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PPC750GX vs. PPC750GL?

Siliconinsider

Well-known member
I honestly though no one would try because the board is exceedingly rare, and also it's a very difficult project. It took me two full month to sand down the PCB and retrace it all, and make verifications. Getting the 8 layer PCB manufactured also took a long time.

Ultimately there was the problem of sourcing the ZIF socket, and I had the idea of using a blank PCB with simple drill holes in it, and purchase some Mill Max pins that are then driven one by one in the PCB holes and surface mounted to the board.

Also, yes, the 750GX and 750GX are identical CPUs, the 750GL being the low power version. The same goes with the 750FX and 750FL, which are also drop in compatible with the 750GX. Note that the 750GL only comes in 800 and 933MHz versions, so they have always been overclocked on the PowerLogix boards. I have added some hardware DIP switches to set the PLL1, but this is probably not very useful as the PLL2 is driven by software. The PowerLogix software does allow to change the clock multiplier on the fly, and after the extension is loaded during boot.

We obviously have the same source, REI is a common source for legacy items that need to be supplied long term, often for military application. They use original masks from the original vendors to manufacture second source parts under licence when the original vendor decide to make the part obsolete. So while these look weird, they are new. The BGA balls are also shallower than the original IBM parts, I expect because they are RoHS solder balls, and the IBM ones are the traditional high melt 90Pb/10Sn balls.

Now the bad news is that the VRM of my PowerMac G3 beige did release the magic smoke when I tried it! I have traced and triple checked everything and could not find anything wrong, but it is possible that when inserted in the socket, there could be a short between either VCCcore, VCCio, or GND. Need to check if it is the high side or low side MOSFET on the VRM that blew up to give me a clue, and hopefully have a simple fix for the PCB (drilling a via on the offending connection).

Well maybe the morale of the story is that I should spend more time here!!! I hope I did not ruin any hopes or expectations, I'll gladly share what I have learned so far, and hopefully will try to resume the project as I had some major deadline to beat lately.
 

herd

Well-known member
Something to look at maybe:
G3ZIF.png

If the vias are also exposed under the CPU and pin arrary there could be a short there...
 

Siliconinsider

Well-known member
Oh no way! I'll check that!
All the vias are tented under the BGA and under the PGA, otherwise.

Edit: I checked that little solder blob and it did not make connection to anything, took it off.
 

LightBulbFun

Well-known member
I'm afraid I have missed the thread, but I have already made a replica of the PowerLogix 750GL accelerator...
It however does not work as I did blow the VRM, so there's possibly a short somewhere in my design that I need to trace.
I had a PowerLogix board that I recovered from a scrap lot, that was destroyed. It's an 8 layer PCB, and it was extremely challenging to both reverse engineer and assemble.

View attachment 38954View attachment 38955
Oh wow thats amazing! hopefully you can get the remaining niggles sorted before not too long and get it booting :)

have added some hardware DIP switches to set the PLL1, but this is probably not very useful as the PLL2 is driven by software. The PowerLogix software does allow to change the clock multiplier on the fly, and after the extension is loaded during boot.
its actually a really good thing you added those PLL dip-switches

because the achilles heel of the PowerLogix 750FX and 750GX CPUs is their base multiplier is only set to about 500Mhz and to clock up to full speed 1Ghz, you need to use as you say their extension and software, to set the software controlled but this software does not work with the last versions of OS X Tiger

and your also SOL if your running other odd ball setups like OS X Server 1.x or Linux etc

so im quite pleased to see hardware control over the multipliers :)


one thing I have always wondered is how PowerLogix got round the Unknown CPU block in BootROM of the PowerMac G3 BW, theres no BootROM patches needed and the CPU cards have no complex logic on them either

so the PowerMac G3 should not boot without patches because the 750GX's PVR is completely different to that of 750

but when you boot one of these you will see in OpenFirmware it reports itself as a regular 750 right down to the PVR value which I find most curious, I almost wonder if the 750GX itself has some sort of "legacy" mode where it pretends to be a 750?
 

Trash80toHP_Mini

NIGHT STALKER
I feel like I've asked this before, but nothing came up in search. Maybe it was lost in the great crash of 2021.
ISTR the discussion predates the sinkhole by a year or two. IIRC it was a lengthy discussion in a thread about a Blackbird CPU Card G3/PBX sandwich?
 

Siliconinsider

Well-known member
Well it was a good plan to add the DIP switches then! Did not thought of the impossibility to use software PLL settings in specific cases.
 

macuserman

Well-known member
My burnt 800mhz zif card arrived today and one of the pins on the zif pin array is snapped off.... I wonder if that was the cause of the initial failure or happened in shipping it wasn't packed terribly well, but I don't see the pin the bag. Not sure if it is repairable or not anymore, I was thinking chip swapping would be the way to go on it, but not sure if the zif pin array could be swapped from a donor card.
 

Siliconinsider

Well-known member
You could try fixing the pin, it would be difficult to swap the entire socket without doing reflow soldering with solder paste, but that's also possible.
 

macuserman

Well-known member
You could try fixing the pin, it would be difficult to swap the entire socket without doing reflow soldering with solder paste, but that's also possible.
I sent you a message, be happy for you to use this as a debug board to compare with your design and possibly even repair.
 

herd

Well-known member
This was posted elsewhere, but for reference:

www.sterpin.net/uk/pismologixuk.htm

In case the link goes dead, here are a couple of the pictures:

750FXa.jpg

750FXb.jpg

Recreating the 7400 to 750FX adapter board might be more versatile than recreating just the ZIF version. Maybe the cheap 750GX chips could be installed in a variety of places...
 

Phipli

Well-known member
It however does not work as I did blow the VRM, so there's possibly a short somewhere in my design that I need to trace.
Hey Siliconinsider, incredible work.

Just double checking which VRM type you had. I'm not 100% sure what the failure mode is, but some Beige G3 VRMs used to stop regulating properly and toast things when you installed an upgraded CPU.

 

Siliconinsider

Well-known member
Hey Siliconinsider, incredible work.

Just double checking which VRM type you had. I'm not 100% sure what the failure mode is, but some Beige G3 VRMs used to stop regulating properly and toast things when you installed an upgraded CPU.

Oh that's interesting, yes I don't have the Royal Tech VRM in my machine, still have to move forward with tracing the problem in my design...
 
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