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Similar to this, among custom keyboard builders it's quite popular to have USB handled connected via TRRS headphone jacks between split keyboard halves [0] [1]

Similar to this, among custom keyboard builders it's quite popular to have USB handled connected via TRRS headphone jacks between split keyboard halves [0] [1] and sometimes even the keyboard -> PC Connection being a custom TRRS -> USB cable or other exotic variations. [2] [0] https://keeb.io/products/trrs-cable https://keeb.io/products/trrs-cable [1] https://docs.splitkb.com/hc/en-us/articles/360010443660-Kyria-Build-Guide-TRRS-Jack https://docs.splitkb.com/hc/en-us/articles/360010443660-Kyri... [2] https://www.kingcables.org/products/custom-straight-usb-cable https://www.kingcables.org/products/custom-straight-usb-cabl...

rollcat 3y ago
USB is usually too complex for something like a simple split keyboard; the TRRS runs I2C with sth like a MCP23018-E/SP IO expander in the other half.
- adolph 3y ago
  You can use a physical connector without using the standard protocol.
- crote 3y ago
  Most split keyboards these days have a microcontroller in each half. IO expanders like the MCP23017/MCP23018 are fairly expensive, quite slow when used with microcontrollers like the ever-popular Pro Micro's ATmega32u4, and they really limit your hardware options on the secondary side. It usually makes more sense to just stick a Pro Micro on each side, and have them talk serial over TRRS. As a bonus, you can connect it to your PC from either half.
serf 3y ago
whats old is new again, I remember 'ergo' point-of-sale KBs from the early 80s that were joined via the same method. it's a good reliable plug if you can cram what you need through it, I can kind of understand why people gravitate towards it in design.
- NikkiA 3y ago
  4-pin mini-din and reusing s-video cables is a much better proposition, and how logitech tend to do things.
justsomehnguy 3y ago
Offtopic: Recently I found myself in the market for a custom keyboard, to my great surprise. Custom as not as DIY but as something different than $10 OTC or even $100 Logitech. After the hours of watching pics I decided to just see it with my own hands and went to a shop which specialize on them. And there I confirmed the feeling what I had before - 99% of these keyboards, despite quite advanced tech gone into them on all layers, are not usable, because they are made to stroke the ego of the owner but not for the actual work. This is especially evident by the crammed right cluster which makes typing brackets, curlies and quotes quite a pain, because the Shift key is gutted and nowhere under the right pinky. Using something else than microUSB or Type-C (for the PC connector) is just a cherry on top.
- tuvan 3y ago
  You could just get a custom keyboard that is comfortable for you. Key mapping is generally an independent step. Most of these keyboards are completely programmable with custom keymaps and layers. Many come with blank keycap options, you can also rearrange a regular keycap set.
  - enormousness 3y ago
    You can't do what OP is looking for without a custom PCB. The switches and caps and mappings might be customizable but their physical position and size on the board isn't. Despite pushing the envelope in many ways, custom keyboards can also be strangely traditionalist at the same time. Making a PCB is a risky venture and most manufacturers don't want to risk it. A huge amount of models still use a huge 7u or larger space bar for example when something as small as 4u would suffice, and leave room for much more ergonomic placement of modifiers. (or other improvements like extra keys, keyed gaps etc.)
    - tuvan 3y ago
      Yes, most keyboards still have the traditional layout. But I believe many options fit what OP needs. A split keyboard such as Ergodox EZ allows access to the maximum amount of keys possible without moving your hand. You can then customize the mapping to properly distribute commonly used keys.
lpribis 3y ago
While it is quite popular, TRRS is one of the worst connectors possible for a power rail (eg usb VBUS) because it all but guarantees that VBUS will be shorted to ground if you ever accidentally hot-plug it.
- JKCalhoun 3y ago
  VCC is on the sleeve, last to enter the jack, GND is a ring deeper inside the jack. It's not clear to me how VCC would ever get grounded unless you pushed the connector in too far somehow. There is a note about a diode across VCC and GND — perhaps that is to prevent this kind of short?
  - lpribis 3y ago
    Think about unplugging from the side thats _delivering_ the power. As you pull it out, the GND (which is further in and needs to travel out as it's unplugged) will contact the live VCC. Even from the power consumer side, there is a chance that a single female side contact (eg VCC) may touch two adjacent TRRS sections as the plug is being removed.
    - NikkiA 3y ago
      Also, 3.5mm jacks and plugs WILL suffer oxidation problems from using them for power.
    - thesh4d0w 3y ago
      Isn't solving this as easy as just not having VCC + GND on the pins next to each other? If you shove data in the middle, you're never going to short VCC + GND together. your data line shouldn't care about being high (5v) or low (gnd).
      - lpribis 3y ago
        No so, even if VCC and GND are on opposite ends of the TRRS, they will pass by eachother as the cable is unplugged.

rollcat 3y ago

USB is usually too complex for something like a simple split keyboard; the TRRS runs I2C with sth like a MCP23018-E/SP IO expander in the other half.

adolph 3y ago
You can use a physical connector without using the standard protocol.
crote 3y ago
Most split keyboards these days have a microcontroller in each half. IO expanders like the MCP23017/MCP23018 are fairly expensive, quite slow when used with microcontrollers like the ever-popular Pro Micro's ATmega32u4, and they really limit your hardware options on the secondary side. It usually makes more sense to just stick a Pro Micro on each side, and have them talk serial over TRRS. As a bonus, you can connect it to your PC from either half.

serf 3y ago

whats old is new again, I remember 'ergo' point-of-sale KBs from the early 80s that were joined via the same method. it's a good reliable plug if you can cram what you need through it, I can kind of understand why people gravitate towards it in design.

NikkiA 3y ago
4-pin mini-din and reusing s-video cables is a much better proposition, and how logitech tend to do things.

justsomehnguy 3y ago

Offtopic: Recently I found myself in the market for a custom keyboard, to my great surprise. Custom as not as DIY but as something different than $10 OTC or even $100 Logitech. After the hours of watching pics I decided to just see it with my own hands and went to a shop which specialize on them. And there I confirmed the feeling what I had before - 99% of these keyboards, despite quite advanced tech gone into them on all layers, are not usable, because they are made to stroke the ego of the owner but not for the actual work. This is especially evident by the crammed right cluster which makes typing brackets, curlies and quotes quite a pain, because the Shift key is gutted and nowhere under the right pinky. Using something else than microUSB or Type-C (for the PC connector) is just a cherry on top.

tuvan 3y ago
You could just get a custom keyboard that is comfortable for you. Key mapping is generally an independent step. Most of these keyboards are completely programmable with custom keymaps and layers. Many come with blank keycap options, you can also rearrange a regular keycap set.
- enormousness 3y ago
  You can't do what OP is looking for without a custom PCB. The switches and caps and mappings might be customizable but their physical position and size on the board isn't. Despite pushing the envelope in many ways, custom keyboards can also be strangely traditionalist at the same time. Making a PCB is a risky venture and most manufacturers don't want to risk it. A huge amount of models still use a huge 7u or larger space bar for example when something as small as 4u would suffice, and leave room for much more ergonomic placement of modifiers. (or other improvements like extra keys, keyed gaps etc.)
  - tuvan 3y ago
    Yes, most keyboards still have the traditional layout. But I believe many options fit what OP needs. A split keyboard such as Ergodox EZ allows access to the maximum amount of keys possible without moving your hand. You can then customize the mapping to properly distribute commonly used keys.

lpribis 3y ago

While it is quite popular, TRRS is one of the worst connectors possible for a power rail (eg usb VBUS) because it all but guarantees that VBUS will be shorted to ground if you ever accidentally hot-plug it.

JKCalhoun 3y ago
VCC is on the sleeve, last to enter the jack, GND is a ring deeper inside the jack. It's not clear to me how VCC would ever get grounded unless you pushed the connector in too far somehow. There is a note about a diode across VCC and GND — perhaps that is to prevent this kind of short?
- lpribis 3y ago
  Think about unplugging from the side thats _delivering_ the power. As you pull it out, the GND (which is further in and needs to travel out as it's unplugged) will contact the live VCC. Even from the power consumer side, there is a chance that a single female side contact (eg VCC) may touch two adjacent TRRS sections as the plug is being removed.
  - NikkiA 3y ago
    Also, 3.5mm jacks and plugs WILL suffer oxidation problems from using them for power.
  - thesh4d0w 3y ago
    Isn't solving this as easy as just not having VCC + GND on the pins next to each other? If you shove data in the middle, you're never going to short VCC + GND together. your data line shouldn't care about being high (5v) or low (gnd).
    - lpribis 3y ago
      No so, even if VCC and GND are on opposite ends of the TRRS, they will pass by eachother as the cable is unplugged.