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For instance, could you get a printed circuit board and connect the cpu to that? I want to connect multiple cheap cpus to each other without spending thousands of dollars on a motherboard that will only hold three or four. If not, what is the actual problem with doing that?

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    \$\begingroup\$ Cpu for Xeon x5650. I think that’s the name. I don’t really know how cpu names work but that was on the Amazon title. \$\endgroup\$ Dec 27 '20 at 8:55
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    \$\begingroup\$ Surely as soon as you create a PCB for your CPUs, then it becomes a motherboard. \$\endgroup\$
    – HandyHowie
    Dec 27 '20 at 8:59
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    \$\begingroup\$ A Xeon? Forget about it then. Modern x86 platforms are so complex you'll find that several thousands of dollars for a commercial server motherboard is pretty cheap. \$\endgroup\$ Dec 27 '20 at 9:04
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    \$\begingroup\$ (For x86) They contain complex switching DC-DC converters to power the various chips. They contain the chipsets, memory slots, I/O expansion slots, etc... All of which work at such insane high speeds these days that designing the PCB for required signal integrity is a art of it's own. Equipment that can measure all this in order to troubleshoot the design costs fortunes. And then you don't even have a working BIOS/various firmwares. \$\endgroup\$ Dec 27 '20 at 9:16
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    \$\begingroup\$ Xeon x5650 is a decade old; a single Ryzen may be more powerful than a dozen of them. \$\endgroup\$
    – pjc50
    Dec 27 '20 at 12:12
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The question is rather conflicted, so I will address each question raised separately.

In the strictest sense, you could use a CPU without a motherboard. You could do things such as wire wrapping and point-to-point wiring. I've done plenty of wiring like that when I was a teen (though with oscillators and makeshift AM radios, not digital electronics). However, that makes things hard to manage, and you have issues with signal integrity. You might have fair luck doing this with old CPUs in DIP packages, but I would not try that with anything that is anywhere close to modern. It would be a nightmare.

Now arguably, as soon as you mount a CPU on a board, you have a motherboard or at least a CPU adaptor board.

Now, you generally cannot directly connect multiple CPUs together on the same bus at the same time. You'd need to know the inputs and outputs of each device separately.

Plus, like in other answers, you don't want to connect lines with conflicting signals. If one device is being driven high, and another is being driven low, and you connect their lines together, you could damage one or both devices or even your PSU. There could even be catastrophic damage such as a fire.

You'd likely need a custom chipset to handle multiple CPUs beyond what the board allows. That would not be a trivial matter.

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Creating a board from a blank PCB for the processor is pretty much unusable and not worth it.

Because:

  • You need to reverse engineer the entire CPU block.
  • You need to understand all I/O
  • You need basically to understand the language code for CPU eg: x86 CPU has much code.
  • It doesn't work with a parallel connection at all, because it's digital input and output, it's like you adding a Positive terminal from DC with a Live Terminal from AC, it will destroy each of them or both.

That's why in server motherboard that has 2 or more CPU have another chipset that specially built for handling the data I/O and separate function of DRAM.

Even if you successfully build the board is still called a motherboard because the final design will be pretty much a motherboard that you buy thousand dollars except this time it will cost you around 5x of what you expected it.

The retail price for a motherboard is pretty much cheaper rather than you build one of yourself. Remember you even need to build a chipset especially for this motherboard which is I doubt any manufacture will make it for you only 1 chipset, the minimum order such as this one is around 1000-5000 pieces.

Sometimes the buying option is better than DIY from scratch.

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  • \$\begingroup\$ Parallel and digital are not at all mutually exclusive. \$\endgroup\$
    – Hearth
    Dec 27 '20 at 16:17
  • \$\begingroup\$ Making an assumption for digital connection especially in a very sophisticated unit like CPU is a bad idea in general. It's impossible without resources telling what when and where the pin communicate with each other. \$\endgroup\$ Dec 27 '20 at 17:34
  • \$\begingroup\$ Yes, trying to do that in this case is not a good idea, but it's not because it's digital. \$\endgroup\$
    – Hearth
    Dec 27 '20 at 17:36
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A motherboard is simply a printed circuit board. What devices you use and what your system design is determines what you put on one or more PCBs. If the system level design for that board requires complicated power or power up/power down timing that can cause more support on that PCB, cplds, dc-dc converters, etc. If you want external I/O, ethernet, pcie, sata, uart, etc. Each of those require both connectors of some sort, plus possibly phys or transceivers or other. More stuff on the PCB.

It is absolutely possible for you to make your own as complicated or not as you like. You have to do your system level design and then "simply" implement it. Getting modern intel or amd processors working, is unlikely to happen any time soon. There is a fair amount of black magic and tribal knowledge that is involved, plus the tens of thousands to hundreds of thousands of dollars of NRE that it takes to do the design, get the boards manufactured, tested, respun, tested, respun, etc. Plus the tens of thousands or more to get a bios.

If you are not talking x86 then your odds are significantly higher, the cost is still quite large somewhere between the cost of a new car and a house.

If you are thinking microcontrollers then you can do that for lunch money...maybe the cost of a few nice dinners at a restaurant.

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