Projetos MSX: Expanding MSX Sony HB-F1 internal memory to 512k

Hello everyone!

All MSX computers that use MSX Engine S1985 are easy to expand, including Sony HB-F1.

So, with the help of Alexandre Tabajara and his post here, I´ve expanded my Sony HB-F1´s internal memory.

I chose to use a SRAM since it doesn´t require extra DRAM refreshing circuitry, and the S1985 has already all internal circuitry in order to provide a total of 512k of mapper ram.

For starters, the logic behind it is that you will need to connect a SRAM to the MSX bus by using "standard" connections:

A0 to A13 of the SRAM connected to the MSX address bus
D0 to D7 of the SRAM connected to the MSX data bus
/OE of the SRAM connected to the /RD pin of the Z80
/WE of the SRAM connected to the /WR pin of the Z80
A14 to A18 of the SRAM connected to pins MA14 to MA18 of the S1985
/CS of the SRAM connected to the slot you want to allocate the RAM at. In HB-F1´s case, it connects to slot 3.3 (slot 3, subslot 3) of the S1985

The catch: The S1985 has pins with dual functions, which are configured at the boot sequence according to keyboard signals. At startup conditions, only 256k of mapper are allowed, unless X5 receives GND for at least 0.2 seconds at startup (this line originally has VCC with a pull-up resistor). With GND at startup, the MA18 line is provided by the S1985, thus allowing for 512 Kb of mapper.

List of components:

512K SRAM (AS6C4008 recommended).
Transistor BC548
Diode 1N4148
Resistor of 1 kΩ x 1/8W
Resistor of 4.7 kΩ x 1/8W
Capacitor of 220uF

Datasheets from Alexandre Tabajara´s webpage:

S1985 Datasheet

AS6C4008 Datasheet

First step: Remove both DRAMs 4464 from the computer´s PCB.

Second step: Fold up the following SRAM pins: 1, 2, 3, 32, 31, 30, 29, 24 and 22.

This is the resulting appearance (first picture from Alexandre Tabajara´s post):

Third step: Place the SRAM directly on top of the main ROM (piggyback), and directly solder all pins that are not folded up (picture from Alexandre Tabajara´s post):

All pins that are folded up will be soldered directly either to the Z80 or to the S1985.

The final result will be:

Pin 24 of the SRAM (/OE) connected to pin 21 of the Z80 (/RD)
Pin 29 of the SRAM (/WE) connected to pin 22 of the Z80 (/WR)
Pin 22 of the SRAM (/CE) connected to the disconnected pin 56 of the S1985 (/SLT33)
Pin 32 of the SRAM (VCC) connected to +5 Volts (pin 28 of the ROM)

Hint: Instead of disconnecting pin 56 of the S1985, as the Sony HB-F1 has jumpers all over the PCB, simply cut/disconnect J8 jumper (the right most jumper on the frontal side of the S1985) and solder its upper position to get a direct signal from pin 56 of the S1985 (/SLT33), as shown on the pictures bellow:

Fourth step: Solder pins 3 to 7 of the S1985 - MA18, MA17, MA16, MA15 and MA14 (picture from Alexandre Tabajara´s post):

Fifth step: Solder MA18, MA17, MA16, MA15 and MA14 from the S1985 to the SRAM:

MA14 to pin 3 of the SRAM
MA15 to pin 31 of the SRAM
MA16 to pin 2 of the SRAM
MA17 to pin 30 of the SRAM
MA18 to pin 1 of the SRAM

After these steps/connections, if you turn the computer on and run a memory test program (as TESTRAM) it should show 256 Kbytes of Mapper RAM.

Sixth step: In order to activate the MA18/KBDIR line, thus activating 512k of memory mapper, you will need a circuit that sends out GND to keyboard´s X5 pin at the boot sequence time.

The RESET line can be found on a jumper the main PCB:

Also, the X5 pin can be found on back side the keyboard connector (RB105). On the back side of the PCB, it is the third pin from right to left, coming directly from pin 78 of the S1985.

The circuit was assembled using the following diagram: