A-A USB serial adapter?
April 5, 2016 9:50 AM   Subscribe

Does an A-A USB serial adapter cable exist? I know I could get two A-DB9 USB serial adapter cables and gang them together. But I want USB A-A. The objective is to run a serial terminal between two linux computers over USB.
posted by GuyZero to Computers & Internet (8 answers total) 1 user marked this as a favorite
They don't exist because that's not an electrical or protocol compatibility. USB-OTG, the thing that lets tablets and mobile devices be both host and device, is a later addition to the spec.

You could get two USB serial adapters, or two USB network adapters...
posted by straw at 9:58 AM on April 5, 2016

Relevant Reddit post from one of the authors of the USB standard

"They had different plugs at each end to prohibit peer-to-peer connections, such as plugging one PC unto another, which wouldn't work and presented power conflicts."
posted by Oktober at 10:09 AM on April 5, 2016 [2 favorites]

It looks like FTDI makes a USB null-modem cable, although it's tricky to find anyone selling it. If you do find it for sale it may well be more expensive than two USB-serial cables and a null modem adapter, which is probably the way to go. Also, you may have a serial port header hidden on the motherboard of one or both of your Linux PCs that would allow you to get by with fewer USB hookups.
posted by pocams at 10:24 AM on April 5, 2016

Q6: You mean I can't make a direct cable connection like a null modem?
A6: Correct. In fact, if you try this with an illegal A to A USB cable, you'll short the two PCs' power supplies together, possibly destroying one or both machines or causing a fire hazard. Even there were no danger to the machines from the problem with two power supplies, there still wouldn't be any way to get the two PCs talking to each other, since USB doesn't support that particular kind of communication. A reasonably priced solution to handle this need is the USB bridge.
-From the USB website

That said, there is something called a "USB link cable" (here's a random one on Amazon) that has extra hardware inside allowing for a Direct Cable Connection on Windows.
posted by Nonsteroidal Anti-Inflammatory Drug at 10:24 AM on April 5, 2016 [1 favorite]

Also, to naysayers: please note that GuyZero is asking for "an A-A USB serial adapter cable", to run a serial terminal between two PCs. This is different from the idea of a USB A-A cable. There's no reason an A-A USB serial adapter cable couldn't exist - it would essentially be two USB-serial cables with their serial chips connected back-to-back, null-modem-style.
posted by pocams at 10:30 AM on April 5, 2016

Doh, thanks for extricating my cranium from my posterior, pocams: Here's the FTDI USB null modem from Digikey.
posted by straw at 10:37 AM on April 5, 2016

Of course FTDI makes one. Thanks!
posted by GuyZero at 12:48 PM on April 5, 2016

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