ASCII-X FlashROM cartridge

Copyright 2024 Laurens Holst

Project information

Open and low-cost FlashROM cartridge design for large cartridge games.

The ASCII16-X cartridge for MSX is an extension to the common ASCII16 mapper. It provides extended memory capacity, FlashROM memory which allows for persisting save games, mirroring of pages and bank select registers across the entire address space, and is backwards compatible with existing software.

The primary goal of this project is to provide the MSX community with open hardware implementations of ASCII mappers with these extended capabilities, using only commonly available parts, and aimed at optimising manufacturing simplicity and cost. As such, this project also contains actual cartridge hardware implementations built with plain logic ICs.

The following cartridge designs are currently available:

These designs are made with KiCad 9.0.2. Their dimensions target the Overrich cartridge cases. Board width is kept under 10 cm to allow for cheap PCB fabrication. The surface mount components are also all on a single side for cheaper PCB assembly, as well as simplified hand-soldering using a stencil.

Media

ASCII16-X Mapper XL 8 MB

Mapper specifications

ASCII16-X

See ASCII16-X specification for documentation.

Cartridge hardware designs

ASCII16-X Mapper Extra Large (XL) 8 MB

This design is suitable for large ROM cartridges of either 4 MB or 8 MB.

Since FlashROMs above 2 MB are only available in 3.3V versions, it requires circuitry for voltage translation between the 5V MSX bus and the 3.3V FlashROM.

The design utilises an Infineon 8 MB FlashROM which is currently the most economical choice, though it also supports smaller capacities. The board has 3 logic ICs for the mapper logic, and 5 additional logic ICs as well as an LDO for the voltage level translation.

PCB fabrication & assembly

Bare and assembled boards can be ordered from online services such as PCBWay. It is recommended to use the assembly service offered by the factory, since their assembly fee is very affordable, and they may also be able to source components at better prices.

Shared community project

For convenience the designs are available as PCBWay community projects, batches of (assembled) boards can be ordered directly from them and part of their fee will be donated to yours truly. Feel free to use that service if you wish.

Cost considerations

The PCB fabrication and assembly cost depends on many factors, such as batch size, component cost, PCB manufacturer fees, tariffs, and several other factors. However as a rough indication of what to expect; a small batch of around 20 XL units produced in March 2024 cost around US$ 15 apiece (ex. VAT). An estimation of the cost for a batch of 200 units amounts to about US$ 10 apiece (ex. VAT).

Of course for the complete package of a final product there are also additional costs to consider, such as cartridge cases, box, manual, etc.

Recommended PCB fabrication settings

For prototyping in low quantities you can get boards cheaply by selecting the HASL finish, however for distribution you should use ENIG finish even if the PCB cost is higher (roughly 1.5x depending on quantity). Gold plated EDGE fingers are very expensive, so that option is not recommended, ENIG is sufficient.

Recommended assembly service settings

When you get a quotation for the assembly, it’s worth cross-checking the parts cost against prices on Digikey or Mouser. You can ask to procure parts from those distributors if there is a significant difference, especially for the expensive parts like the FlashROM.