Toshiba HX-51

Supporter (8)

13-07-2022, 18:54

As promised, I finish the reverse engineering of the above MSX computer. The schematic is in the download repository, in Documents, MSX Engine (there's no location for schematics or other kind of hardware info, or I haven't found it. In case, the administrators can move wherever they want). Now it's here
As in the description, I found some more indication for the T7937 MSX Engine that were not reported in a doc file in brasilian that's the only I found on the net. Two pins are marked as ??? because I don't know the function up to now.
If anyone have any further information, please give to me and I will update the file accordingly.

For that machine, that I'm trying to let it work again, I have some doubt on the original ROM (masked and possibly failed).
Does anyone have a dump of that ROM, because I think it's a little different from the standard Basic ROM? If yes, please send it to me and I will burn an EPROM.

If anyone have a MSX (or others) computers with one or two layers PCBs, I will be happy to reverse engineer it. I already made that job on old arcarde from '80...

By mars2000you

Enlighted (6502)

13-07-2022, 19:03

This machine is emulated by openMSX as Toshiba-HX51I.

It means that the ROM can be found on some specific sites (I can't give a link here).

(filename)hx51-basic-bios.rom
(sha1)829c00c3114f25b3dae5157c0a238b52a3ac37db

By gdx

Enlighted (6223)

14-07-2022, 03:09

Thank you, I added a link in the wiki page.

By AndyPF

Supporter (8)

14-07-2022, 07:25

Ok, so that's the ROM I'm disassembling right now. Thank you for confirmation.

By gdx

Enlighted (6223)

15-07-2022, 02:18

You seem to know this hardware well. Do you think it's possible to modify this MSX so that the RAM is entirely in slot 3? It would make this MSX more interesting.

By AndyPF

Supporter (8)

15-07-2022, 07:17

Unfortunately I think it's not possible. All the address decoding circuitry is fully embedded in the T7937 Engine, and there's no external access to separate the buses in some way.
Looking at the shematic, the RAM has a separate address bus (RA0..7) and the dedicated chip select SCAS that are coming from T7937, I really do not understand why they decided to split the RAM that way.

By gdx

Enlighted (6223)

15-07-2022, 16:56

Okay I see. Everything is in the MSX Engine and the RAM addressing is multiplexed.
So the only solution would be to remove the DRAMs, use an SRAM instead and get the addressing and data signals from the ROM as well as /WR and the one from slot 3-3 that is unused. Is not?

By AndyPF

Supporter (8)

16-07-2022, 10:24

It should be an idea. That pin is not used on HX-51, and seems to be the Slot 3 Page 3.
But being probably only the page 3 select, so only 16K mapped.
Another problem is that the PPI device is inside, and there's no possibility to access the Slot 3 decoding circuitry, so I think is a hopeless desire... It's not only a DRAM problem but that the mapping is fully internally decoded without modification possibilities.