C64 With Black Screen

Got an issue?
User avatar
eslapion
Member
Member
Posts: 932
Joined: Mon Jul 20, 2015 10:11 am
Location: Canada
Contact:

Re: C64 With Black Screen

Post by eslapion »

I didn't have the energy to sift through the previous posts but a naive question here: Could it be a problem with the drive ?


Wealth, like happiness, is never attained directly. It comes as a by-product of providing a useful service. -Harland D. Sanders

rmzalbar
Member
Member
Posts: 148
Joined: Wed May 08, 2019 9:06 am
Contact:

Re: C64 With Black Screen

Post by rmzalbar »

It could be. C64person has two C64's and two drives (or rather, two logic boards for the drive he has) but hasn't been able to get a working combination by swapping things around. Occam's razor thinks it's a bad cable, but we think that's been checked and two bad logic boards is not outside the realm of statistical possibility. We're currently trying to see if the same signals are being seen on both ends of the line, at the 6522 and the 6526.

C64Person
Member
Member
Posts: 66
Joined: Thu Feb 13, 2020 2:52 pm
Contact:

Re: C64 With Black Screen

Post by C64Person »

I have 2 drives, with one working mechanically (the other not) and the state of the 2 logic boards is not promising. One of the boards looks like it has had a good bit of repair work done (poorly), so I’m a bit less hopeful with that one, though they both yield the same result: a crash whenever trying to access the drive. Both C64’s do the same as well, so I personally suspect the drive boards but have not ruled anything out. I just double-checked the cable for continuity and it’s fine.

In terms of testing with the 1541 on, I have started on that but am definitely not seeing what I see in your pictures. Once I have gotten a little further along, I will share my results.

rmzalbar
Member
Member
Posts: 148
Joined: Wed May 08, 2019 9:06 am
Contact:

Re: C64 With Black Screen

Post by rmzalbar »

FYI, I no longer have access to an oscilloscope (it was a long-term borrow, and I've returned it to its owner. Looking for recommendations on a solid scope for reasonable money!) so I won't be able to retest anything for a while, though the captures I did already should carry us pretty far.

C64Person
Member
Member
Posts: 66
Joined: Thu Feb 13, 2020 2:52 pm
Contact:

Re: C64 With Black Screen

Post by C64Person »

The only pin I actually caught a signal on when running the program with 1541 on was pin 5. Interestingly, the signal seems the same as your image up to a certain point, where it just continues straight. It seems to me that this could indicate it behaving normally and then crashing when it stops following the expected output.

I saw nothing at all on pins 7 or 9, likely because no data was ever sent due to a failure in the handshake (or at least that’s my guess). On pins 6 and 8, I saw nothing until after I pressed RUN/STOP+RESTORE, which probably is not useful but I figured I should include it anyway. I suppose I will start checking signals on the 1541 VIA, but considering the low success with U2, I’m not sure if I will see anything useful.

In terms of the oscilloscope, I’m using a RIGOL DS1054Z. I’m new to oscilloscopes but this one has been working really well for me. I looked up the pricing and it seems reasonable. Just a recommendation but I hope it’s helpful!

U2 Pin 5
U2 Pin 5
Pins 6 and 8 after pressing RUN/STOP+RESTORE.
Pins 6 and 8 after pressing RUN/STOP+RESTORE.

rmzalbar
Member
Member
Posts: 148
Joined: Wed May 08, 2019 9:06 am
Contact:

Re: C64 With Black Screen

Post by rmzalbar »

I may be able to get the scope back shortly, or acquire one anyway. The Rigol is on my list for a closer look. I read that a lot of features are locked away behind an aftersale license upgrade, which initially scared me away, but I intend to give it a thorough consideration when I get a chance.

Sorry, I've been wondering what to try next. Bottom line.. when I run the program with nothing plugged into the IEC port, I get an immediate result of NOT PRESENT. There is no freezing at all, always a ready prompt. I suppose it's best to try it with the cable unplugged completely to rule out any effect from being plugged into a powered off device.

So if your C64 is acting differently I don't think we need to consider the drive yet. Are the data in/out lines sitting at the expected levels when the C64 is just doing nothing? 5,6, and 7 low, 8 and 9 high?

Also, do you still have a 2nd C64 and is it also freezing when the program is run with nothing plugged into the IEC?

There is a logic IC sitting on ATN, CLK, and DATAOUT pin 7, an inverter. The output of DATAOUT on the IC is going back to DATAIN pin 9 of the CIA, so if there's a fault it might have those two lines in disagreement. Basically 9 and 7 should always be in the opposite state during this test, pin 9 should be high when 7 is low, and low when 7 is high. You can hook up two inputs and capture their states together...
Last edited by rmzalbar on Thu Aug 06, 2020 10:24 pm, edited 1 time in total.

C64Person
Member
Member
Posts: 66
Joined: Thu Feb 13, 2020 2:52 pm
Contact:

Re: C64 With Black Screen

Post by C64Person »

Actually, it freezes when the drive is ON and plugged in; if the drive is off or unplugged from IEC, I do get a NOT PRESENT and READY message.

I do have both C64’s and both have the same results all around, which is why I suspect the drive.

Unfortunately, I can’t test because I have had no power in several days, but once it’s back up I will continue.

I have seen no locked features with my oscilloscope, though I borrowed it so I have no idea if the buyer had issues with this.

rmzalbar
Member
Member
Posts: 148
Joined: Wed May 08, 2019 9:06 am
Contact:

Re: C64 With Black Screen

Post by rmzalbar »

Then I would try this...

Since some of the lines do change levels once before freezing up, that should still be enough to trace the signal from the 6526 in the C64 over to the 6522 in the 1541. You should see the same level change on the corresponding pin at the 6522. This is where you can try out hooking up two input probes at once for a nice comprehensive view of what's happening on both ends at the same time.

If all level changes at the C64 are seen at the 1541 end, then we can see if a response is being generated by the 1541 and if those signals are making it back. SOMETHING must be happening in response, and the freeze is just the C64 waiting for the data promised by the 1541's response.

I found this guide which I've just started to look at: IEC Dissected

http://www.zimmers.net/anonftp/pub/cbm/ ... al-bus.pdf

Post Reply Previous topicNext topic

Who is online

Users browsing this forum: No registered users and 2 guests