Daughterboard Pinouts

Daughterboard Pinouts

How to quickly determine the JST Pinout of a daughterboard

When retrofitting a PCB into a keyboard case, one of the most common issues is ensuring that the daughterboard is compatible with the PCB. If the pinout of the daughterboard doesn’t match that of the PCB (or vice versa), you risk frying your PCB—that's no bueno.

Not every keyboard uses a Unified Daughterboard; many use proprietary daughterboards of various shapes and sizes that aren’t easily replaceable with a Unified Daughterboard.

Here's how you can quickly and easily determine the pinout of any daughterboard. 

All you'll need is the following:

  • A multimeter with continuity testing mode - this checks for a complete circuit (or continuity) with the two probes, and should beep or indicate when touched together. 
  • The USB-C connector datasheet - we need this for identifying the pins on the JST connector. This one is pretty standard. 
  • Magnifying glass if your eyesight is poor like mine.

Step 1: Identify the Relevant Pins on the USB-C Connector

We’re mainly interested in the VBUS, DP(+), DN(-), and GND pins because these correspond to the four JST connector pins on the daughterboard.


For your standard USB-C connector: 

  • GND: A1/B12 and A12/B1; located on the leftmost and rightmost pins (continuity also usually through the legs and housing).
  • DP(+): A6 and B6
  • DN(-): A7 and B7
  • VBUS: A4/B9 and A9/B4

This means each JST pin will have continuity with the corresponding USB-C pins above. We just need to discover which USB-C pins each JST pin connects to.

Step 2: Test Continuity to Identify JST Pins

Put your multimeter in continuity mode (check by touching probes together to hear a beep indicating a complete circuit).

Now we can start identifying each JST pin by finding the corresponding USB-C pin. 

  1. Place one probe on a JST pin and use the other probe to test each of the USB-C pins.
  2. When the multimeter beeps, you’ve found the matching pin.
  3. Refer to your datasheet to confirm the USB-C pin function.

In this example (a DB60 daughterboard), while holding a probe to the second JST pin (white wire) and sweeping the USB C pins with the other probe, the multimeter beeps on the 6th USB C pin from the left and 5th USB C pin from the right. This corresponds to A6 and B6 according to the datasheet--so the second pin (white wire) is the Data Positive pin. 

Do this for all the JST pins mapping each one as you go. The GND and VBus pins on the USB-C connector should be easy to identify as they're on the ends of the pins array, while DP and DN pins may need a magnifying glass and good lighting. 

Step 3: Map the Pins

You can map it however you like. I recommend drawing a little diagram for clarity. My BD60 daughterboard (pictured) pinout diagram looks like this: 

Step 4: Reorder the JST Cable Pins

Once all pins have been mapped, you'll likely need to reorder the JST cable pins/wires to match the daughterboard's pinout so it connects correctly to the main PCB. You can find a quick guide for removing JST cable wires here.

What if You Don’t Know the JST Pinout on Your PCB?

If you do not know the JST pinout of your PCB, you can often get a general idea of what each pin is with these basic steps (definitely reach out to the designer first to see if they can provide that information). 

(Pins are labeled on this PCB but it's just a demonstration).

For instance, if you’re working with an Atmega32u4 chip, its datasheet will tell you:

  • Pin 2 is 5V power
  • Pin 3 is DN
  • Pin 4 is DP
  • Pins 5, 15, 23, 35, and 43 are GND

You can use your multimeter to probe each JST pin and check for continuity with these chip pins.

The caveat here is the complexities of different chips and PCB designs. For something like the RP2040 chip, you can usually find the DP and DN pins on pins 47 and 46, but for the power pin you may need to find the trace that leads to a voltage regulator stepping down the 5v to 3.3v.  

Conclusion

I hope this helps those struggling with proprietary daughterboards and JST pinouts. It's a fairly simple process that can save you from bricking your PCB and opens up the possibility of using your favorite PCB or layout in many more cases. 



Back to blog