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+++ title = "Stop Using TRRS for Split-Keyboard Interconnects!" date = 2025-03-20T14:10:01+01:00 draft = true +++
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. They are cheap, compact, and thanks to their popularity, come in a variety of aesthetic styles.
However, TRRS jacks were only designed for passive electrical components, and expose one large flaw when used actively. When a TRRS cable is (dis)connected, the tip of the plug will slide past every single contact of the jack. Likewise, the first contact of the jack will slide past every contact of the plug.
To illustrate this, let us consider a TRRS setup where 5v is applied to the tip. In this example, assume this plug is on the passive side of the board, receiving power from the active side plugged into USB. When fully plugged in @5v_plugged_in, everything is connected properly. However, when pulled out, 5v immediately makes contact with the TX line @5v_partial_plug.
5v tip TRRS fully plugged in
A 5v tip TRRS starting to be pulled out. Notice the short between 5v and Tx
When the 5v Aurdino Pro Micro dominated as a keyboard MCU, a brief short between 5v and Tx/Rx may have been acceptable. However, due to the emergence of RP2040 powered drop in replacements for the Pro Micro, such as the Elite-pi or KB2040, 3.3v logic levels are now commonplace among keyboards. 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.
Now, what if we put the 5v at the base, so that it is the first pin disconnected?
A GND tip TRRS fully plugged in
A GND tip TRRS starting to be pulled out. Notice the short between 5v and Rx.
In this case, we are looking at the active side of the board, connected to USB, and supplying power to the passive side. Now, when unplugged the 5v contact of the jack will immediately make contact with the Rx line, pulling it up to 5v and damaging the pin on the passive side of the board.
No matter what order we put the contacts in, one end of the TRRS cable will be unsafe to unplug while powered. No other electronics found in your home suffer permanent damage from simply being unplugged in the wrong order. In a moment of carelessness or forgetfulness, damage to hardware could easily happen.
So what are the alternatives? USB-C, while almost as small as TRRS, are more expensive component wise and and having the same connector for board-to-board and PC-to-board connections may lead to user error. There are also a wide variety of JST and Molex connectors, some of which rival TRRS in size, but premade cables are not readily available, and many connectors have a tendency to work themselves loose over time. My personal favorite are 4P4C connectors, also known as RJ9, RJ10, or RJ22. While bulky on the PCB, the connection is sturdy, cables are availible, and one can make ones own cables with a cheap crimping tool. {{
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.
(This article was originally published in issue #6 of the Paged Out! magazine.)