Sony HB-20P
This page was last modified 01:14, 2 April 2018 by Mars2000you. Based on work by Gdx and Rderooy and others.



The Sony HB-20P (P for PAL) is a HB-10P modified for an unknown reason. The monitor connector has been removed. This model was aimed at the Spanish marked only. The keyboard is QWERTY with a ñ key.

The case is available in black. The power supply is external and its connector is proprietary, being similar in shape but different in size to the mini-IEC used in Sony HB-F1 and Panasonic FS-A1 computers. The voltages also are not the same (6V-900mA, AC16V-160mA).

Brand Sony
Type HB-20P
Year 1986
RAM 64kB in slot 3
Media Cartridges
Video Toshiba T6950
Audio PSG (YM2149 integrated in MSX-Engine S3527)
Chipset Yamaha S3527


Sony HB-20P
Sony HB-20P box
Sony HB-20P AC adapter
Sony HB-20P inside
Sony HB-20P rear
Sony HB-20P video board


The HB-20P is a MSX1 with a Z80A from NEC (DA780C-1) or Sharp (LH0080A). The MSX-Engine comes from Yamaha (S3527).

The used VDP is the Toshiba T6950, which does not support some undocumented features of the Texas Instruments TMS9918, TMS9928 and TMS9929 VDP's.

The machine pictures above include a video board with the VDP, the VRAMs and some glue logic. It's intended to allow a different type of VRAM to be used, either PSRAM or SRAM, since the motherboard only accepts 4116 DRAMs. Only some batches of HP-20P feature such daughterboard, while the majority has the T6950 VDP and 4116 DRAMs directly installed on the motherboard. This daughterboard causes absolutely no perceptible changes from the software point of view.

Slot Map

Slot 0 Slot 1 Slot 2 Slot 3
Bank C000h~FFFFh RAM mirror* Cartridge
Slot 1
Slot 2
Bank 8000h~BFFFh
Bank 4000h~7FFFh Main-ROM
Bank 0000h~3FFFh

(*) RAM mirror quirk on early models


  • RF output
  • Tape data recorder connector
  • Centronics compatible Parallel port for a printer
  • 2 general connectors (Joysticks, Mouse, Graphic pad, Paddle controlers, etc)
  • 2 cartridge slots

Quirk on port A8h

Just like the HB-10P computer, some early versions of the HB-20P also have a quirk on port A8h that makes RAM bank 3 appear both in slot 0 and in slot 3. This mirroring effect is emulated in openMSX for the Sony HB-10P, it can be very annoying as it can play havoc with games that need more than 16kB RAM, using the 16kB RAM mirror to store system variables and assuming that all the ram is in the same slot 0 as page 3.

This configuration can cause poorly written software to crash. Nevertheless, you can run these games if you type the following command before loading:

OUT &hA8,240

For cartridges, the trick is to plug them in after booting the MSX using a slot expander that has switches to disable each slot and prevent them from auto booting (WARNING!! Do not insert a cartridge when power is ON. It can damage the machine). Then type the following instructions on BASIC after re-enabling the slot:

OUT&hA8,240: DEFUSR=&H7D75: ?USR(0)

Alternatively, you can use ROM loaders like Execrom, LoadROM or OPFX /X. Since they start the game from MSX-DOS, this problem will be automatically solved as DOS already selects the proper slots for the RAM.

Fixes, Repair and Upgrades