Address Jumper Block. It also has flyouts to various points on the motherboard. There is also some pin cutting needed. Basically, there are 4 multiplexer chips that are controlled by a switch. These multiplexers act like jumpers, modifying the PET video generation circuitry as needed. The board also controls one address line of the Editor ROM to select between two different Editor ROMs - One for 40 column and one for 80 column depending on the state of the switch. Finally, whenever the switch is changed the computer is automatically reset in order to activate the proper Editor ROM routines.
In the revised/enhanced board my goal is to add a connection to the Editor ROM socket. This will eliminate 3 flyouts (+5V, GND, and ROM select). Then I will add the Mult-EditorROM capabilities and the allow the Editor ROM to control the 40/80 setting via software. I will need to add additional control for auto or manual 40/80 switching in conjunction with multiple ROMs.
I converted my hand-drawn board diagrams into a schematic which was entered into Kicad and routed. The Gerbers were sent to DirtyPCB.com and have arrived! I am in the process of populating and testing the boards.
Here is the CBM 40/80 Switcher schematic diagram and PCB:
Kicad files and gerbers are available on Github here.
Coming soon.
Coming soon...
In the TORPET Newsletter, Issue 7 there is an article on doing the 40/80 conversion yourself. That's great, but at the end of the article the author suggests advanced users can do it another way. I'm intrigued with the possibility of having 2K ram available in 40 column, and possibly a simpler circuit than the Comspec board cloned here. Unfortunately (for me) there is not enough information to figure out exactly how it's done. If anyone has any ideas please let me know. You can read the article here.
Page Created: Jul 15/2017. Last updated: Aug 6/2021, 9:20pm EST
Send comments or feedback to Steve Gray(sjgray@rogers.com)