A500 plus black screen

[Steve_w]

I don't see anything on the serial port.

[Steve_w]

OK so i was assuming you're testing with DiagROM? If not do that. It doesnt need any RAM to output on serial or screen what the issue is.

OVR should be high with the machine held in reset. If its not then there is no way anything else will work. Start there before changing anything else.

There are 2-3 ways to do this. I personally tie off the reset pin to ground but You can also hold down the required keys on the keyboard or tie the KB_RESET line on the A500 keyboard connector.

If OVR is not high then no matter how sure you are of a connection there is a fault. Remember it will go low very very early in the boot cycle so you need to measure all this will reset HELD.

After that use DiagROM rather than doing potentially more damage to the machine.

OVR is permanently high even when I carry out KB_RESET, it stays high continuously. Reset is working as when I carry out a KB_RESET the RST signal momentarily drops low then goes high.

[terriblefire]

I don't see anything on the serial port.

With DiagROM you should always see stuff on the serial port even with broken RAM. 9600 8N1

[Steve_w]

Yes, but I think the ROM code is not being executed which is why the is no serial output.

Speaking to Chucky, he said OVL is set by diagrom code and I should see some initial activity on the address bus before OVL is set.

So I guess I'm not sure why the address bus has no activity.

[stephen_usher]

If there's a clock, power and the reset goes high then the CPU should do something. It should at least put 0 on the address bus and assert /AS to read the first byte of memory as it will attempt four memory reads before starting to execute code.

If /AS doesn't go low with all address lines zero then the CPU is dead.

If it does this and the the /DTACK line on the CPU doesn't follow then it's something to do with the bus arbitration lines.

I would suggest the you set your 'scope to trigger on a falling edge and do a single capture on the /AS pin on the CPU.

[Steve_w]

Hi,

I tested the CPU in a working board, the CPU worked fine in the other working board.

I'll test AS and DTACK tomorrow, I did have a brief look, AS started high then went low, but did not seem consistent so want to test a bit more tomorrow as I could not see activity on AS after a keyboard reset. DTACK seemed to do the opposite to AS....

[terriblefire]

If OVR transitions from HIGH to low the CPU is running. I've no idea why you would not see anything on the screen or serial in that circumstance.

[Steve_w]

OVR stays high though, the CPU is working when placed in another board. Does the CPU assert the OVR signal?

Does the CPU also assert AS? Just wondering if I should test CPU controlled signals on initial power up to try and isolate and ascertain the CPU connectivity in this board is okay.

[terriblefire]

OVR stays high though, the CPU is working when placed in another board. Does the CPU assert the OVR signal?

But you said...

OVL is low and stays low which I believe means the ROM is not being mapped to the start of memory and maybe why the CPU is not doing anything.

Seriously which is it?

OVR gets set low when the CPU pokes the CIA after about 5 instructions.

[Steve_w]

OVR and OVL are different signals, or am I missing something? Sorry if I'm being stupid and missing something obvious.