2x20 Connector Breakout 2 by Lance Benson (lizby) (HARDWARE, updated 26-05-2021 19:14:12) UPDATE
Experimenter Board

This PCB breaks out the pins in the 2x20 expansion port on the CMM2 with particular emphasis on the ports: COM1 & 2, SPI & SPI2, I2C & I2C2. Eleven individual pins are brought out to 3-pin headers (0V, 3V3, and signal) as suited to the "37 sensors" type kits. More information and links to the EagleCad gerber and .sch and .pcb files are here.

That site also provides an MMBasic program which exercises most of the pins.

The gerber zip file can be sent directly to Five PCBs can be ordered for $2US--probably $10 or so with shipping.

3D case by Jonathan Wright (Neino) (HARDWARE, updated 27-02-2021 12:25:32) UPDATE
Nice Retromax case

Front ports for: audio jack, Micro SD card, power button, reset button, power LED, SD card LED, user LED.
Rear ports for: VGA, USB-B, USB-A, 2 Wii Nunchuck ports.
Snaps together.
Board can be mounted using M2 x 12 mm screws and nuts. Printable washers included.
Printable buttons for power and reset buttons.
This version does not have a port on top for the header pins. A future version is planned with the option of a removable port cover.

Maximite Expansion System v02 by PS labs (HARDWARE, updated 18-01-2022 14:35:57) UPDATE
Maximite Expansion System

PS Labs is preparing to introduce new Maximite Deluxe model called GEN2, since IDC40 pin layout will be changed in GEN2, You have to double check pin layout before connecting. Please refer to image below to find out how IDC40 connector should be placed on older and future (GEN2) models

NONchuk - Joystick/I2C/keypad/rotary interface 1 by CircuitGizmos (GAME/HARDWARE, updated 01-06-2021 22:46:29) UPDATE
HW interface

The NONchuk board provides a way to interface joysticks and other devices to the RetroMax. The intention is to use the I2C port on the NONchuk to connect to the Nunchuk interface on the RetroMax (or CMM2).

The NONchuk allows the Nunchuk port of the RetroMax to connect to joysticks and several other devices. Since there are a variety of different devices that can be connected, the NONchuk uses a MicroMite chip (the '170 with MMBasic installed) to run code for the task.

The NONchuck has a console port that can be used as the serial interface for programming the MMBasic code specific to how the NONchuk will be used.

The NONchuk features:
• Based on MMBasic '170 so it can be user re/programmed
• Support Atari (digital/switch) joystick
• Support PC (analog/potentiometer) joystick
• Operate as I2C slave interface
• Operate as serial interface
• Support 16 key keypad
• Support rotary module
• Support analog joystick module

The NONchuck board could also be used as a stand-alone MicroMite ('170 chip) board. For example it could be used with a joystick and various pins from the microcontroller could control relays, LEDs, etc.