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Re: Fixing my C128D
Posted: 17 Sep 2023 18:13
by PhilC
And does it boot? Nope.
Fitting the PLA has once again helped. It looks like it’s something to do with the swap from Z80 to 8502 which is done with a whole load of logic to only ever keep one cpu online at a time.
I also had to fix the reset circuit as it wasn’t resetting properly, so two new caps were required there too.
Re: Fixing my C128D
Posted: 17 Sep 2023 18:52
by stephen_usher
Well, one step forward at least.
Re: Fixing my C128D
Posted: 17 Sep 2023 18:54
by rubber_jonnie
PhilC wrote: 17 Sep 2023 18:13
And does it boot? Nope.
Fitting the PLA has once again helped. It looks like it’s something to do with the swap from Z80 to 8502 which is done with a whole load of logic to only ever keep one cpu online at a time.
I also had to fix the reset circuit as it wasn’t resetting properly, so two new caps were required there too.
Ahh damn, that's a pain. Given the first stage boot is to Z80 and CP/M if you have a disk installed, what is the Z80 doing during the boot process? Do the address and data lines look normal?
Also, I know you said the ROMs read ok, but would it be worth trying replacements?
Re: Fixing my C128D
Posted: 17 Sep 2023 21:12
by PhilC
I’ve managed to get a second C128, I’m hoping I can use this to fix the C128D. At the very least I can swap over the motherboard but I’m hoping a working one will let me scope out the startup of the working one to get the 128D up and running, then I’ll sell the new one on again.
Or that’s the plan anyway.
Re: Fixing my C128D
Posted: 17 Sep 2023 21:40
by JezC
PhilC wrote: 17 Sep 2023 21:12
Or that’s the plan anyway.
@PhilC Good luck with that plan...it never seems to work out that way when I try to follow tat approach.
Mind you, that might just be me being a compete & utter hoarder...but then I'm not the only forum member with that problem eh
@rubber_jonnie ?
:fire: :fire: :fire: :hide:
Re: Fixing my C128D
Posted: 18 Sep 2023 08:33
by rubber_jonnie
JezC wrote: 17 Sep 2023 21:40
PhilC wrote: 17 Sep 2023 21:12
Or that’s the plan anyway.
@PhilC Good luck with that plan...it never seems to work out that way when I try to follow tat approach.
Mind you, that might just be me being a compete & utter hoarder...but then I'm not the only forum member with that problem eh @rubber_jonnie ?
:fire: :fire: :fire: :hide:
I cannot deny the truth...
It is very useful to have a 2nd machine, it can give important pointers and certainly helped with my first BBC B. Good luck
@PhilC, I hope you can get both to function :)
Re: Fixing my C128D
Posted: 18 Sep 2023 10:10
by PhilC
From the photos the motherboard looks the same other than the power supply and internal disk connections, which are actually on the board, just not populated.
We will see what time I get before cyber legends to get it all done, otherwise it’s plan b.
Re: Fixing my C128D
Posted: 18 Sep 2023 11:53
by rubber_jonnie
So out of curiosity, do you see any FDD activity when you power on? Surely if it's trying to boot to CP/m using the Z80 prior to handoff to the 8502, it must try to boot from the FDD, so if it isn't then it is something to do with the Z80 initialising and trying to start the system.
Re: Fixing my C128D
Posted: 18 Sep 2023 16:18
by PhilC
rubber_jonnie wrote: 18 Sep 2023 11:53
So out of curiosity, do you see any FDD activity when you power on? Surely if it's trying to boot to CP/m using the Z80 prior to handoff to the 8502, it must try to boot from the FDD, so if it isn't then it is something to do with the Z80 initialising and trying to start the system.
No all I get is the 1571 resetting and it clicks a couple of times with a disk in and then does nothing.
It’ll be good to get the second machine as I have no idea exactly what you should see when booting cpm and this will let me know.
That and it’s got rims to swap over etc etc.
Re: Fixing my C128D
Posted: 18 Sep 2023 16:31
by rubber_jonnie
PhilC wrote: 18 Sep 2023 16:18
rubber_jonnie wrote: 18 Sep 2023 11:53
So out of curiosity, do you see any FDD activity when you power on? Surely if it's trying to boot to CP/m using the Z80 prior to handoff to the 8502, it must try to boot from the FDD, so if it isn't then it is something to do with the Z80 initialising and trying to start the system.
No all I get is the 1571 resetting and it clicks a couple of times with a disk in and then does nothing.
It’ll be good to get the second machine as I have no idea exactly what you should see when booting cpm and this will let me know.
That and it’s got rims to swap over etc etc.
So I'd guess that the Z80 isn't even initialising, which is not good.