Author Topic: 464+ controller port issue  (Read 2376 times)

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Offline Cwiiis

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Re: 464+ controller port issue
« Reply #25 on: 20:10, 30 July 21 »
Back to the topic, I just socketed the AY chip - not received the new one yet, but the old one is acting identically to before, so good to know I didn't screw it up! Will update once I get the new AY...

Offline Cwiiis

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Re: 464+ controller port issue
« Reply #26 on: 15:00, 31 July 21 »
And bad news for me, new AY chip arrived and unfortunately the behaviour is exactly the same :( I guess at this point it's likely to be the ASIC... Is there a circuit diagram/chip listing somewhere handy for the 464+? It'd be good to check everything and make sure that it's definitely the ASIC and not something simpler - I don't want to give up hope just yet!

Edit: Found the service manual - now to decide how deep I want to go here...
« Last Edit: 15:49, 31 July 21 by Cwiiis »

Offline Bryce

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Re: 464+ controller port issue
« Reply #27 on: 16:26, 31 July 21 »
If you have a logic analyser you can connect it up to the ASIC keyboard outputs and observe what happens. It should run in an endless loop of all bits high except for one going low. ie:
11111110
11111101
11111011


Bryce.
 

Offline Cwiiis

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Re: 464+ controller port issue
« Reply #28 on: 16:35, 31 July 21 »
If you have a logic analyser you can connect it up to the ASIC keyboard outputs and observe what happens. It should run in an endless loop of all bits high except for one going low. ie:
11111110
11111101
11111011


Bryce.
Now, do I buy a logic analyser to debug this...  :D
I assume that what might happen is that on one of the scans, it might erroneously have all of the bits low?
If it was scanning correctly, is there a part of the circuit outside of the ASIC that might be worth checking? I guess IC16, given it's connected directly to those lines - are the diode and resistor bodges on there standard?

Offline Cwiiis

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Re: 464+ controller port issue
« Reply #29 on: 17:06, 31 July 21 »
I was looking around for pictures of other people's 464+'s... I notice on this photo*, there are no bodges on IC16 at all... (unless they're on the underside I suppose - which seems unlikely?) I'm wondering if this is some kind of fix or mitigation that didn't quite work out? Here's mine**

* https://www.nightfallcrew.com/wp-content/gallery/amstrad_cpc_464_plus/img_1582.jpg?1ad88f
** https://photos.app.goo.gl/5BUETo78oZQePGpi9
« Last Edit: 17:09, 31 July 21 by Cwiiis »

Offline Bryce

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Re: 464+ controller port issue
« Reply #30 on: 17:20, 31 July 21 »
IC16?? The keyboard outputs of the ASIC go directly to the keyboard connector. Those traces from the ASIC only go under IC16, they are not connected to it.

Bryce.
« Last Edit: 17:41, 31 July 21 by Bryce »

Offline Cwiiis

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Re: 464+ controller port issue
« Reply #31 on: 17:31, 31 July 21 »
ah, ok, I guess they just go under that chip, I assumed there must be vias connected to it but obviously a bad assumption :) Also I see that the differences on that IC are down to different board revisions and the chip is actually different.
I don't have or have access to an oscilloscope at the moment, I'd like to eliminate everything possible without one if possible, but perhaps I have already...

Offline Cwiiis

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Re: 464+ controller port issue
« Reply #32 on: 00:57, 01 August 21 »
Not wanting to let go of this without a fight, I've been reading, studying schematics, searching... I found this video of someone with the exact same problem on a CPC6128 -
https://www.youtube.com/watch?v=PU_AIW6kJSU

The Y keyboard lines go to a 74LS chip on there though, which seems to have been subsumed by the ASIC on the Plus, so more evidence pointing at that :(

Having looked at all the schematics in the service manual, the only things the keyboard Y lines are connected to are the ASIC, the keyboard connector and COM1 on the joystick ports. I've checked for shorts between pins on CP04, NR02 and any unexpected ground/5V shorts, all came up negative. Nothing is wrong with keyboard X, but I've checked everything there anyway. The X lines are connected to the AY (replaced and no change, so presumably fine) and NR02 (removed and no change, so presumably fine).


I think I might be out of ideas for things I can check with the equipment I have... Everything points to the ASIC right now, but it seems like such an unlikely thing to happen with no trace of any other issues anywhere else... If anyone has any ideas, I'm all ears. If it is the ASIC, I guess I'm just out of luck? I'd happily sacrifice my GX4000 to have a fully working 464 Plus if there's anyone that has the skills to desolder/resolder that chip... I have the equipment, but I'm certain I don't have the talent and I'd rather not test that on something like this... Any help gratefully received! (alternatively, anyone looking to get rid of a 464 Plus on the cheap...)

Offline RetroCPC

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Re: 464+ controller port issue
« Reply #33 on: 10:15, 01 August 21 »
Hi,


Its a chance that the ASICS Joystick / keyboard port has been Damged by ESD (Joystick cable).

I can confirm condition of the ASICs internal ESD protection diodes with lab equipment here and no problem to swap the ASIC SMD IC if required (I have full SMD rework equipment) - trouble is during these covid times I'm based in Czech Rep. so shipping is not so easy post Brexit... but happy to do the work for free if you cover the shipping costs...
« Last Edit: 10:17, 01 August 21 by RetroCPC »

Offline Cwiiis

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Re: 464+ controller port issue
« Reply #34 on: 12:20, 01 August 21 »
Thanks, that's a very generous offer! Postage is actually not too expensive, at least on the UK side, so once I've exhausted a couple more avenues I'll send you a PM 🙂

It's a shame the design didn't include more protection from this kind of thing...

Hi,


Its a chance that the ASICS Joystick / keyboard port has been Damged by ESD (Joystick cable).

I can confirm condition of the ASICs internal ESD protection diodes with lab equipment here and no problem to swap the ASIC SMD IC if required (I have full SMD rework equipment) - trouble is during these covid times I'm based in Czech Rep. so shipping is not so easy post Brexit... but happy to do the work for free if you cover the shipping costs...

Offline Cwiiis

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Re: 464+ controller port issue
« Reply #35 on: 15:29, 01 August 21 »
Ok, one last desperate idea... Would it be possible to bypass the ASIC for keyboard handling using a 74LS145 like on the non-Plus models?
I can't work out from the schematics if it's possible and what exactly you'd need to bodge where to do it, but it seems like it might be vaguely possible? I suppose that if you could figure out what corresponds to the 4 input lines on that chip, you could cut the traces for KY1-10 on the ASIC, connect those 10 lines to the 74LS145 (along with the 4 inputs, assuming they have corresponding lines somewhere), find points for VCC/GND and that'd work around the ASIC for input?

Offline Cwiiis

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Re: 464+ controller port issue
« Reply #36 on: 15:43, 01 August 21 »
Ok, one last desperate idea... Would it be possible to bypass the ASIC for keyboard handling using a 74LS145 like on the non-Plus models?
I can't work out from the schematics if it's possible and what exactly you'd need to bodge where to do it, but it seems like it might be vaguely possible? I suppose that if you could figure out what corresponds to the 4 input lines on that chip, you could cut the traces for KY1-10 on the ASIC, connect those 10 lines to the 74LS145 (along with the 4 inputs, assuming they have corresponding lines somewhere), find points for VCC/GND and that'd work around the ASIC for input?
More reading and I can see that that's probably not feasible - those input lines are on an 8225 on the CPC and the strobing of those 4 pins is likely also completely internal to the ASIC... Maybe it's time to give up :( It's a shame to sacrifice a GX4000, but I'd rather have the home computer than the console...

Offline eto

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Re: 464+ controller port issue
« Reply #37 on: 15:44, 02 August 21 »
It's a shame to sacrifice a GX4000, but I'd rather have the home computer than the console...

but what if this doesn't work or if you fry the other Acid? If you buy a broken Plus or GX, it's fine, but please don't kill a very rare working machine.

Offline Cwiiis

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Re: 464+ controller port issue
« Reply #38 on: 16:16, 02 August 21 »
but what if this doesn't work or if you fry the other Acid? If you buy a broken Plus or GX, it's fine, but please don't kill a very rare working machine.
I don't like the idea either, but I trust anyone offering to do this that they have the skills necessary, I suppose. I'm looking at other Pluses on sale in case I can get a reasonable deal and avoid this.

Offline Cwiiis

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Re: 464+ controller port issue
« Reply #39 on: 20:48, 02 August 21 »
Crisis averted, just got a decent deal on a fully working 6128+, so I can even use the RAM I got to upgrade my GX4000 :)