C64 With Black Screen

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C64Person
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Re: C64 With Black Screen

Post by C64Person »

U2 pin 6
U2 pin 6
U2 pin 7. This is the only one that triggers, but it triggers the moment I touch the lead to the pin, and no change when running the program.
U2 pin 7. This is the only one that triggers, but it triggers the moment I touch the lead to the pin, and no change when running the program.
I apologize for my lack of oscilloscope background. I’m learning as I go!


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Re: C64 With Black Screen

Post by rmzalbar »

It looks like your time scale is set to 200 nanoseconds per division. I'm using 2.5 milliseconds per div, which means you are zoomed in 12.5x relative to me, and the signals would be too large for you to capture the whole message. You could fit maybe one pulse at most. So you're seeing a different type of activity altogether. From a glance it looks like you're seeing address or data activity, but we shouldn't be seeing anything on these pins unless IO is occurring on the serial port.

Make sure you are looking at the correct chip and the right pins. U2 is the CIA on the right, and the pins are numbered counterclockwise from the notch end - 1-20 on the left side top to bottom, 21-40 on the right side bottom to top. If you're reading the right side instead of the left for example, you might be reading pins 32-40 and the address lines to the CPU are there. If you're reading the wrong CIA you might be seeing keyboard polling signals.

Another possibility is that you are reading noise from elsewhere in the board. What are you attaching the probe's grounding point to?
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Re: C64 With Black Screen

Post by rmzalbar »

Also, remember to reset the trigger capture after you connect to the pin. There would naturally be a spike on first contact with the pin.

Your pin 7 screenshot looks like straight up noise. None of the logic on the board operates at 7 mhz.
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Re: C64 With Black Screen

Post by C64Person »

Yeah, that time scale was too far zoomed in. Looking much better now; pins 5, 6 and 8 all look almost exactly like yours. That's a good sign! Pictures below.

The ground lead is attached to the heatsink for VR2 on the C64, which should be fine. I didn't realize how to reset the trigger, so that was my other issue. I'm definitely looking at the right chip though.

Unfortunately, now I'm not even getting a signal on pins 7 and 9. No trigger, or any onscreen activity when firing the program on these pins. I know that I'm just looking for activity on these pins, but is there something I need to do differently here?

U2 Pin 5
U2 Pin 5
U2 Pin 6
U2 Pin 6
U2 Pin 8
U2 Pin 8
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Re: C64 With Black Screen

Post by rmzalbar »

Pins 7 and 9 of the CIA in the 64 are supposed to show no activity with the 1541 turned off in this case. Pin 9 is active-low, so it normally will sit at 5V when there's no activity.

With the 1541 turned on, the C64 will see the 1541 acknowledge the handshake, so you'd then see outgoing data on pin 7, which would also be seen (but inverted) on the incoming data pin 9.
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Re: C64 With Black Screen

Post by C64Person »

Well, it's been a while, but I finally got around to looking at the signals on pins 7 and 9 of the CIA. With 1541 off, I'm seeing nothing on pin 7 (which makes sense) but 2.7V on pin 9, which is lower than the 5V at which it should be. I'm still not 100% sure that I am using the oscilloscope right for these two pins (I used the same trigger settings as you did and reset the capture before running the program and it didn't trigger here), so I used the multimeter to check voltage.

I tried testing these with the 1541 on, but of course the system freezes when running the program and I'm not seeing any action from the oscilloscope on these pins.

Does any of this indicate anything?
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Re: C64 With Black Screen

Post by eslapion »

C64Person wrote: Tue Jul 28, 2020 3:06 am Well, it's been a while, but I finally got around to looking at the signals on pins 7 and 9 of the CIA. With 1541 off, I'm seeing nothing on pin 7 (which makes sense) but 2.7V on pin 9, which is lower than the 5V at which it should be.
Normally, NMOS and TTL chips can't signal higher than about 4.3V which is 5V minus the voltage drop of one P-N junction (a silicon diode is the simplest form of P-N junction). So 5V it should not be.

If you read 2.7V on pin 9 then clearly there is something pulling down but remember in TTL levels, signaling a logic high is guaranteed to provide you with 2.4V or higher and this guarantee is met with 2.7V.

NMOS chips have a weird characteristics which encouraged them getting replaced with other technology. When signaling a logic low, they can sink as much as 15mA but when signaling a logic high, they can source about only 100uA or one hundred times less.
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Re: C64 With Black Screen

Post by rmzalbar »

You won't see anything triggering on pins 7 or 9 with the 1541 off, because with the 1541 off, there will be no data activity at all. Pin 7 will sit low (near zero) and pin 9 will sit high. 5v nominal, but as eslapion says anything above 2.4 volts should be valid. It may be that with the 1541 plugged in but turned off, it's loading the data line pin 9 causing the voltage to drop like that, which would not be a cause for concern. You could unplug it and see if you get closer to 5v.

In normal use, it's recommended if you have a chain of devices plugged into IEC (multiple disk drives) that you turn all of them on for reliable operation, and that's because the powered off devices might load down the signal lines too much.
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Re: C64 With Black Screen

Post by C64Person »

Unplugging the 1541 raised pin 9 up to 4.39V, so I'm good with that. So given this, I think I can be mostly confident that U2 is properly trying to make contact with the drive.

Now I will try to see what the signals on both devices look like with the 1541 on, but as you suggested earlier, this may be difficult considering the program freezes when the drive is powered on.
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Re: C64 With Black Screen

Post by rmzalbar »

Right. Well.. you should have the probe connected and the trigger armed so that when you hit Return, you catch the first message even if it freezes. The idea is that one of the messages isn't getting through either on the handshake or the data side.

If it freezes with the 1541 on, then my guess is that the handshake is at least partially working on the ATN/CLK lines, so the computer sees this and is now waiting for the next part of the message that never arrives (which may be the data line.) Anyway, the freezing shouldn't really change your testing except that you have to RUNSTOP+RESTORE between attempts.
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