Mega ST4 - no video output at all

[frank.lukas]

An atari diagnosis cartridge is needed ...!

I would measure based on the circuit diagram whether there are all data and address connections between the CPU, Glue, MMU and MFP? And whether all control signals at the MMU are particularly there for the four Latches 244/373.

Check the soldering side of the board to cold solder joints and use the leading soldering solder ...

[asapreta]

An atari diagnosis cartridge is needed ...!

I would measure based on the circuit diagram whether there are all data and address connections between the CPU, Glue, MMU and MFP? And whether all control signals at the MMU are particularly there for the four Latches 244/373.

Check the soldering side of the board to cold solder joints and use the leading soldering solder ...

Hello frank.lukas!

Yesterday I have finished checking the all connections between MMU, GLUE and CPU.
Today I will do the MFP ones.

Also, If needed, will remove it and test in my 520ST.
I didn't do that yet as It is soldered on both machines.

Unfortunately I can't get a diags cartridge right now.
In Brazil these stuff aren't available.
I didn't search, but is it possible to build the cartridge using another one? (The board of an Atari ST cartridge)

But by the weekend I can have the DiagRoms installed.

Thanks for your reply.

[stephen_usher]

The diag cart is merely a ROM cartridge. The Exxos one is more complex as it has a larger ROM and jumpers to select different ROM images from it for the different types of machine.

As its a cartridge it needs the machine to be able to access the TOS ROMs and run, I think, four instructions before it tests to see if the ROM cartridge is there and then jump to the code in the cartridge ROM. Unfortunately it's not like the C64 dead test cartridge.

[asapreta]

The diag cart is merely a ROM cartridge. The Exxos one is more complex as it has a larger ROM and jumpers to select different ROM images from it for the different types of machine.

As its a cartridge it needs the machine to be able to access the TOS ROMs and run, I think, four instructions before it tests to see if the ROM cartridge is there and then jump to the code in the cartridge ROM. Unfortunately it's not like the C64 dead test cartridge.

Thanks for the explanation
In my case I do think, even the DiagRoms won't help, but will give a try.

Now, I never asked, do RTC Power and the keyboard must be connected for these testings?

[stephen_usher]

No, there's no need as the machine isn't getting that far to even notice if they're there or not.

Without the keyboard the machine will boot to the desktop but make a lot of "keyclick" noises. Without the clock battery it'll just set the date and time to the default for that version of TOS.

[frank.lukas]

Unfortunately I can't get a diags cartridge right now.
In Brazil these stuff aren't available.

I can give you the Gerber file and the Eprom bin Files ...

DIAG_ST.zip

[asapreta]

Unfortunately I can't get a diags cartridge right now.
In Brazil these stuff aren't available.

I can give you the Gerber file and the Eprom bin Files ...


DIAG_ST.zip

Thank you very much.
I will see if a local PCB maker is cheaper and faster than our Chinese ones.. :D

Meanwhile, can these ROMs be installed internally or do they only run from a Cartridge?

[stephen_usher]

Only cartridge.

By the way, you never said if you tested the chip enable lines on the ROMs to see if they go low. This is a fundamental step as otherwise the CPU will be just reading garbage from the data bus when it tries to read the first four words when it comes out of reset.

[asapreta]

Only cartridge.

By the way, you never said if you tested the chip enable lines on the ROMs to see if they go low. This is a fundamental step as otherwise the CPU will be just reading garbage from the data bus when it tries to read the first four words when it comes out of reset.

Is this the /CE signal?
If yes, this is what I could get when power on or reset the computer in ROM PIN 20, /CE:

WhatsApp Image 2024-10-23 at 18.06.22.jpeg

I had to film, pause and export the frame.

[stephen_usher]

OK, looks like the ROM is being selected then and it's being read a number of times. This is very good news!

Unfortunately without being able to monitor the whole of the data bus and/or address bus with a logic analyser we can't tell if the CPU is getting valid data from the ROMs or the ROMs are getting a valid address.

IF the ROMs are OK then a diag cart with a computer connected to the serial port could well give you useful information.