Dumping/reverse engineering MSX models thread

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Par bsittler

Master (217)

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27-04-2020, 08:28

Thanks! I tried to open the case but failed and ran out of time, but I will try again soon following your directions, hopefully in the next week.

Par bsittler

Master (217)

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03-05-2020, 10:02

I was able to open the AX-150 this time, though unfortunately some of the locking tabs were so brittle they snapped apart despite my attempt to be careful. I did not open the RF encoder shielding because I wasn't sure I'd be able to do so nondestructively. Here are some photos taken with my phone — let me know if you want more information on any part, or if more photos are needed:

https://www.dropbox.com/sh/4knmkphn52z88tc/AADX8SZ1LHIYkVXXt...

This MSX is currently in pretty bad condition — it showed signs of previous insect infestation, spills, a fairly ugly power supply repair, and the power switch was placed in reversed orientation. Also the whole thing was really dusty. I tried to gently brush away enough of the dust to at least read markings on the components, but I will need to get some cleaning tools to really clean it up later. Also I discovered the joypad/general purpose port connectors were non-functional because they had been snapped off of the PCB, so I tightened the bolts holding them in place and re-soldered them, and now they appear to work.

Edit: also the left arrow keycap had been pointing rightward

Edit 2: I read elsewhere that this video chip (YM2220 VDP) may produce RGB and CSYNC in addition to composite video (PAL). Is there any information on this signal? Can it be adapted to RGB-SCART somehow? And does anyone know how the RGB signal quality compares to the already-connected PAL signal?

Par Manuel

Ascended (17869)

Portrait de Manuel

03-05-2020, 21:36

bsittler: thanks a lot for the outstanding pictures! Very useful!

Par pgimeno

Champion (288)

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03-05-2020, 21:55

@bsittler thanks a lot indeed! Being in that condition, I'm surprised that the cartridge switches are working. See this thread: https://www.msx.org/forum/msx-talk/hardware/sakhr-ax-150-car... in case you have resetting problems.

Par Manuel

Ascended (17869)

Portrait de Manuel

03-05-2020, 21:57

Patrick wrote:

I have dumped the ROM of my Olympia PHC-2 with the SaveROM 1.2 tool.
The file is here.

Thanks. What is exactly in these slots? Did you use a cartridge to dump it? Which?
In which slot was it? Can you also run MSXMEM2.BAS?
Slot 1 seems to contain RAM and slot 2 a disk ROM of some kind. MSXMEM2.BAS and some more info about your setup can explain how to interpret these dumps :)

The BASIC/BIOS ROM seems to be identical to the Sanyo PHC-28L, its sha1sum is d3af963e2529662eae63f04a2530454685a1989f

Par Manuel

Ascended (17869)

Portrait de Manuel

03-05-2020, 22:12

dproldan wrote:

Got it, thanks! I dumped the ROMS as 64KB files and put them here: https://drive.google.com/drive/folders/1_4Tvv1YcH99fyE8MnQuZrTJE3MhYicwI?usp=sharing

Thanks! Which of these dumps are from which slot?

Par Manuel

Ascended (17869)

Portrait de Manuel

03-05-2020, 22:14

sdsnatcher73 wrote:
xesco wrote:

Dumps of System ROMs can be found here

Added many photos of internals and screenshots of execution results for all the tools. Only missing part is keyboard matrix which requires pressing combinations of keys and taking pictures at the same time which is not possible by myself :)

From your rom dumps and the existing YIS-805/128R2 machine config I created a machine config and systemroms. I bundled them here.

I guess you quoted the wrong post... xesco had VG 8020/40 ROMs. I assume you used the Yamaha YIS-805/256 dumps. But I don't think it's wise to base the machine on the Russian YIS-805/128. It might be quite different, despite the name.

Par bsittler

Master (217)

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03-05-2020, 23:22

pgimeno wrote:

@bsittler thanks a lot indeed! Being in that condition, I'm surprised that the cartridge switches are working. See this thread: https://www.msx.org/forum/msx-talk/hardware/sakhr-ax-150-car... in case you have resetting problems.

Hah! I had that same problem when I first got it, but I think it was due to dust as after repeated activations it seems to work reliably now. I had noticed it was due to the cartridge detection switches as when I first got the machine only slot 2 worked well, but after some repeated trial slot 1 also started working more reliably instead of causing random resets and audio+video "pops". Thanks for the pointer, though! Now I know I can fix it easily either by bypassing the switch, or by replacing it.

Also you're very welcome, and I hope this can lead to a robust emulation of this hardware. I am also curious about the YM2220, is its color palette known? And can it be switched to NTSC and/or 60 Hz somehow? I am somewhat interested in trying such a modification with a switch and two different crystals, and of course interested in possible RGB output as mentioned earlier. I also have a (non-MSX) Sega SG-1000 Mark II, and am interested in the same questions for it, as I understand the VDP there may be somewhat related.

Par Piter Punk

Master (222)

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04-05-2020, 02:32

bsittler wrote:

Edit 2: I read elsewhere that this video chip (YM2220 VDP) may produce RGB and CSYNC in addition to composite video (PAL). Is there any information on this signal? Can it be adapted to RGB-SCART somehow? And does anyone know how the RGB signal quality compares to the already-connected PAL signal?

I put the YM2220 pinout here.

Take a look if your MSX isn't already using the RGB pins to build the composite video. If not, maybe is good to compare the not identified pins to see if it does some kind of PAL/NTSC selection, because in Victor HC-30 the composite output is NTSC.

Par bsittler

Master (217)

Portrait de bsittler

04-05-2020, 06:26

Thanks! I think your implication is right, these signals are used to feed some sort of separate composite (PAL) video encoder, in this case a Motorola MC1377P.

I attempted to decipher my photos and it looks like the R, G and B pins are indeed being used. Not sure I'm reading the resistor color codes correctly, but it looks like each color channel is connected to ground through a resistor (R202, R203, and R204, maybe each is 2.2kΩ ±5%?) and also each is shorted to a separate one of the other pins — 19 is R, 18 is G, and 17 is B. Maybe it's just a "staging area"/routing convenience, though, since you have those all listed as not connected, and from there they connect to a five-wire cable, alongside RFVID and GND (from YM2220 pin 32).

This video cable has five wires laid out like this:
white wire: R
white wire: G
white wire: B
white wire: GND
red wire: RFVID

CSYNC seems to be routed to an unpopulated two-wire connector location, CN215, as is the same GND that is present in the five-wire video cable.

The other end of the five-wire video cable touches down at CN214 in the audio-visual output section of the main PCB. From there the signals (R, G, B, and RFVID) go two different places:

1. To an (unpopulated) video connector section which has labels, traces, and holes for a bunch of small components (resistors, capacitors, etc. — some labelled "N.P" which I take to mean "not populated", though it could instead indicate those components should be present for the video connector for both NTSC and PAL models) and space for some sort of video connector (including a prepared hole in the back of the case, but with a molded-in partial dust cover which is still intact).

2. By way of some capacitors (their labels are unreadable due to a previous spill) to a Motorola MC1377P, which is indeed a PAL/NTSC encoder IC according to its datasheet. I presume from there I could trace the composite video onward to the connectors on the back of the case and likely also into the adjacent RF encoder section.

Given this, it seems RGB output is achievable from this VDP. The simplest way might be to determine what the unpopulated passives in the RGB connector section should be, populate them, and find a suitable connector and install it.

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