Demystifying the Brazilian MSX Plus & DD-Plus

Both machines have the MSX Engine SMT chip in its core, combining three chips in one and replacing a fair amount of extra discrete components that in previous Expert versions were main sources of bad connectivity problems in the socketed microchips.

However, an unforeseen problem related to the MSX engine chip is that it mapped the main RAM to a internal secondary Slot. It should have been a non-issue as it is predicted in the MSX standard, but due to lack of proper documentation in the country most Brazilian software ware written assuming all MSX computers would have similar stot configurations to the old Experts and Hotbits (both are machines without secondary Slots). Such misleading assumption resulted in many programs simply not functioning as expected in the MSX Plus and DD-Plus, either rebooting the computer or entirely crashing the system. For the same reason, incompatibility issue also impacts those programs preventing them from running on MSX machines produced overseas. 

The DD-Plus uses a internal disk controller board that was quite similar to the external cartridge interfaces sold in the Brazilian market at the time. It uses 16Kb for MSX DOS, resulting in 48Kb of ROM (probably the reason why the Slots configuration changed). The physical drive itself was very good quality, but for many technical reasons related to the interface itself it was painfully slow and many users opted to use external FDD interface drives instead.

The board configuration switches and jumpers

In 2009, B@SS (Rafael Santos Lopes) pulled information from two board revisions of the PCI-483, revisions A1 and B, then wrote a small tutorial and published in his blog. These information were somewhat important to allow some level of maintenance in those MSX and also could potentially solve some of the challenges encountered with badly written software, including converting a Plus board into DD-Plus and change the internal Slot configuration, among other possibilities.

Please, notice some information differs from one revision to the other (which will be highlighted).

The Main Board Jumpers

• 5j257: Sets the OE of the EPROM (CI-12) in /CS1
• 5j258: Sets the OE of the EPROM (CI-12) in /CS2
• 5j259: Sets the OE of the EPROM (CI-12) in /CS12
• 3j114: Connects the Pin 27 of the EPROM (CI-12) to the VCC
• 15j261: Connects the Pin 27 of the EPROM (CI-12) to the A14 of the MSX
• 5j233: Connects the OE of the EPROM (CI-12) to the /CS(X)
• 3j113 (Rrevision A1) or 5J113 (Revision B): Connects the OE of the EPROM (CI-12) to the Pin 17 of the CN9
• 35j104: SLOT 1
• 25j108: SLOT 2
• 25j109: SLOT 3.3

The CN9 Interface Pins

The FDD Interface Jumpers (DD-Plus only)

• OFF: 15J233, 5J258, 5J259 and 15J261
• CONNECTED: 5J257, 3J114 and 3J113 (or 5J113)

The Demo ROM Jumpers (Plus only)

• OFF: 3j113 (or 5J113), 5j257, 5j259 and 5j261
• CONNECTED: 5J258, 3J114 and 15j233

Exclusive on Revision B Boards

• R27: 220 Ohms
• CN 13: LED CAPS (Pin 106 of the MSX Engine SMT chip)