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If 19.5v is requested, 20v is supplied.

You’re misreading the blog post. That adapter has 19.5V written on it, but it requests 20V. 19.5V is not a PD voltage.

You can have a USB→barrel and you can have a barrel→USB.

The latter is pretty much impossible to make to spec, because it can’t negotiate voltage. A barrel jack is single-voltage by necessity.

The interesting case is what I described, when the appliance and PSU are compatible in terms of voltage but still fail because of the protocol.

Respectfully, I’m going to have to disagree. It’s much more interesting when it catches fire, and I’d rather have a protocol that sometimes doesn’t operate in order to avoid the excitement of one that sometimes explodes

Also consider that a 20v barrel PSU could feed a dumb usb-c adapter which then fries a 5v device.

No matter what charging standard you use, some idiot could cut off the end, wire it to a stun gun, and send 100,000V into your phone. That doesn’t make the standard bad, it makes the adapter bad.

And in both situations (usb→barrel and barrel→usb), it’s not foolproof.

If you do it correctly, a USB power supply being adapted to a barrel jack won’t destroy any device that can tolerate 5V. It may not operate the device, but as long as nothing breaks, I’d consider it as foolproof as anything reasonably can be.

Countless consumers cannot handle the voltage matching task so if the plugs fit, they will try it.

That’s just how electricity works. You can get 5V devices that use 5.5x2.5 barrel jacks and you can get 20V 5.5x2.5 power supplies. You can get C13 leads that connect to 240V wall outlets and you can get C14 devices that only accept 100V. If someone plugged things in without reading before USB, they broke things and started fires. PD simply doing nothing when incompatible devices are connected is a massive upgrade over that, even if it can be frustrating sometimes.

I’m getting the sense from your comments that you think that USB-PD will deliver a higher voltage than requested. This is not accurate. A device can request any of the voltages specified by PD, and the power supply will supply that voltage if it is capable of doing so. If not, it will provide 5V. It’s very possible to get a sippy that won’t power your device, but a working, PD-compliant power supply will not damage a working, PD-compliant device.

Obviously, if you get a barrel jack adapter that requests 20V and then plug it into a 5V device, you’re going to let the magic smoke out, but that’s because barrel jacks don’t do negotiation. The power supply correctly supplies 20V as requested, and then you plug that 20V into the wrong thing.

Huh, TIL that B-keyed slots can have any PCIe at all. I thought it meant SATA-only.

In other words, if it’s correctly labeled, it will work unless Dell actively prevented it from doing so, but that isn’t out of the question.