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Apple Lisa 2 I/O board problem - garbage on the screen

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greniu:
Dear all,
I am restoring my old Apple Lisa 2/5 I/O board. I have a problem that I cannot get a correct boot image after turning on the Lisa but instead the screen displays garbage. During the turn on Lisa one beep appears (sometimes two short beeps). The board worked correctly before. I have a second good I/O board and after inserting it into the same Lisa there are no problems. So the issue is definitely somewhere in I/O board. Maybe someone knows this problem and knows hints where I should start with measurements on the oscilloscope?
Screen from boot process below. Thanks

sigma7:

--- Quote from: greniu on January 21, 2025, 04:01:21 pm ---... the screen displays garbage. During the turn on Lisa one beep appears (sometimes two short beeps).
I have a second good I/O board and after inserting it into the same Lisa there are no problems. So the issue is definitely somewhere in I/O board.

--- End quote ---

Since it works with a different I/O Board, I agree the CPU and RAM are (probably) ok.

Since the board used to work, progressing corrosion damage may be a likely suspect, so a visual examination of the area where the battery used to be, down towards the card edge may reveal something.

IIRC, one beep means the CPU didn't find any RAM, which, if caused by the I/O board, suggests a bus contention issue. I imagine there are quite a few possible causes, so another Lisa with similar symptoms might have different causes.

A clue is that the CPU can control the I/O board enough to generate a beep, which suggests the address and data bus are at least partially working for writing data.

The video circuitry reads the screen directly from RAM, so in cases where the 68k CPU is not running but the address and data bus are working, the screen can have a steady but random image. In your case, is the screen image changing/moving, or is it steady with one random image?

If it is changing, then the I/O board would appear to be interfering with reading RAM in general, but if the screen is static, then the I/O board may be interfering only with 68k access to RAM.

It is useful to know you have an oscilloscope:

As a first suggestion, I would set the scope to trigger on the falling edge of /AS (Pin 3 of U8A) with a time scale that permits viewing through to the following rising edge, then probe the signals on the I/O board that connect to the CPU board, looking for signs of contention (signals that are between 0.8 V and 2.0 V for more than a moment) while /AS is low, or any that are permanently stuck high or low.

Signals to check:

 Data lines
 Address lines
 UDS
 LDS
 DTACK
 VPA
 READ
 INTIO

Hopefully a further clue will appear, or perhaps someone else has some more specific advice?

HTH & good luck!

greniu:

--- Quote from: sigma7 on January 22, 2025, 12:06:23 am ---
It is useful to know you have an oscilloscope:

As a first suggestion, I would set the scope to trigger on the falling edge of /AS (Pin 3 of U8A) with a time scale that permits viewing through to the following rising edge, then probe the signals on the I/O board that connect to the CPU board, looking for signs of contention (signals that are between 0.8 V and 2.0 V for more than a moment) while /AS is low, or any that are permanently stuck high or low.

Signals to check:

 Data lines
 Address lines
 UDS
 LDS
 DTACK
 VPA
 READ
 INTIO

Hopefully a further clue will appear, or perhaps someone else has some more specific advice?

HTH & good luck!

--- End quote ---


Hi, Thank you for advices. The image is random and always static. I have two beeps soon after I power Lisa on. I/O board interfering only with 68k access to RAM sounds a good path to check.

sigma7:

--- Quote from: greniu on January 23, 2025, 02:40:02 pm ---... I/O board interfering only with 68k access to RAM sounds a good path to check.

--- End quote ---

I forgot to mention that for some conditions, you need to press reset each time you probe a new signal to reproduce the problem.

Once the CPU decides there is a hardware problem it may loop in ROM doing nothing, or it may enter a troubleshooting loop where it repeats the operation that is failing.

In this case, the contention problem may be brief before you hear the beep(s), or it may continue after that (depending on the cause).

In the 2.48 Boot ROM listing, there is a loop at $0666 which (after detecting a memory error) repeats writing $A55A to one RAM address then reading it back. If the conditions are right, this will continuously repeat the problem reading/writing RAM, but I can imagine failure modes where it would not (requiring a reset each time you wish to reproduce the problem).

greniu:
thank you. Any idea which component to check? I suspected U4E, but I test it on IC tester and it is ok. The image is static and I get 2 beeps very quickly soon after  power on.

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