I have since decided that, to prevent devices expecting lower voltage from being plugged into higher-voltage ports and getting fried, it's better to use GX12-4 connectors (slightly smaller sibling of GX16-4) for 3.3V connections and GX16 for 5V connections, which could be USB or connections to I²C devices that expect 5V.
I want to wire up some I²C devices using GX16-4 connectors, also known as 'aviation connectors'. Which pins should go to which I²C wires? If possible I would like to follow existing wiring / pinout conventions.
(Which I am calling "WSTYPE-4106", which is short for urn:oid:1.3.6.1.4.1.44868.260.4106)
| Pin | Color | I²C role |
|---|---|---|
| 1 | Red | Positive |
| 2 | Yellow | SCL |
| 3 | Green | SDA |
| 4 | Black | Ground |
Looking at the front of the male GX16-4 connector,
pins are numbered counterclockwise starting from the bump, 1, 2, 3, 4,
corresponding to +, SCL, SDA, and -,
respectively.
Warning! As noted by the unicycle charging port page, The numbering order on GX16-X connectors is different from that of GX12-X ones (they are mirrored from left to right). If you adapt this standard to GX12-4s, make sure to use the GX12-4 pin number to position mapping. i.e. pin 1 should still be + and so on, but the wiring will be mirrored from the GX16 version.
None of these use aviation connectors.
+, -, SCL, SDA.+, -, SCL, SDA (red, black, yellow, green, respectively).Some people like to use GX12-4 or GX16-4 for USB keyboards. They seem to have standardized on the following pinout:
| Pin | Color | USB role |
|---|---|---|
| 1 | Red | +5V |
| 2 | Yellow | Data- |
| 3 | Blue | Data+ |
| 4 | Black | Ground |
This matches the pinout (by number) of regular USB connectors.
These are a thing, and apparently use GX16-3 connectors with pin 1 carrying the positive voltage, and pin 2 the negative (maybe the third pin is used for ground?). See this electricunicycle forum post or this page about charger customization.