63 lines
3.8 KiB
Markdown
63 lines
3.8 KiB
Markdown
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title = "Stop Using TRRS for Split-Keyboard Interconnects!"
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date = 2025-04-05T14:10:01+01:00
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draft = false
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TRRS (Tip Ring Ring Sleeve, or, as you may know it, "headphone jack with microphone support") cables have long been the go to connector between split keyboard halves.
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They are cheap, compact, and thanks to their popularity, come in a variety of aesthetic styles.
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However, TRRS jacks were only designed for passive electrical components, and expose one large flaw when used actively.
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When a TRRS cable is (dis)connected, the tip of the plug will slide past every single contact of the jack.
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Likewise, the first contact of the jack will slide past every contact of the plug.
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To illustrate this, let us consider a TRRS setup where 5v is applied to the tip.
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In this example, assume this plug is on the passive side of the board, receiving power from the active side plugged into USB.
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When fully plugged in, everything is connected properly.
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However, when pulled out, 5v immediately makes contact with the TX line.
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5v tip TRRS fully plugged in
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A 5v tip TRRS starting to be pulled out. Notice the short between 5v and Tx
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When the 5v Aurdino Pro Micro dominated as a keyboard MCU, a brief short between 5v and Tx/Rx may have been acceptable.
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However, due to the emergence of RP2040 powered drop in replacements for the Pro Micro, such as the Elite-pi or KB2040,
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3.3v logic levels are now commonplace among keyboards.
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Thus, shorting the 5v power line with a logic pin is a surefire way to burn out at least a GPIO, if not your whole MCU.
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Now, what if we put the 5v at the base, so that it is the first pin disconnected?
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A GND tip TRRS fully plugged in
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A GND tip TRRS starting to be pulled out. Notice the short between 5v and Rx.
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In this case, we are looking at the active side of the board, connected to USB, and supplying power to the passive side.
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Now, when unplugged the 5v contact of the jack will immediately make contact with the Rx line,
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pulling it up to 5v and damaging the pin on the passive side of the board.
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No matter what order we put the contacts in,
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one end of the TRRS cable will be unsafe to unplug while powered.
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No other electronics found in your home suffer permanent damage from simply being unplugged in the wrong order.
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In a moment of carelessness or forgetfulness, damage to hardware could easily happen.
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So what are the alternatives?
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USB-C, while almost as small as TRRS, are more expensive component wise and and
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having the same connector for board-to-board and PC-to-board connections may lead to user error.
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There are also a wide variety of JST and Molex connectors, some of which rival TRRS in size,
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but premade cables are not readily available,
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and many connectors have a tendency to work themselves loose over time.
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My personal favorite are 4P4C connectors, also known as RJ9, RJ10, or RJ22.
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While bulky on the PCB, the connection is sturdy, cables are availible, and one can make ones own cables with a cheap crimping tool.
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{{<figure src="./4C4P.jpg" title="A 4C4P connector (Wikimedia, CC-BY-SA 3.0)" width="50%">}}
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There are of course other connectors, and any with at least 4 conductors will work for a split keyboard.
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Unfortunately, there does not seem to be a perfect connector, but there are many alternatives better than TRRS.
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*This article is dedicated to the late pin D26 of Jonathan's Ferris Sweep. He is forever grateful that the Elite-pi has extra GPIOs.*
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*(This article was originally published in issue #6 of the [Paged Out!](https://pagedout.institute/) magazine.)*
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