MEGA ST 4 - blank screen boot

[macsonny]

Ok. So got my ERxxos diagnostic cart today. Have checked the jumpers are for Atari ST and have connected the serial cable from the Atari printer (Serial) port to a PC and using TeraTerm with all correct settings and the PC sees the serial cable port.

Boot the Atari and nothing on the terminal.

What now?

FYI - I tested the cart and terminal output on a working Atari ST and everything is set correctly

[exxos]

I stopped using a lot of programs on the PC, I cannot remember why, I just went with the standard putty and works fine with the diagnostic cartridge.

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[rubber_jonnie]

Ok. So got my ERxxos diagnostic cart today. Have checked the jumpers are for Atari ST and have connected the serial cable from the Atari printer (Serial) port to a PC and using TeraTerm with all correct settings and the PC sees the serial cable port.

Boot the Atari and nothing on the terminal.

What now?

FYI - I tested the cart and terminal output on a working Atari ST and everything is set correctly

If you have truly connected to the parallel port you won't see anything.

How do you have Teraterm set? There are details of the required settings here: Basic troubleshhoting - Diagnostic Cart.

You should be set to 9600 baud, 8 bit, no parity and 1 stop bit. If not you likely won't see anything, and of course if your PC serial port isn't working then you'll have the same issue.

You can change the baud rate later in the diag cart menu but need the above set to begin with.

Also, if you are running the Mega of just 5v then your serial port won't work either as 12v is needed for serial.

If there is still nothing then it's going to be down to good old fashioned troubleshooting I'm afraid.

Is the machine stock and un-modded at this point? I can't recall, but you should aim to get the machine running as standard before looking at any problems with mods.

[macsonny]

SO managed to get some testing done today. Confirmed Y1 and Y2 operational. I did a bit of testing per the diagnostics manual for U2 and everything seems ok there. Also got the right readings on the 68000 pin 17 (HALT) and pin 15 (CLOCK) and I think it's 8 MHz.

Have also checked PIN 22 to be high.

Also saw a few other forums on U2 so checked pins 2,3,4 all see to behaving fine.

What's next?

[macsonny]

I've been doing some further diagnostics after getting myself ran oscilloscope.

Here's where I'm at:

1. Verified the 8 MHz clock is working fine
2. Check PIN 17 (HALT) of the 68000 and it was LOW - which indicates the CPU is not operating.
3. Have replaced 68000 CPU with known good CPU.
4. Have replaced both GLUE and MEMORY CONTROLLER chips with known good units (pulled from working 1040 ST)
5. Have checked all chips for heat - all cool to touch.
6. Have re-socketedd all chips in sockets.
7. Have tried different TOS ROMs that are known to be good.
8. Im now wondering if it's the 68901 or DMA chips?

I'm supposed to check the two inverters on the RESET circuit. Can someone please advise how I do that?

Thanks

[russellnash]

That is a good question. I replaced the 74LS05 on several boards where I was having problems because they are still readily available to buy. I have never seen a Mega ST schematic but from the 520ST schematic you can test the output of the inverters pins 1-2, 3-4, 11-10 and 13-12, but the output can be pulled low from the CPU. I never could confirm a problem with the reset circuit.

[PhilC]

If your DMA is removable, then remove it, then power up the ST, the reset circuit should now operate correctly. On press goes low, then about half a second later /reset and /halt should now go high.

Obviously you'll not get it to boot without a DMA but at least you'll know if the reset works or not.

[macsonny]

So removed the DMA and get the same result of a white screen.

Assume that means it's something to do with the reset circuit then?

[PhilC]

So removed the DMA and get the same result of a white screen.

Assume that means it's something to do with the reset circuit then?

Did you measure the halt and reset lines as id suggested? Press and hold reset and they go low. After releasing them it should go high and stay there with the dma removed.

[macsonny]

If your DMA is removable, then remove it, then power up the ST, the reset circuit should now operate correctly. On press goes low, then about half a second later /reset and /halt should now go high.

Obviously you'll not get it to boot without a DMA but at least you'll know if the reset works or not.

Ok. With the DMA removed I could confirm the /resset and /halt go low when the reset is pressed and then both return to high when the reset is released.

When I put the DMA back in, following the same process, /halt goes from low to high, but /reset stays on high even when then reset button is pressed.

Does that help?