A500 plus black screen

[Steve_w]

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

[stephen_usher]

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.

No, the CPU does not assert OVR. That's controlled by one of the PIAs, U7.

[terriblefire]

OVR is irrelevant. Should always be high. Focus on OVL. Does it go high then low?

EDIT: I've realised i've typod' OVR for OVL.. Ignore OVR. Just OVL is important.

[Steve_w]

Ah okay, I'll check OVL again tomorrow morning and report back :)

[Steve_w]

I tested OVL (I tested pin 30 Gary and pin 2 on the Odd CIA) and it is high and stays high at 4.89v. It also stays high on keyboard reset.

I observed the RST signal being momentarily pulled low upon Keyboard reset for the CPU, ODD_CIA, PAULA, AGNUS, and GARY as I wanted to double check the signal was getting through to all the chips it needs to.

[stephen_usher]

Is there any activity on /AS at the CPU?

[Steve_w]

I just checked /AS on the CPU (pin 6), on power on from cold it's high at 4.9v with no activity, if I then do a keyboard reset, it goes low and stays low with no activity.

[terriblefire]

I just checked /AS on the CPU (pin 6), on power on from cold it's high at 4.9v with no activity, if I then do a keyboard reset, it goes low and stays low with no activity.

OK OVL is working. My bet is DTACK isnt working at this point

[Steve_w]

Just checked DTACK

Power on from cold
1. keyboard power LED dim
2. DTACK high for several seconds at 4.95v then goes low to 170mv
3. Sometimes DTACK will repeat step 2 once more before staying low
4. After about 10-15 seconds the keyboard power LED will go from dim to bright green
5. A few seconds after the power LED goes bright the floppy LED will light up and stay lit

Once on power up I saw a brief square wave on DTACK, maybe 3 or 4 waves on the scope before no activity.

I checked R101 and that is reading 27 Ohms

After observing DTACK going low after a number of seconds after powering up the board (sometime it took 6 seconds, sometime as low as 2 seconds), I thought I'd checked /AS again to see if it was the keyboard reset triggering /AS to go low or if it was just coincidence. /AS starts high and goes low by itself after a number of seconds so I think the keyboard reset being a trigger is a red herring.

[stephen_usher]

Well, 4 times would make sense as the CPU is blindly reading the first four addresses to get the set-up data before executing code.

What does the BERR pin on the CPU then do? If the CPU is reading an incorrect value for the address where to start execution then it'll jump to an inappropriate location which cause a bus error.

Which revision of A500 motherboard do you have? Early ones need an adapter and/or motherboard changes before they can read the newer ROMs/EPROMs which would mean that the DiagROM wouldn't work.