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Galaksija typing random characters by itself after power-on
- Thread starter Desperado
- Start date
daver2
10k Member
There is supposed to be an LS32 and an LS38 fitted to separate IC sockets.
What have you got fitted and to where?
Dave
What have you got fitted and to where?
Dave
daver2
10k Member
That doesn't really answer the question I asked...
>>> There is supposed to be an LS32 and an LS38 fitted to separate IC sockets.
>>> What have you got fitted and to where?
Dave
>>> There is supposed to be an LS32 and an LS38 fitted to separate IC sockets.
>>> What have you got fitted and to where?
Dave
daver2
10k Member
That is correct.
Where did you have the HC part previously?
Dave
Where did you have the HC part previously?
Dave
Desperado
Veteran Member
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- Nov 25, 2017
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HC00 in U15 sir!
daver2
10k Member
As previously stated, the HC00 should not be present...
The interesting thing (from looking at the used gates of U4 - 74LS32) is that only 2 of the 4 gates are used. The unused gates do not have their inputs connected to anything (either 0V or pulled up to +5V). This can introduce noise and strange behaviour into the used gates on the package - and this IC drives the address decoder (U6) that selects the ROM, RAM, and keyboard.
This is poor (original) design - and a faithful copy of the original will contain the same fault. I would be tempted (myself) to connect the unused input pins of U4 to 0V.
If the CPU cannot correctly read the instructions out of the ROM, it will go loopy and can halt.
The same address decoder (U6) is used for everything.
So, if the address decoder fails (or there is noise or timing issues) then all sorts of random things will go wrong (i.e. the computer will be unstable).
This is where I was heading in looking for the timing error(s) that are causing misreads of the keyboard (and hence the ghost writing).
This can be either U6 or U4 to blame.
However, in order to check the signal timing, we have to have a Z80 CPU that runs (the chicken and the egg scenario).
If the Z80 has stopped running, it is pointless to complain that you can't measure the /MREQ signal on pin 19 of the Z80 CPU.
You will have to power down the machine, count 10 seconds, and then power it back up again to (hopefully) reset the Z80 CPU and kick it into running.
If you get the READY prompt and the ghost writing then you can use your oscilloscope. If you get a picture of a black cat in a coal cellar at midnight, the Z80 is not running. This design requires an operational Z80 CPU to generate the video characters.
Remember the trick with the PET and the 6502 CPU? We always check pin 7 (SYNC) for signs of activity. If none are present, the CPU has halted.
Likewise with the Z80 CPU, if /MREQ is not pulsing away - the CPU is not fetching and executing instructions, so your machine is a door stop and there is no point in poking around with an oscilloscope.
Dave
The interesting thing (from looking at the used gates of U4 - 74LS32) is that only 2 of the 4 gates are used. The unused gates do not have their inputs connected to anything (either 0V or pulled up to +5V). This can introduce noise and strange behaviour into the used gates on the package - and this IC drives the address decoder (U6) that selects the ROM, RAM, and keyboard.
This is poor (original) design - and a faithful copy of the original will contain the same fault. I would be tempted (myself) to connect the unused input pins of U4 to 0V.
If the CPU cannot correctly read the instructions out of the ROM, it will go loopy and can halt.
The same address decoder (U6) is used for everything.
So, if the address decoder fails (or there is noise or timing issues) then all sorts of random things will go wrong (i.e. the computer will be unstable).
This is where I was heading in looking for the timing error(s) that are causing misreads of the keyboard (and hence the ghost writing).
This can be either U6 or U4 to blame.
However, in order to check the signal timing, we have to have a Z80 CPU that runs (the chicken and the egg scenario).
If the Z80 has stopped running, it is pointless to complain that you can't measure the /MREQ signal on pin 19 of the Z80 CPU.
You will have to power down the machine, count 10 seconds, and then power it back up again to (hopefully) reset the Z80 CPU and kick it into running.
If you get the READY prompt and the ghost writing then you can use your oscilloscope. If you get a picture of a black cat in a coal cellar at midnight, the Z80 is not running. This design requires an operational Z80 CPU to generate the video characters.
Remember the trick with the PET and the 6502 CPU? We always check pin 7 (SYNC) for signs of activity. If none are present, the CPU has halted.
Likewise with the Z80 CPU, if /MREQ is not pulsing away - the CPU is not fetching and executing instructions, so your machine is a door stop and there is no point in poking around with an oscilloscope.
Dave
A 5V 1A switching power supply was used, but the noise interfered with the computer. Since there is no dedicated section for the integrated power supply on the motherboard, but an external linear power supply was originally planned, I recommend using linear or switching power supplies for audio, or at least a power supply with low ripple.
Desperado
Veteran Member
- Joined
- Nov 25, 2017
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- 8,108
Hi Doc, I can't believe it...we went crazy for days with this Galaksija only to find out it was the power supply? I used this 1A power supply I bought on Amazon...I don't understand how it could have caused that problem. Why, if I removed the LS251, it stopped writing on its own? Anyway, thanks so much for fixing it so quickly... Now which power supply should I buy?
daver2
10k Member
So much crap on the power supply, like I asked you to check ages ago...
Dave
Dave
Desperado
Veteran Member
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Hi Dave, I couldn't see any noise from the power supply...how is it possible that a power supply can make a computer write by itself??? Why would the problem go away if I removed the LS251? I'm desperate!
daver2
10k Member
I asked you to do this back in post #209.
Have you got a bench power supply you could use as a test.
When building a piece of equipment, I always use my trusty bench power supply first. It provides for current limiting and all sorts.
If there is noise on the power supply - and the voltage pins are dipping below the minimum recommended by the IC manufacturer - then all sorts of weird stuff happens.
However, this doesn't (potentially) answer some of the strange oscilloscope traces you were reading... But this may be down to how you are reading them of course...
There are decoupling capacitors on the board, but the PCB layout may not be amenable to adequately keep the noise at bay.
This may be a red herring of course, but a lot of your oscilloscope traces (especially the later ones) do not (still) make sense.
Dave
Have you got a bench power supply you could use as a test.
When building a piece of equipment, I always use my trusty bench power supply first. It provides for current limiting and all sorts.
If there is noise on the power supply - and the voltage pins are dipping below the minimum recommended by the IC manufacturer - then all sorts of weird stuff happens.
However, this doesn't (potentially) answer some of the strange oscilloscope traces you were reading... But this may be down to how you are reading them of course...
There are decoupling capacitors on the board, but the PCB layout may not be amenable to adequately keep the noise at bay.
This may be a red herring of course, but a lot of your oscilloscope traces (especially the later ones) do not (still) make sense.
Dave
Desperado
Veteran Member
- Joined
- Nov 25, 2017
- Messages
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Unfortunately i haven't a power supply bench
daver2
10k Member
Well, see if you can buy a decent second-hand one.
You keep building equipment - and this is the sort of thing you require.
Failing that, buy a DECENT power supply and use that for project work.
If you buy stuff from Amazon (and any similar marketplace) it is a cr*p shoot as to whether you get a decent bit of equipment at a cheap price, or a pile of cr*p...
If you end up buying a pile of cr*p, this just causes anything you power from them to (potentially) not work.
Likewise with IC chips. If you buy from an approved supplier, and get a CoC (Certificate of Conformance) with it, you should be purchasing good parts. If not, you are playing roulette.
Note that IC testers can identify counterfeit or clearly faulty parts. They cannot identify substandard parts (e.g. remarked lower clock speed parts as higher clock speed parts). Also, these IC testers depend upon the test coverage of the the test vectors. The parts testers that I have (commercial ones) publish their test vectors so that you can ascertain the suitability for a particular case (and they can be modified if deficiencies are found).
Dave
You keep building equipment - and this is the sort of thing you require.
Failing that, buy a DECENT power supply and use that for project work.
If you buy stuff from Amazon (and any similar marketplace) it is a cr*p shoot as to whether you get a decent bit of equipment at a cheap price, or a pile of cr*p...
If you end up buying a pile of cr*p, this just causes anything you power from them to (potentially) not work.
Likewise with IC chips. If you buy from an approved supplier, and get a CoC (Certificate of Conformance) with it, you should be purchasing good parts. If not, you are playing roulette.
Note that IC testers can identify counterfeit or clearly faulty parts. They cannot identify substandard parts (e.g. remarked lower clock speed parts as higher clock speed parts). Also, these IC testers depend upon the test coverage of the the test vectors. The parts testers that I have (commercial ones) publish their test vectors so that you can ascertain the suitability for a particular case (and they can be modified if deficiencies are found).
Dave
In this case linear or switching describes the means by which the desired output voltage is obtained from the input voltage.
A simple linear voltage regulator like the LM7805 takes a higher input voltage and drops it to a lower output, ~5V in this case, shedding a large part of the excess energy as heat.
By contrast, a switching power supply is rather more complex and actually switches the power on and off constantly to control the output.
Somebody else might be able to explain some of the finer points of the difference.
If I understand correctly, it seems that switching power supplies:
- rectify incoming AC power (to DC)
- filter that DC power with a capacitor (because otherwise it would be a pulse of DC followed by no electricity at all and so on)
- recreate an AC waveform with a much higher frequency from the DC
- rectify (2) the new waveform to get back to DC at the desired voltage
- filter (?) again to smooth out pulses
So you could have some sort of electrical coupling that is creating spurious signals in your circuit wiring or it might just be that the adapter's output voltage dips occasionally causing your logic chips to behave weirdly.
Since your computer reproduces an original design that likely used a power supply design involving a step-down transformer and a linear voltage regulator to get the 5V output, sticking to that should eliminate any chance the power supply is responsible for the glitches.
If I understand correctly, it seems that switching power supplies:
- rectify incoming AC power (to DC)
- filter that DC power with a capacitor (because otherwise it would be a pulse of DC followed by no electricity at all and so on)
- recreate an AC waveform with a much higher frequency from the DC
- rectify (2) the new waveform to get back to DC at the desired voltage
- filter (?) again to smooth out pulses
So you could have some sort of electrical coupling that is creating spurious signals in your circuit wiring or it might just be that the adapter's output voltage dips occasionally causing your logic chips to behave weirdly.
Since your computer reproduces an original design that likely used a power supply design involving a step-down transformer and a linear voltage regulator to get the 5V output, sticking to that should eliminate any chance the power supply is responsible for the glitches.
Desperado
Veteran Member
- Joined
- Nov 25, 2017
- Messages
- 8,108
Today i receveid the Galaksijan pcb and the new power supply from my friend @Xchip !
Now the keys are working!
I've already assembled the add-on board for the tasty Cherry and purchased some vintage green Cherry keys. Unfortunately, some of the keys have the wrong serigraphy, and I'll have to apply labels if I can find them. Xchip also gave me a memory expansion for this computer! Thank you all so much.
PS: I haven't installed the RAM expansion yet, what you see on the screen is the base memory.
Now the keys are working!
I've already assembled the add-on board for the tasty Cherry and purchased some vintage green Cherry keys. Unfortunately, some of the keys have the wrong serigraphy, and I'll have to apply labels if I can find them. Xchip also gave me a memory expansion for this computer! Thank you all so much.
PS: I haven't installed the RAM expansion yet, what you see on the screen is the base memory.
Attachments
daver2
10k Member
What power supply do you have now?
Can you post a photograph of the label of the PSU that doesn't work and the PSU that does.
Dave
Can you post a photograph of the label of the PSU that doesn't work and the PSU that does.
Dave