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Re: MEGA ST 4 - blank screen boot
Posted: 24 Apr 2026 08:15
by olivier.jan
So your machine is PAL but outputs an NTSC signal, meaning the ROM doesn’t execute or doesn’t reach the code where it switches to PAL, very early on in the boot process.
You can try the diag cart, using auto run mode of the sidecartridge (
https://docs.sidecartridge.com/sidecart ... _emulator/).
The image is available in the side cartridge only ROM repository, you want a Diag ROM for ST, not STE, Mega STE or TT.
Still, I’m not sure you will get anything if the machine doesn’t start at all: did you check Reset line ? Did you check ROM or CPU data/address lines for activity?
Re: MEGA ST 4 - blank screen boot
Posted: 24 Apr 2026 09:58
by macsonny
olivier.jan wrote: 24 Apr 2026 08:15
So your machine is PAL but outputs an NTSC signal, meaning the ROM doesn’t execute or doesn’t reach the code where it switches to PAL, very early on in the boot process.
You can try the diag cart, using auto run mode of the sidecartridge (
https://docs.sidecartridge.com/sidecart ... _emulator/).
The image is available in the side cartridge only ROM repository, you want a Diag ROM for ST, not STE, Mega STE or TT.
Still, I’m not sure you will get anything if the machine doesn’t start at all: did you check Reset line ? Did you check ROM or CPU data/address lines for activity?
So tried putting the dial image on the SideCart and using .autorun in ROMS folder. Still grey screen.
Have checked every data and address line on 68000 and I can see something happing driving 5 volts. Can't see the signal exactly as I don't have scope right now.
How do I check the reset line? On the 68000 it's showing 5 vdc.
Re: MEGA ST 4 - blank screen boot
Posted: 24 Apr 2026 11:14
by rubber_jonnie
macsonny wrote: 24 Apr 2026 09:58
How do I check the reset line? On the 68000 it's showing 5 vdc.
Press the reset button while monitoring the RESET pin on the 68000. The voltage should drop to 0v then go back up to 5v when you release the reset button.
Re: MEGA ST 4 - blank screen boot
Posted: 24 Apr 2026 11:25
by macsonny
rubber_jonnie wrote: 24 Apr 2026 11:14
macsonny wrote: 24 Apr 2026 09:58
How do I check the reset line? On the 68000 it's showing 5 vdc.
Press the reset button while monitoring the RESET pin on the 68000. The voltage should drop to 0v then go back up to 5v when you release the reset button.
RESET checked and drops to 0v on reset and then goes back to 5v.
Re: MEGA ST 4 - blank screen boot
Posted: 24 Apr 2026 11:38
by olivier.jan
For the diag cart, check it’s working as intended by plugging it in another ST if you have one.
If it works fine in another ST, you need to check using serial port on the Mega 4. Easiest way to do that is to connect the ST serial port to a PC or other RS232 capable device, open a serial terminal on the PC and connect it to the proper serial port. If the ST is actually running code you should have some information on the serial console.
If you don’t have an RS232 capable device(recent PCs don’t), you can still use an RS232 to USB cable or a Serial to USB adapter. Be careful, the Serial to USB adapter must not be plugged to the ST serial port, you have to connect it directly to pins 8 and 9 of the ACIA (6850) inside the ST.
Re: MEGA ST 4 - blank screen boot
Posted: 25 Apr 2026 10:32
by MasterOfGizmo
olivier.jan wrote: 24 Apr 2026 11:38
Be careful, the Serial to USB adapter must not be plugged to the ST serial port, you have to connect it directly to pins 8 and 9 of the ACIA (6850) inside the ST.
The ST's serial is done by the mfp68901. The two acias are for midi and keyboard. Connecting to them will not help here unless the diag cart is able to communicate over midi which I doubt.
Furthermore, connecting directly to one of these chips would cause your serial adapter to drive against the rs232 line receivers inside your st. You probably won't cause damage that way, but it's still not a good idea.
To summarize: just use a usb-rs232 adapter and connect it to the regular rs232 port of your st.
Re: MEGA ST 4 - blank screen boot
Posted: 25 Apr 2026 11:50
by macsonny
olivier.jan wrote: 24 Apr 2026 11:38
For the diag cart, check it’s working as intended by plugging it in another ST if you have one.
If it works fine in another ST, you need to check using serial port on the Mega 4. Easiest way to do that is to connect the ST serial port to a PC or other RS232 capable device, open a serial terminal on the PC and connect it to the proper serial port. If the ST is actually running code you should have some information on the serial console.
If you don’t have an RS232 capable device(recent PCs don’t), you can still use an RS232 to USB cable or a Serial to USB adapter. Be careful, the Serial to USB adapter must not be plugged to the ST serial port, you have to connect it directly to pins 8 and 9 of the ACIA (6850) inside the ST.
I confirmed the SideCart is booting into diagnostics ROM on my Atari 520ST.
I was playing around with a heap of different TOS's I have. I noticed no matter what TOS I used, it booted the exact same way. In addition, if I booted with no TOS ROMs it also booted the same way.
Make me think it's not even getting to loading the TOS?
Waiting for a Diag cart and cable to arrive from Exxos. Hopefully not too long to wait :-)
Re: MEGA ST 4 - blank screen boot
Posted: 25 Apr 2026 12:21
by olivier.jan
MasterOfGizmo wrote: 25 Apr 2026 10:32
olivier.jan wrote: 24 Apr 2026 11:38
Be careful, the Serial to USB adapter must not be plugged to the ST serial port, you have to connect it directly to pins 8 and 9 of the ACIA (6850) inside the ST.
The ST's serial is done by the mfp68901. The two acias are for midi and keyboard. Connecting to them will not help here unless the diag cart is able to communicate over midi which I doubt.
Furthermore, connecting directly to one of these chips would cause your serial adapter to drive against the rs232 line receivers inside your st. You probably won't cause damage that way, but it's still not a good idea.
To summarize: just use a usb-rs232 adapter and connect it to the regular rs232 port of your st.
You are totally right it’s the MFP, still pin 8 and 9 but of the MFP. I’ve been troubleshooting too many keyboard issues recently it seems.
Re: MEGA ST 4 - blank screen boot
Posted: 25 Apr 2026 13:20
by macsonny
Dumb question but is the serial port the modem port (male) or the printer port (female)?
Re: MEGA ST 4 - blank screen boot
Posted: 25 Apr 2026 14:18
by rubber_jonnie
macsonny wrote: 25 Apr 2026 13:20
Dumb question but is the serial port the modem port (male) or the printer port (female)?
Modem.