MD 32X RAM Cart

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The MD-32X-RAM-Cart and MD-32X-RAM-Cart-32Bit are experimental SRAM cartridges for the Sega 32X that load game binaries and utilities from the Sega CD.

Usage

Normal Genesis Games

Boot from Sega CD > Load ROM > Verify ROM > Switch to MD > Power Cycle Console

32X Games

Boot from SegaCD > Load ROM > Verify ROM > Write Protect Switch > Switch to MD > Reset Console

Creating a Sega CD Boot Disc

Example ISO: MARS Diagnostic program

To setup your environment to build CDs:

Download and compile Chilly Willy's Sega MD/CD/32X devkit. You can do this by:
Opening a terminal in the "sega-toolchain-12.1/sega" folder using WSL or Ubuntu and running:
sudo apt install build-essential

sudo apt install libgmp-dev libmpfr-dev libmpc-dev

sudo apt update && sudo apt install genisoimage

sudo make
Then change directory to the Demo folder
sudo make
Put ROMs in the Roms folder.
sudo make cd
The iso will be created in the demo folder.

To build another CD:

sudo make clean

sudo make cd

Rinse, wash, and repeat

SRAM Write Protection

How the Write Protection Works

Strobe Interruption (B28/B29): The circuit severs the LDS (Lower Data Strobe) and UDSW (Upper Data Strobe Write) lines from the cartridge edge. These signals are required for the CPU to initiate a "Write" command to the SRAM.
The 10k resistors act as pull-ups to +5V (A2/A31). When the switch is moved away from the "Normal" position, these resistors pull the SRAM's strobe pins High. In logic terms, a High signal on these active-low pins means "Do Not Write."
Write Protect Switch Positions:
- Pos 1 (Normal): Completes the circuit between the console and the SRAM, allowing the game to save normally.
- Pos 3 (Hard Lock): Disconnects the console's write signals and relies on the pull-up resistors to keep the chips in a permanent "Read-Only" state.

Component Role in Protection

CY62167ELL Pin 11: In a write-protect context, this connection is used to manipulate a specific address bit or a chip enable line to "lock" the memory at a hardware level, ensuring the 32X and the 68000 CPU cannot modify the bits.
Pull-up Resistors: These are essential because "floating" pins (pins not connected to anything) can pick up electrical noise and trigger accidental writes. 10k ohms is the standard value for a stable pull-up on Genesis hardware.

SRAM Write Protect Modification for 32X Games

A manual write-protect switch can be added which toggles the SRAM on the SegaCD32X cartridge. This switch physically disconnects the control signals and pulls them high, effectively locking the save data on the two CY62167ELL chips to prevent accidental overwriting.

To modify the current PCB to load 32X games, use two DPDT switches as shown below. One will switch between the Sega Genesis and the Sega CD, the other will toggle Write Protect.