IIGS not resetting

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IIGS not resetting
I have been running this IIGS for years as a home controller. I wrote the software and have been modifying and expanding it for years. All of a sudden, it stopped working. When I listed the program, it seemed to stop listing before it reached the end. Applesoft was not locked up, just the program did not list or run. While I was trying to troubleshoot this issue, the whole computer stopped booting. Now when you turn it on there is a small click on the speaker and that is about it. My initial thought before much research was that some of the RAM was bad. Bad RAM may have caused the program to not load completely was my thinking. I have an inexpensive oscilloscope and after watching several you tube videos tried to troubleshoot the problem. As I stated in the summary, the reset signal is not being sent out. What I don't know is if these two observation could be related and bad RAM is the root of all my issues. Or is my RAM theory nonsense and my ADB chip is defective. I never noticed any keyboard issues. I have pulled and reseated all the socketed chips and actually bought a new CPU but that did not do anything.
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Have you checked the power

Have you checked the power supply voltages?

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IIGS not resetting
Yes, has a brand new power supply which I have checked for all the proper voltages.
ggb
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When you say reset isn't

When you say reset isn't being sent out, what is the voltage on the CPU pin 40?

Is the board a rom 01 or 03?

Regards
Geoff B

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IIGS won't reset
Pin 40 is moving high and low between something above 0 to about 4,8 volts. The board is a ROM 01 version. I have tracked the problem so far to the ADB chip. The reset voltage is showing up on pin 16 as it should but the voltage on pin 49 is fluctuating as I described.
ggb
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IIGS not resetting

Is there any display visible on the screen either text or graphics when powered on?

While the ADB chip can generate a reset, the Mega II also has a power on reset circuit and generates a reset signal when first turned on.

With all cards removed, can you capture the reset on pin 40 of the CPU with your oscilloscope and upload a picture.
Is the reset signal different with or without the ADB keyboard/mouse connected?

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IIGS not resetting

I have screen shots of everything but I guess I don't know how to attach them to a comment.

The screen does change. The bottom third is white. The middle third is right side only white. The top third is blue.

The keyboard disconnected or attached does not seem to have any affect on the startup of the computer.

My schematic does not show a reset circuit on the MEGA II chip. The one pin that is labeled /reset is connected to the same trace that is on pin 49 of the ADB. The only "reset circuitry" that I have been able to locate is at pin 16 of the ADB.

There is a slight uptick in the line on the oscilloscope when the computer is turned on. However, my oscilloscope has about 12-15 mv of noise on the line all the time and the uptick is only about 145 mv. The reset voltage on pin 16 is just under 5V so I guess I thought the reset voltage on pin 49 would be similar. I have attached printouts.(again couldn't figure out how to do this)

All my testing is being done with all cards removed.

ggb
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I think when your creating

I think when your creating message you select media browser to upload file(s)

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IIGS not resetting

Sorry, I don't see "media browser" when I open this reply to your message. In the navigation window there is a line to upload a file which I did. However, I don't know how to reference it so that someone can see it.

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IIGS not resetting

Since my original post I have also tried tying in +5 V to the reset trace. This has no affect. As a matter of fact, the voltage on the reset pin 40 of the CPU does not change. Does that mean there is something in the circuit drawing the voltage down below the required reset voltage. Does anyone have any ideas what to check next?

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If you REALLY tied the reset

If you REALLY tied the reset trace to +5 and pin 40 of the CPU is still fluctuating between 0 and +5, the clearly you either have a broken trace or (more likely) the CPU socket is bad!

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IIGS not resetting

I have 2 motherboards and mixed up which one I was talking about when I said the voltage was moving between 0 and 5V. That board has many things wrong with it. The board I have been trying to fix seems to be in better shape. The voltage on pin 40 is moving from 0 to about 140 mv. It holds there but nothing happens, This is the board I attached a 5V source on the board through a resistor to pin 40. Still no action and as I said the voltage at pin 40 was the same 140 mv.

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/RESET

If the CPU pin 40 is stuck at 0 V or nearly 0, then something in the computer is pulling down /RESET.

As long as /RESET is asserted, there's no way anything can happen!

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IIGS not resetting

I am not an electrical engineer with the knowledge to troubleshoot this problem. I am looking to try an engage someone with this expertise. I was hoping that this person existed on Applefritter.

Am I in the wrong place for this type of help?

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The /RESET line is connected to all of the major chips in the IIgs, but it has no pullup or RC timer connected to it. So while it's possible for any of the chips to pull the line down to force the computer to reset, it can only become high (and thus, allow the machine to start working) if it is pulled high by one of the chips. The chip that pulls /RESET high must be the ADB microcontroller M50740, for reasons that it is the only chip with its own power-on-reset timing circuit attached. After the ADB microcontroller itself comes out of reset, it can begin executing instructions which probably involve a self-test or sanity check, possibly involving communication with ADB GLU, after which it pulls its P25 pin 49 high to bring the rest of the computer out of reset.

So the reasons for the /RESET to be stuck low are that either the ADB micro is broken internally, its connections to the board are broken, or that it sees some fault that prevents it from completing its self-test, or that one of the other chips is pulling /RESET low and preventing this line from coming to a high state. You said you measured an intermediate voltage on this line, something like 200 mV, which means that the last case is likely happening (ADB is pulling the line high but another chip is pulling it down, fighting each other). Determining which chip it is may be difficult although there are techniques using inductive probes to find which chip pin is sinking this current. If two chips are fighting each other there will always be current flowing from one to the other.

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<blockquote><strong

<blockquote><strong>robespierre wrote:</strong> <p>The /RESET line is connected to all of the major chips in the IIgs, but it has no pullup or RC timer connected to it. So while it's possible for any of the chips to pull the line down to force the computer to reset, it can only become high (and thus, allow the machine to start working) if it is pulled high by one of the chips. The chip that pulls /RESET high must be the ADB microcontroller M507</blockquote><p><br/></p>

robespierre - thanks for the insight. I don't have an inductive testing device. However, I think the next step would be to remove all the chips in sockets that are connected to the reset line. Maybe I'll get lucky and one of them is the culprit. After that, I may have to try to unsolder the reset pin connection at other chips.

Does this sound like an approach that may work?

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pcbishaf wrote:I think the
pcbishaf wrote:
I think the next step would be to remove all the chips in sockets that are connected to the reset line. 

 

This sounds a bit odd as the approach, may be better to see if you can find if reset has a short to ground somewhere on the board. This should be easy check.

Similar to what jeffmazur said, if you tied reset to 5V directly and the system didn't work that's a concern and I would not try that again before checking reset doesn't have a short to ground. You can kill your supply if there is a short to ground and you connet 5V to ir.... that would be bad.

 

Any chance you tried the self-test before it stopped booting? 

 

Did you do or notice anything betweeen when it was working and when it started doing this? Do you have cards installed in the system, if so get them out for the troubleshooting. 

 

roespierre covered some good points on the reset line but know this, the GS is quite a complex system to troubleshoot. There's so much going on between the mega, the 816, fast and slow RAM, the ADB controller and ADB glu... this can be a very difficult task even when you know a lot about the GS. 

 

When you said the voltages are good, where are you checking this? Is your scope 2 or 4 channel? If you monitor all voltages at the same time (4 channels) this would be best but if you only have two just wastch 5 and 12V because those are the main ones ones used when starting up.  The speaker click you're hearing could be something but also not uncommon for ROM01 motherboards.

 

You are correct RAM could easily be your problem, but only after power is ruled out.  The most important thing about power is stable voltages, meaning not a lot of movement they should read flat all the time. Even mometary dips are not good.  There may be a minor ~0.75V dip on 12 when the drive spins up ( you may see the same same dip on -12 and -5) but that's nothing to worry about and that's the only significant movement you should see. 

 

For pictures, the media browser should be here:

 

Unfortunately it's not as easy as drag and drop, you need to load the file, upload the file, click next, next, submit, save or something along those lines... it's clunky at best. The image will be inserted where the cursor was and don't worry about the size in the  editing window it will be sized better when you post.

 

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Jeff D. - Thanks for all your

Jeff D. - Thanks for all your input. I finally found the "media browser". My ad blocker was preventing it from displaying.

I have done some more work on the system. I thought I had it resolved when I removed the CPU chip and the sound GLU. All of a sudden the RESET pin was at 4.9 V. So now I needed to determine which chip was causing the problem. It turned out that it was neither. After about 20 seconds from powering on the computer the reset line drops to the 120 mv. The drop is not one second it is at 5 V and the next at 120 mv. All sorts of noise appears as it drops to the low level. I have checked the power supply voltages and they are rock steady. I have a 2 channel oscilloscope and am moving the probe between the 2.

I have all the slots empty. Only the keyboard and the monitor are connected.

As I said earlier, the program did not appear to be loading fully and then the computer just stopped working. I did not run a self test. At the time I was thinking I had a disk drive or floppy disk problem.

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If checking for a short between the /reset line and ground is checked by ussing a multimeter to check the resistance between the two lines, then there is no short.

The way the voltage is dropping, makes me suspect an overheating issue. I was thinking I could power up the GS for an hour or two and see if anything is hot. My understanding of the /reset line is that if it falls below a specific value then the IC stops running and is once again waiting for the voltage on the line to go high. I have checked the reset pin on the ADB micro and it is rock steady at 4.9V.

Does anyone have any other ideas for checking before I start what might be my destructive troubleshooting?

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expected

A reset signal is not a pause or wait signal. Its purpose is to initialize state machines to a known state, from which operations can proceed deterministically. As a simple example, a flip-flop can assume a state of '0' or '1' when power is applied. A computer might contain thousands or millions of flip-flops, so you can see that it is no good if it were simply to start running after being switched on. It is necessary to have a reset signal to clear all of these flip-flops so that their behavior can predictably proceed.

A computer typically has a watchdog reset circuit, a counter which is constantly counting down towards zero. Periodically, some task in the machine writes to the counter, to "fill it up" again so that it won't reach zero. But if the computer is stuck or hung, nothing will fill up the counter, it will continue counting down to zero, and when it reaches zero it will assert the reset signal to bring the computer back to a known state where it isn't stuck.

Earlier you said that the /RESET line dropped "after about 20 seconds". I would expect a watchdog reset to time-out in a much shorter time, but there you go. Without any CPU activity (because you removed the CPU) the firmware inside the ADB microcontroller gets stuck waiting for a response that never arrives, and its watchdog reset activates which brings it to a known state. It stops pulling P25 high because the output pin is reset to its initial state at that time. Most of the time, the known initial state of output pins is "hi-Z" or no output, effectively disconnecting the chip from that wire.

With nothing connected to a wire, it will hold the same voltage for a while just from its parasitic capacitance, but it will soon bleed away. A scope probe on the wire will pick up noise, just like it does when you wave it around in the air. Depending on how the probe is grounded there could be a lot of noise.

So what you discovered when removing those chips is what would be expected to happen.

ggb
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The RESET on Apple II's is

The RESET on Apple II's is normally pulled high via pullup resistor to 5V. Then any source that wants to reset can connect collector of an NPN transistor to RESET and emitter to GND, then when the transistor is turned on it will pull the collector down towards ground (RESET low). If no device is pulling it low then RESET would be pullup to 5V.

As far as I am aware of the Mega II chip, the reset pin is both an input and output.
During power on it pulls the Reset low until power is considered stable by internal Power on circuit and will reset internally if reset is pulled low externally to Mega II.
The ADB micro can also pull the Reset low due to the correct keyboard combination being pressed.

You could try shorting Pin 16 of ADB micro temporarily to GND to discharge C39 and see if this changes reset going to the 65816.
This would cause the ADB Micro to reset and start up correctly if it had not when the IIgs was first powered on.
Also check voltage on ADB Micro Pin 48, which would be connected to IIe keyboard header Pin 15, which is RESET from IIe keyboard if it was connected.

Three chips that could be removed from sockets on ROM1 to see if they impact RESET, are 65816, Socketed VGC, DOC5309.

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More checking to do

robespierre and ggb: Thanks for the additional help. I will post again when I have worked through your messages.

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responses and picture

<p>https://www.applefritter.com/files/2024/06/15/pin%2040%20with%20cpu.pdf</p> not sure how to get picture instead of link

This is the oscilloscope from pin40 when I put the cpu back into the board. All chips are installed.

I grounded pin 16 on ADB to discharge C39 when the cpu was removed and there was no change.

The voltage on pin48 of ADB micro is 4.5 V. This is with the keyboard attached.

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More information

The output was from a new CPU that I purchased. I also monitored BE on the CPU and this is high until the wider section of the graph drops towards zero. The narrower section of high output on the graph did not affect this pin.

I am thinking this means the software is starting up and something else is not working. This leads me back to my original thinking that a FAST RAM chip is not functioning. I am going to buy 8 new RAM chips and see what happens.

I would appreciate any input anyone has. Especially if you think I am going in the wrong direction.

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pcbishaf wrote: The output
pcbishaf wrote: The output was from a new CPU that I purchased. I also monitored BE on the CPU and this is high until the wider section of the graph drops towards zero. The narrower section of high output on the graph did not affect this pin. I am thinking this means the software is starting up and something else is not working. This leads me back to my original thinking that a FAST RAM chip is not functi
 

 

I see the scope trace says 200ns but I can't make out what that time is, it it a division or full frame?Do you have timing for all the states? It looks like ther's a reset loop that includes both a system reset (guessing this is the short low which was 8 cycles on the other IIs and I didn't check the GS reference. But then there's the longer "high" which I am guessing is the system starting up until something decided it needs to reset so pulls it low and then the process repeates.  The question still seems to be which component is pulling it low. I also haven't checked if /RST is pulled up to 5V but would assume that may be the case, but I don't see the plot gettng to 5V, looks like it hangs below 4.8V for all those pulses, if pulled up that would be near the lower end of acceptable and may be something to look at.  Are all cards still removed?  Could you set all slots to "your card" to disable everything but ADB controller? 

Can you clarify what these unknowns are?

I am intrigued by the small rise to the reset line and assuming that's at 5V, or does that rise indicate some component has pulled itself offline which would cause the voltage rise. 

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pcbishaf wrote: The output
pcbishaf wrote: The output was from a new CPU that I purchased. I also monitored BE on the CPU and this is high until the wider section of the graph drops towards zero. The narrower section of high output on the graph did not affect this pin. I am thinking this means the software is starting up and something else is not working. This leads me back to my original thinking that a FAST RAM chip is not functi
 

 

I see the scope trace says 200ns but I can't make out what that time is, it it a division or full frame?Do you have timing for all the states? It looks like ther's a reset loop that includes both a system reset (guessing this is the short low which was 8 cycles on the other IIs and I didn't check the GS reference. But then there's the longer "high" which I am guessing is the system starting up until something decided it needs to reset so pulls it low and then the process repeates.  The question still seems to be which component is pulling it low. I also haven't checked if /RST is pulled up to 5V but would assume that may be the case, but I don't see the plot gettng to 5V, looks like it hangs below 4.8V for all those pulses, if pulled up that would be near the lower end of acceptable and may be something to look at.  Are all cards still removed?  Could you set all slots to "your card" to disable everything but ADB controller? 

Can you clarify what these unknowns are?

I am intrigued by the small rise to the reset line and assuming that's at 5V, or does that rise indicate some component has pulled itself offline which would cause the voltage rise. 

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pcbishaf wrote: The output
pcbishaf wrote: The output was from a new CPU that I purchased. I also monitored BE on the CPU and this is high until the wider section of the graph drops towards zero. The narrower section of high output on the graph did not affect this pin. I am thinking this means the software is starting up and something else is not working. This leads me back to my original thinking that a FAST RAM chip is not functi
 

 

I see the scope trace says 200ns but I can't make out what that time is, it it a division or full frame?Do you have timing for all the states? It looks like ther's a reset loop that includes both a system reset (guessing this is the short low which was 8 cycles on the other IIs and I didn't check the GS reference. But then there's the longer "high" which I am guessing is the system starting up until something decided it needs to reset so pulls it low and then the process repeates.  The question still seems to be which component is pulling it low. I also haven't checked if /RST is pulled up to 5V but would assume that may be the case, but I don't see the plot gettng to 5V, looks like it hangs below 4.8V for all those pulses, if pulled up that would be near the lower end of acceptable and may be something to look at.  Are all cards still removed?  Could you set all slots to "your card" to disable everything but ADB controller? 

Can you clarify what these unknowns are?

I am intrigued by the small rise to the reset line and assuming that's at 5V, or does that rise indicate some component has pulled itself offline which would cause the voltage rise. 

 

RAM would not be my first suspect, because it looks like something is pulling reset low and AFAIK that's not a RAM thing... system keeps running with bad RAM and may crash but not usually reset. 

In addition, both fast and slow RAM are soldered in place an getting them out is not easy so should be done by someone with good rework skills. It's way too easy to destroy stuff and I've seen some horrible examples recently... so sad.

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Reply to JeffD and new find
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JeffD - you make an excellent point about what type of error a bad RAM chip would cause. Withy that in mind, I scoured the motherboad and found this odd looking connection at the ADB micro. I was checking always at the CPU reset pin and not the ADB micro itself(above picture at the RED arrow). When I checked the resistance between this pin and pin 33 on the ADB GLU, it was not zero as it was on my other bad board. I think I need to fix this before I go much further.

However, I did get the measurements you marked on the test report.

1st t = 293 ns
2nd t = 380 ns
3rd t = 327 ns
4th t = 322 ns all numbers a close but may not be exact.

The peak voltage was 5.02

Does anyone know how to fix such a small lead coming off the IC? I am afraid my solder will just join several of the pins together!

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That's your problem

Considering that's pin 49 (Reset), I think you found your problem. Did you perhaps dislodge that pin with a probe when you were checking it?

 

You need some fine tools to work on these small-pitched smd devices. Try to gain the skills working on a spare board you don't care about. Or get someone with the skill to repair it. Definitely fixable though; just need a very small soldering iron tip and try to nudge the pin back in place. Don't try to add solder. You should be able to just tack it down with what's there.

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