Home » RBC Forums » General Discussion » MSX2 Compatible Computer Project
| Re: MSX2 Compatible Computer Project [message #6858 is a reply to message #6857] |
Wed, 04 December 2019 14:32   |
Sergey
Messages: 236
Registered: October 2015
Location: Portland, OR
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Jordi,
It shouldn't be any issues with using 74HC670 as long as the other ICs you're using are CMOS. Particularly Z84C00 CPU and U37 74AHCT32.
The reason I mention 74HC670 in the BOM is that it is four times cheaper than 74HCT670 at Mouser ($1.69 vs. $6.90). I have no idea why a simple CMOS logic IC would cost that much...
Omega uses a fairly low clock, so you can use 74HCT instead of 74AHCT.
Low power TTL ICs (74ALS, 74F) should work too. Just do not mix them together with 74HC parts. Also if you're installing U43 (e.g. for PAL clock), it must be a CMOS part.
Even 74LS should do... but if you use them throughout the board the power consumption will go up.
The MSX standard specifies that the MSX cartridge interface should use TTL levels, and that is the reason for using 74F there. I presume 74HCT would work just as well...
Thanks,
Sergey
[Updated on: Wed, 04 December 2019 14:35] Report message to a moderator |
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| Re: MSX2 Compatible Computer Project [message #6866 is a reply to message #6862] |
Mon, 09 December 2019 16:41 |
Sergey
Messages: 236
Registered: October 2015
Location: Portland, OR
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Senior Member |
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jordi.solis wrote on Sun, 08 December 2019 07:19
Is there possible you mean R30 and not R3?
You're absolutely correct. I'll fix the documentation.
jordi.solis wrote on Sun, 08 December 2019 07:19
Another question, I would like to test NTSC and PAL. Is actually needed to plug those components out?
let me know.
You can keep these components for NTSC. Just move the JP5 and JP6 jumpers. Although, for a longer term use, if you decide to stay with NTSC (and you're not using V9938 VDP), I would recommend to remove U43, so that it doesn't generate extra noise.
Note that the recommended values of R9 and C91 differ between NTSC and PAL. With that being said NTSC values worked for me with PAL frequencies, but perhaps I'd get better video quality when using the recommended PAL values (I haven't tested that).
Thanks,
Sergey
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| Re: MSX2 Compatible Computer Project [message #6916 is a reply to message #6915] |
Fri, 10 January 2020 11:45 |
Sergey
Messages: 236
Registered: October 2015
Location: Portland, OR
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Senior Member |
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jordi.solis wrote on Fri, 10 January 2020 08:39
I think there is already partial compatibility and could be expanded by updating the bios.
I found a page showing how to update a Phillips NMS 8245, which is a model very similar to the NMS 8250 (or so I think).
I hope someone helps me update the current omega bios.
This is the page mentioned:
https://www.msxinfo.net/tag/plus/
Of course all the V9958 VDP stuff described on the page is irrelevant, since it is possible to install V9958 directly in Omega.
I think the easiest route would be grabbing an existing MSX2+ BIOS, and trying to run it on Omega.
There are several possible issues with such approach:
1. All MSX2+ machines have memory (RAM) mapper in Slot 3-0, Omega has it in slot 3-2. It should be possible to slightly modify U13 SPLD code to change the mapping.
2. MSX2+ machines have 0xF4 I/O port, one of the purposes is to indicate hard or soft reset. I am not exactly sure what MSX2+ BIOS would do if it can't read back this port. It might be possible to patch the BIOS to ignore it 
3. MSX2+ normally have MSX-MUSIC (OPLL), it is not a part of the MSX2+ standard, but perhaps the lack of it will confuse the BIOS.
For starters we can try MSX2+ C-BIOS, it shouldn't be sensitive for any of the issues above
Quote:
I failed to get PAL video for SCART and I will retest via RGB to SCART. Is it expected that the video quality will be better when connected by RGB?
Yes, I'd expect a better video quality using SCART/RGB. I don't have a TV set with SCART, so I can't really test. But, I've tested PAL timing (note, it requires PAL-specific MSX BIOS), and obviously RGB signals are properly produced, otherwise CXA1645 wouldn't work.
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| Re: MSX2 Compatible Computer Project [message #7203 is a reply to message #7202] |
Sat, 07 March 2020 13:38 |
jordi.solis
Messages: 119
Registered: March 2017
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Senior Member |
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Hi,
If at first they worked but they stopped doing it, three times, I would not try another chip until locate the fault.
Check all the tracks on that chip until you find a scratch, a cold weld or something making contact between its pins.
If you can't find it, try turning it on without that chip and observe its pins with an oscilloscope.
Check that no chip heats up, and all voltage levels.
If your power supply has +12 and -12 try connecting only + 5v
Check for anything inside the slot connectors, clean it with a brush.
If you have a cooling spray, spray on the circuit and power on to locate something with an abnormal temperature.
It is also possible that after checking the circuit you have a CI in an inverted position or with one of its bent pin.
Without further testing, I would not insert another memory chip, which has broken a third chip, breaks a fourth.
You can also replace all the chips you have with others, eliminate all that are optional, or change positions that are the same to find any changes.
Nothing changes if you don't change anything.
To have a different result, try a different source.
I hope this helps.
J S
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| Re: MSX2 Compatible Computer Project [message #7206 is a reply to message #7202] |
Sun, 08 March 2020 12:16 |
Sergey
Messages: 236
Registered: October 2015
Location: Portland, OR
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Senior Member |
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Hi Borut,
What type/manufacturer of the 8255 you are using? I've been using Intersil CP82C55A, and I am wondering if using an older NMOS part would create any issue... It shouldn't. It only used to control the keyboard and primary slot selection. Both are not time-sensitive and either one should work fine if you are using 74HCT/74AHCT/74LS logic. There potentially might be an issue if you're using NMOS 8255A with 74HC or 74AHC logic.
The computer should not destroy SRAMs...
I personally suspect power supply issue. Try using a different power supply 5V 3A or so. I recommend against using cheap USB chargers. They don't filter the output...
128K or 512K should work fine with the modifications you've mentioned.
I don't think MSX1 BIOS will boot. The VDP initialization is a bit different, and, possibly MSX1 will not work with paged memory (C-BIOS actually might support that). What is the reason for using the MSX1 BIOS?
Cartridges - again, if you're are using 74HC or 74AHC for the buffers/transceivers, that might cause issues, especially if the cartridge uses older NMOS ROM or TTL logic.
Thanks,
Sergey
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| Re: MSX2 Compatible Computer Project [message #7262 is a reply to message #7260] |
Mon, 16 March 2020 12:01 |
Sergey
Messages: 236
Registered: October 2015
Location: Portland, OR
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Senior Member |
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Hi Nicholas,
Thanks for sharing the photo of your build! I like the keycaps, they look very 80's
The type of 74HC670's shouldn't matter, as long as your CPU and U37 (74HCT32) are CMOS. They don't have much to do with cartridge's operation anyhow.
I don't have these particular cartridges, I do have Metal Gear, Vampire Killer, USAS, and Aleste, and all are working fine. The only issue I observe is with Metal Gear and C-BIOS, for whatever reason this combination, makes Omega reboot. But it works just fine with MSX2 BIOS.
I am wondering if any of these cartridges require +/-12V? Another simple thing to try is replacing the power supply, check if you have 5V/3A or so power supply around that you can use.
Cheers,
Sergey
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| Re: MSX2 Compatible Computer Project [message #7264 is a reply to message #7263] |
Mon, 16 March 2020 16:59 |
Sergey
Messages: 236
Registered: October 2015
Location: Portland, OR
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Senior Member |
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jordi.solis wrote on Mon, 16 March 2020 15:36Hi everyone,
I have another problem. When I use the NTSC configuration I can connect a composite video TV to SCART or super video to SCART.
The video signal is not perfect but you can see something on the screen. I can also connect DIN-8 to scart and then the signal is wonderful.
On the other hand, if I configure the board in PAL mode, I can only get a video signal using the DIN-8 connector because with the other connectors I don't receive a video signal.
DIN-8/SCART uses Composite for sync, so if Composite doesn't work, RGB will not work either. I suspect something if wrong with the SCART adapter (I suspect you are using one to connect S-Video or Composite to your TV). Can you try using a different input on the TV (e.g. use RCA connectors, or S-Video instead of SCART with an adapter), or a different monitor / TV?
With that being said, I've tested S-Video and Composite in both PAL and NTSC modes. I don't have an RGB/SCART input, so I haven't tested that.
By the way, if you switch between NTSC and PAL, CXA1645 would need slightly different values of R9 and C91. PAL worked for me with NTSC component values, but the video quality wasn't that great.
Quote:
Also, for some reason, some games (or all) cannot be loaded with PAL settings (or perhaps I would say with PAL BIOS). For example Galaga restarts constantly, while with NTSC it always works).
Games are normally written for either NTSC or PAL, but not both. That is because the periodic interrupt frequency is different (60 Hz vs. 50 Hz), and games commonly use this interrupt for timing. I don't think it would make a program or a game fail, but the timing might be off, e.g. the game will feel "slower".
Quote:
Finally, the best thing about the PAL BIOS is to have a keyboard configuration that matches the European keys. If someone knows in which area of the BIOS the character map is located, I will try to make an msx2 + BIOS with PAL character map.
Not 100% sure, but perhaps just change the value of bits 0-3 at address #002C in the BIOS
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