Sunday, 20 February 2022

The Mini PET 40/80

Introducing the Mini PET 40/80.


This is an improved version of the original Mini PET, and includes all of the optional features of that kit, the colourful deluxe keyboard and perspex baseplate and clear top panel.

The kit is still based on all modern through hole parts. The heart of which is the WDC W65C02S CPU. It is compatible with most software for the Commodore PET from the 2001 series through to the 8032. 32K RAM is provided as standard, the maximum the PET can support (without going into paged RAM). It does not have a 6545 CRTC, so some graphical demos will not run, but pretty much everything else will.

This side hasn't changed much, address decoding, IO and CPU. I swapped out a 74HC10 that used to be there for another 74HC138. It does the same job, but is one fewer part on the BOM.


This section doesn't look very different, but lots of work has gone on here. The video RAM is now 2K bytes, and supports 40 or 80 character modes, with a 74HC257 being used to select the dot clock. Gone is the 74HC74, with the character inversion now hard coded inside the character ROM. There are more DIP switches, as there are more video mode options. For a 40 column display, the CRTC microcontroller has 16 clock cycles to read the character code from the video RAM, looking that up in the font ROM to get a bit pattern and load that into the shift register. For an 80 column display, that is only 8 clock cycles, so the firmware had to be completely rewritten to achieve that. It is now using only 6 cycles per character to allow loops to be used rather than the previously unrolled loop code.

This section is mostly new. Some old school 4000 series logic running at 9V providing soft power on/off, and also controlling the 40/80 column selection. The new style 8 pin regulators replace the 7805 and TIP29s. Sad to seem them go, but would have needed additional MOSFETs to switch those off during standby. The two red LEDs show when the motor power is activated on the two datasette ports.

The Mini PET 40/80 is wider than the previous Mini PET, adding a second datasette connector to the rear. That allows you to use an SD2PET SD card disk drive and a datasette drive without having to unplug anything.


There are more video modes, monochrome composite video in PAL and NTSC, support for 9" or 12" PET monitors and a new 9 way D monitor connector to the side for RGBI/CGA/MDA monitors. I have been surprised that I have not been asked more often if that is VGA. It is not, that would have needed a second crystal oscillator and a faster microcontroller to deal with the line and frame rates required for VGA. Power is from a 9V DC centre negative supply (included in the kit).

There are now four function keys down the side of the keyboard (I wonder where I got that idea from - maybe I should make them orange or grey?). Soft power on / off controls the new power circuitry, the reset button and NMI Menu button are now full size keys and there is a new 40/80 button which changes video mode, changes the ROM set and resets the Mini PET so you can easily switch between 40 and 80 column systems.

The Mini PET 40/80 has it's own version of BASIC, Mini PET BASIC 4.1, adapted from Commodore BASIC 4.0 with a few extra commands, and built in features including a file browser, a full self test and a DOS Wedge which is permanently active (no need for a SYS call or to load from disk). 

The self test can be started from any of the versions of BASIC, or selected via the DIP switches. This will cycle continuously to act as a burn in test.

Pressing the Menu button brings up the built in file browser to allow you to load files more quickly from an SD2PET or other disk drive.

The big change is the 80 column mode. Great for text adventures, programming and business software. Seen here with the optional 2 pixel gap between lines enabled.

The extra space makes text heavy screens easier to read.

But of course, 40 column mode is still the default, ready for all the usual games.


Or new ones

Attack of the PETSCII Robots - The best game ever written for the Commodore PET [citation needed]

Or there are my own PET remakes of Tut-Tut

and 3D Monster Maze


And with 80 column mode, you can use the cheat mode in 3D Monster Maze to get a Pterodactyl's eye view of the maze.

See how the maze is drawn and how Rex moves and even find the exit.

Or fully show off the 80 column mode with Cheese and Chive

The best game ever written for the 80 column PET.


The Mini PET 40/80 is a self assembly kit, not for the novice, some soldering experience is necessary.


To help you along with the process, the kit comes with a lovely spiral bound manual


Being spiral bound is idea so you can lay it flat on the bench as you are working.


The Mini PET 40/80 kits are available now from The Future Was 8 bit


I don't know at this stage if this will follow the Minstrel kits in being the Final Edition. We are will waiting for IC availability will return to previous levels. When I will see this many microcontrollers again?


So if you want one, get yourself along to the The Future Was 8 bit now