AX-230 photos/ROMs - How to update WiKi and BlueMSX?

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By Tabajara-Labs

Expert (85)

Tabajara-Labs's picture

13-06-2018, 22:17

Manuel wrote:

Thanks, all is fine now. Just wondering what is the serial number, for reference in the XML file. Then we know on which machine the file was based.

S/N: 00 AA432877

By Manuel

Ascended (14468)

Manuel's picture

13-06-2018, 22:22

Oh, what is the thing with the CPU clock that is stated on the Wiki? Are we sure about that, because it means the thing is not MSX compatible, I think...

By Tabajara-Labs

Expert (85)

Tabajara-Labs's picture

13-06-2018, 22:33

Manuel wrote:

Oh, what is the thing with the CPU clock that is stated on the Wiki? Are we sure about that, because it means the thing is not MSX compatible, I think...

Good question, I've never seen it before.
I have only one design reference (with proper schematics and even a service manual) of a computer with the T7967A chip, which is the Expert Plus from Gradiente. In the manual, it says the /clock line (pin 133 if I'm not mistaken) is an OUTPUT and in my schematic, it goes from inside the msx engine to the slots.

In this ax230 computer, /clock line is an INPUT (an oscillator composed of a crystal + 74HC02). Also, this clock signal goes to slot.

I BELIEVE the different clock is used because the video output is encoded inside the msx engine, and this 22.168.0xx crystal is divided by 6 and used to create the PAL subcarrier inside the engine. The exact value differs a bit from the PAL standard used in Kuwait, but there is little details avaiable about the color system used where this computer was sold (which I ASSUME it was Kuwait). So, I'd not classify it as a "not MSX compatible", but would insert a remark inside the XML with the cristal value and the detail of the 2 clock generators.

By Manuel

Ascended (14468)

Manuel's picture

13-06-2018, 23:19

But if it can output the correct clock, why couldn't it then also run the CPU at this clock?

Or am I totally missing the point? Perhaps you can explain again in all detail, from the start?

By Tabajara-Labs

Expert (85)

Tabajara-Labs's picture

13-06-2018, 23:32

No problem, we can try again Big smile

Lets say (in a basic way) that we have two clock "users" in the T7967A, one is the "master clock", lets say this way, which is the 3.579546MHz (NTSC subcarrier frequency, save this name). The other is the NTSC color encoder, that takes the RGB+Sync signals and encode it to Composite video in NTSC. MSX was created "around" the NTSC subcarrier frequency, because it is easy enough to have ONE clock generator, and this clock generator distributes the clock signal for all subsystems on the board: VDP, CPU, slots, etc.

But in some places, the composite output isn't NTSC. It is a different beast with different subcarrier frequencies.

So, in european PAL/SECAM computers, you usually have a 3.579575 MHz clock for the digital part of the computer (CPU, slots, VDP, etc) and ANOTHER clock (4.4xxx, I don't remember the exact frequency, sorry) for the color encoder. It is used ONLY on the color encoder.

In case of the MSX engine, the color encoder sits INSIDE of it. So, a master clock of 21.477270 MHz is divided by 6 and, here you are, color subcarrier frequency for everyone Smile One crystal commands all the circuits inside the MSX Engine.

BUT what happens when you have a different color system?

Well, I've never seen it on a computer with the T7967A. But now, analysing this computer, I know. There is a (22.something, sorry, I'm on bed and I don't have my writtings handy) which generates the correct color subcarrier frequency for PAL, and ANOTHER clock generator that generates the 3.579545MHz that our CPU, our slots and our beloved MSX peripherals hope to see.

So, it mantains compatibility. The CPU/VDP/Slots/Etc runs at 3.579545MHz and the PAL video is generated with the correct 4.something subcarrier frequency, and everyone gets happy Smile

Got it? If you don't, ask me that I'll try to explain in other ways. English isn't my first language so I may make some mistakes on the way Big smile

By Manuel

Ascended (14468)

Manuel's picture

13-06-2018, 23:37

OK, so the CPU clock is nothing special?
I ask because the Wiki article says currently:

Quote:

The CPU clock is something strange, the crystal used is 22.1680 MHz, which gives us a clock of 3.6946 MHz (instead of 3.579 MHz).

But from you I understand there is an extra clock generator for the CPU (etc.), to get the normal 3.58Mhz...

By Tabajara-Labs

Expert (85)

Tabajara-Labs's picture

14-06-2018, 00:20

Yep, this is right. AFAIU the cpu clock is derived from the /clock input, being the usual 3.579545MHz. The higher clock is used for PAL video encoding.

Is there a timing test that can be run on MSX1 machines?

By mars2000you

Enlighted (5350)

mars2000you's picture

14-06-2018, 00:34

I guess the best is to remove this very technical info from the Wiki page about the AX-230.

If someone wants to write an apart Wiki page about the difference between the CPU clock (and the ways to 'generate' it) with the specific ways to get an higher clock for the PAL video encoding, he's of course welcome.

By gdx

Prophet (2157)

gdx's picture

14-06-2018, 00:49

About RGB connector what are the type and the pinout?

By Tabajara-Labs

Expert (85)

Tabajara-Labs's picture

14-06-2018, 14:12

gdx wrote:

About RGB connector what are the type and the pinout?

Standard RGB pinout

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