Connecting a controller with only a USB-C port to a daughterboard that only has a JST port

Etherial · August 9, 2023, 8:31pm

I have a TKC M0lly that I love but almost never use, because it doesn’t have dedicated arrow keys. (Yes, I can use the numpad 8/4/6/2 keys, but it feels weird. And without legends printed on the keys, I forget they can even be arrows.)

I want to rebuild the board with a custom plate to add arrow support (and a split spacebar, but that’s mostly just for looking cool). I have the official plate file, and made the necessary tweaks to it. But since that’s not a supported layout for the original PCB, I’ll need to handwire everything.

M0lly, now with arrows and split spacebar:

No problem, I’ve handwired before! Except, the M0lly uses a daughterboard, that its PCB connects to using what looks to be a 4-pin JST cable.

Here’s the daughterboard side:

And here’s the original PCB’s side:

The daughterboard is weirdly-shaped, and not only has the USB-C port for the external cable, but is also a hub with an additional USB 3.1 port on it. I feel like I’ll likely have to make use of the original daughterboard, since nothing else(?) will fit into the space.

I have no idea how to connect a handwired board’s controller to a daughterboard that only has a JST connector on it. I’m only familiar with RP2040-based controllers, none of which seem to come with the connector I’d need.

Example of the type of controller I’d typically use:

Can I take a short USB-C cable, plug that into to the controller’s USB-C port, snip off the other end of the cable, put four wires into a JST header, and plug that into the daughterboard?

Is the fact that the daughterboard is also a hub going to complicate anything?
What if there’s more than 4 wires in the cable - which ones do I ignore?
How do even I know if the pins on the JST end are in the right order?
Or, is there another daughterboard out there that I could use?

My only other idea is to remove the daughterboard altogether, ignore the 3.1 port, run a short USB-C extension cable from the controller to the USB-C hole in the case, and hot-glue it so it stays put. But I don’t want to make any permanent case modifications, should I ever decide to put the original plate and PCB back in.

Or, I can just try to use the numpad to navigate. But it’s been a couple years now, and while I love this board otherwise, I can’t get used to this one issue.

insolentpotato · August 9, 2023, 8:51pm

i’m not (yet) fully good at this topic but the first thing i’d look for:

pin out for pcb and daughterboard can probably be provided by the company or the pcb designer
you have the db, jst cable, and original pcb that should tell us a lot and also talk about how the hub is powered

id always try to start with what is there before going exotic with the soldering/bridging

Can I take a short USB-C cable, plug that into to the controller’s USB-C port, snip off the other end of the cable, put four wires into a JST header, and plug that into the daughterboard?

cc: @techbeezin see above line

also a quick google search didn’t net much about the pcb, so i wonder who designed this to start. cause a lot of your pinout qs can probably be asap answered and you can eliminate some of the choices

Etherial · August 9, 2023, 8:56pm

I know who designed it, BlindAssassin111. Though he’s since parted ways with TKC, and TKC won’t give any technical info about it.

I get the sense the board is essentially an orphan since it was a joint collaboration, and their partnership ended.

Etherial · August 9, 2023, 9:37pm

Part of me wonders what it would cost to just commission four custom arrow keys in NN black and WA white from Signature Plastics - 1.75u r4 Up, and 1.25u r3 Left, Down, and Right; like the SA Laser Exotic kit used to include, to match the SA Spectra caps on there now.

Would it be cheaper and less maddening than totally rebuilding this thing?

insolentpotato · August 10, 2023, 3:01am

ill be the shit head that says do both
the custom caps

and also your handwiring thing

Etherial · August 10, 2023, 11:08am

Your ideas intrigue me, and I wish to subscribe to your newsletter.

Honestly, if I could get those four weird-sized arrows even for a couple hundred, I would. I’m hesitant to pester Signature Plastics for a quote, though. I feel like I’d be wasting their time if I got sticker shock over a price higher than that.

insolentpotato · August 10, 2023, 5:53pm

launch query
receive quote
pursue custom caps or nah based on budget/time to delivery

side tangent to that: find a resin caster willing to take on your idea, why not. won’t be the same sound as ? pbt? abs but gets you the profile.

i’m vibing with this post bc its similar to the thought process i go thru with my own projects. if im not willing to pursue all the avenues (and also the associated costs) that helps me grieve the board and ultimately make a decision if i keep it or nah. so keep going, keeb friend. and im happy to wargame anytime

Etherial · August 10, 2023, 8:25pm

I broke down and asked for a quote. I couldn’t find anything posted already about pricing for a handful of doubleshot ABS keys using existing molds, other than the minimum order limit.

I feel bad potentially wasting their time. But who knows? If it’s somehow magically cheaper than a plate, switches, and everything else that would go into a handwired rebuild, it’d be worth it. And I wouldn’t need to figure out this JST issue.

Otherwise, I’ve already compiled the firmware for a rebuild.

Etherial · August 11, 2023, 1:09am

I may have found a far simpler solution. The Symbiosis gamer keys match with Spectra, since they use the same cadet molds. Boom. Color-differentiated “arrow” keys.

Still curious what SP would charge for custom 1.25 arrow keys, though.

insolentpotato · August 11, 2023, 3:49am

gotta post the whole board now man

Etherial · August 11, 2023, 12:45pm

The light isn’t optimal right now, but here it is, in its current state.

insolentpotato · August 11, 2023, 2:57pm

well done!!

LASERman71 · August 16, 2023, 8:38am

I feel you, been in pretty similar situation (with pretty same keeb) and decided to… skip redo of existing keeb and just commission full bespoke kit reducing overall layout footprint in the process.

Etherial · August 16, 2023, 11:00am

SP wouldn’t give me a quote, so I’m back to considering a handwired rebuild after I do more research on the JST daughterboard issue. The colored WASD keys will do in the interim, but I need a fix that doesn’t involve layers.

LASERman71 · August 16, 2023, 11:22am

I take this is to use right mods as arrows but do you really need to have arrow legends on them?
Right Shift is clear to remember as Up and the 3 keys below are naturally Left Down and Right.
You can even keep their function as tap mods with QMK codes RSFT_T(KC_UP), RALT_T(KC_LEFT) etc.
You can also just get those 4 mods in different colourway (from different keyset) to make your arrows cluster stand out like you did with WASD without commissioning keycaps.

Etherial · August 16, 2023, 12:19pm

Yeah that’s pretty much it; I just get bothered by keys not having the right legends on them for what they do (I’m autistic ), and I also need to match an existing set I love on there (SA Spectra). I can’t really type on anything shorter than SA or MT3 caps. I had asked Signature Plastics for pricing on custom color-matched SP keys, but they wouldn’t provide it.

So I’m cutting a custom plate that can accommodate the 1u arrow keys SA Spectra includes, and handwiring everything since the original PCB doesn’t support that layout.

I think(?) I’ll just need to make a USB-C to 4-pin JST SH cable to connect my handwired controller to the existing daughterboard, since I can’t find any for sale. I still need to conclusively confirm it’s a JST SH connector on there. There’s no public documentation for it, so I’m holding up photos and comparing things visually.

At this point it’s personal, and I want to do it just to prove I can make this board work the way I want. It was the very first board I ever built, and the only one I have without dedicated arrows.