EuroBusboard

A Eurorack bus board with 8 module slots, distributing ±12V + GND across multiple connector formats. The project has three PCB variants in the same KiCad project: 10-pin IDC + JST combined, 10-pin IDC only, and JST only.

For the Doepfer-standard 16-pin IDC variant (with +5V, CV, Gate rails additionally distributed), see the sibling repo: EuroBusboard-8-IDC-16PIN.

Variants

The KiCad project contains four configurations. Pick one when fabricating the PCB:

Variant	KiCad file	Per-slot connectors	Use case
16-pin IDC (Doepfer A-100)	(in separate repo: EuroBusboard-8-IDC-16PIN)	1× 16-pin IDC	Modules with full Doepfer ribbon cables (±12V + GND + +5V + CV + Gate)
10-pin IDC + JST (combined)	eurobusboard_idc_jst_8.kicad_pcb	1× 10-pin IDC + 1× 3-pin JST	Maximum flexibility — accepts both connector types per slot
10-pin IDC only	eurobusboard_idc_8.kicad_pcb	1× 10-pin IDC	Compact, IDC-only modules
JST only	eurobusboard_jst_8.kicad_pcb	1× 3-pin JST	DIY modules that use only the 3-pin JST power input

All variants share the same power-input + reverse-protection + filter section. Only the per-slot connectors differ.

Pinout (10-pin IDC, Eurorack subset of Doepfer A-100)

Looking at the IDC connector with the keyway (red stripe) on the bottom:

Pin   10  9   8   7   6   5   4   3   2   1
      +12 +12 GND GND GND GND GND GND -12 -12

-12V is on pin 1 / 2 (the red-stripe side) — Eurorack convention. +12V is on pin 9 / 10 (opposite side). Six GND pins in the middle provide low-impedance ground return.

The 3-pin JST is:

JST pin	Net
1	+12V
2	GND
3	-12V

Schematic

• Latest: Rev 0.1 (EuroBusboard-Schematic-rev0.1.pdf) — 3 sheets, one per non-16-pin variant

Power input / protection

Same circuit on all three sheets, just renamed nets per variant:

External ±12V supply
   │
   ├──[D3 / D14 / D9   1N4007 reverse-polarity diode on +12V]──┬── +12V bus
   │                                                            │
   │                                                            [C1 / C7 / C5   100µF bulk cap]
   │                                                            │
   │                                                            [D1 / D13 / D11  +12V indicator LED + R1/R7/R5 10K]
   │
   └──[D4 / D16 / D10  1N4007 reverse-polarity diode on -12V]──┬── -12V bus
                                                                │
                                                                [C2 / C8 / C6   100µF bulk cap]
                                                                │
                                                                [D2 / D15 / D12  -12V indicator LED + R2/R8/R6 10K]

Additional features (combined-variant only):

• Power Thru (J20, J21) — daisy-chain to another bus board
• Test points TP1 (+12V), TP2 (GND), TP3 (-12V) for DMM probing

Design notes

1N4007 diodes have a real voltage drop. At a peak transient current of 1 A across each rail (with all 8 modules pulling momentarily), the diode drops about 1.0 V — modules see only ~11 V on the rail instead of 12 V. For most analog modules this is harmless; for tight regulators (e.g. an LM7805 regulating to +5V from +12V supply that's actually 11V), it can reduce headroom. A 1N5817 Schottky (~0.4 V drop) would be a near-drop-in upgrade. See issue.

No +5V or CV/Gate rails. The 10-pin IDC variants don't distribute +5V (which most digital modules need) or CV/Gate (which the Doepfer A-100 standard provides for keyboard-bus signals). Modules requiring +5V must generate their own (via a 78M05 + the +12V rail — which is what most DIYSynthMNL modules do). For full Doepfer compatibility, use the 16-pin IDC variant in EuroBusboard-8-IDC-16PIN.

Bulk capacitance. 100 µF at the input is reasonable for a small case. For a fully-loaded board with current-hungry modules (digital sequencers, OLEDs, several VCAs), consider adding ~10 µF ceramics near each IDC connector for transient suppression.

License

GPLv3 — strong copyleft. Note: this is different from most DIYSynthMNL modules (which use CC-BY-NC-SA 4.0 for hardware, MIT for firmware). Confirm this license is intentional — for a passive bus board, MIT or CC-BY-SA may be more appropriate. See issue.

Build status

What's ready for builders today, and what's still on the TODO list:

• Schematic — Rev 0.1 (EuroBusboard-Schematic-rev0.1.pdf)
• PCB layout — in progress — four variants in kicad/, no rev marker yet
• Gerber files for fabrication — none yet
• BOM — bus boards are simple; the schematic doubles as the BOM (LED color, diode replacement to Schottky if you take the recommendation above, etc.)
• Photos of an assembled board — see photos/
• License — LICENSE (GPLv3 — see note above)