EPCOM 80 Columns Cartridge Schematics4 min read
The Sharp HB-4000 was a cartridge released in 1987 by Brazilian Sharp that expanded MSX 1 computers to have 80 column in text mode. It was based on a V9938 video chip (the same used on most MSX 2 computers) and was produced with 16kB VRAM. The official product name in Brazil was “Expansão 80 Colunas“, model HB-4000.
Contrary to the Gradiente CT-80E and C-80NET cartridges, it does not have a ROM with BIOS. Even then, it strictly follows the MSX standard by using the I/O port 88h.
This cartridge only works in systems that recognize its presence. It is automatically found and enabled by both HB-DOS (MSX-DOS compatible) and HB-MCP (CP/M compatible) operating systems, released by Sharp.
Before B@SS reproduce its schematics and make adjustments to it, there was no way to access the 80 column text mode from MSX-BASIC. It would require users to extend their MSX 1 computer with the HB-3600 drive interface and a compatible drive before using this cartridge.
This schematics was produced and tweaked by B@SS MSX in 2009, and published in his blog here.
This version of the 80 Columns cartridge by EPCOM was designed to operate in the auxiliary video port 88h and 8Bh. That is the main reason it will only display images when used together with the HB-DOS and HOTPLAN, as above explained.
However, it is possible to make it compatible with most MSX computers by cutting and isolating the PIN 5 of the IC4 and grounding it (by connecting it direct to the GND). It will change the cartridge’s behavior to operate in both addresses, thus becoming identifiable by most MSX.
However, to use it on the Brazilian MSX Gradient Plus and DD-Plus machines, a small additional modification is required by installing a 10K resistor connecting the PIN 25 of the VDP to the VCC.
As MSX 2 and superior machines already have 80 columns by design, this cartridge will not add any special functionality to those machines. Nevertheless, it is possible to use this cartridges on an MSX 2 or superior as far as the memory banks are fully replaced because those machines’ BIOS require at least 64Kb of VRAM (Video-RAM) to turn on.
Despite its limitations, it is a simple and beautiful project anyone can build, either for learning or for fun. Enjoy.