Compute blade dda adapter

Compute Blade ↔ DDA GPS/RTC adapter — parallel-DDA revision

This repository contains a passive two-connector KiCad adapter that remaps the Dark Dragons Astronomy GPS PPS signal while keeping the unmodified DDA PCB parallel to the Compute Blade PCB. Physical connector pin numbers are authoritative; GPIO names are descriptions and are never interpreted as BCM pin numbers.

The project targets KiCad 10.0.5 through the pinned kicad/kicad:10.0.5-full CI image.

Passing connectivity, ERC, DRC, CAD collision, and clearance checks is required but does not make this design fabrication-ready. Connector seating forces and final BladeRunner fit still require a physical prototype.

Approved architecture

• J1: Samtec HLE-105-02-L-DV-PE-BE, 2×5, 2.54 mm, bottom-entry/pass-through receptacle.
• J2: Samtec MTLW-106-05-G-D-140, 2×6, 2.54 mm, conventionally mounted vertical male header; its manufacturer-designated gold-plated mating post points toward the DDA.
• Adapter: 2-layer 1.0 mm FR-4, nominal 1 oz / 35 µm copper.
• Compute Blade, adapter, and DDA PCB planes are all XY.
• DDA socket/battery face points inward (−Z), toward the adapter and Compute Blade.
• DDA GNSS face points outward (+Z), toward the neighboring blade.
• The DDA remains electrically and mechanically unmodified.

The exact footprints are project-local in pcb/Adapter.pretty/. They use the manufacturer 2.54 mm patterns, explicit fab outlines, courtyards, 1.0 mm drills, and 1.8 mm pads. That gives a nominal 0.40 mm annular ring. The 1.0 mm board provides a conventional, stiff interposer, but the selected fabricator must still confirm 1.0 mm FR-4, 1.0 mm plated drills, and its finished-hole/annular-ring rules before ordering.

The generated connector STEP models are manufacturer-dimensioned approximations because exact configured Samtec CAD is not available to deterministic unauthenticated CI. They are not substitutes for a first-article fit test.

Coordinate and orientation contract

X = Compute Blade long axis
Y = Compute Blade width
Z = outward normal from the Compute Blade PCB

Compute Blade PCB plane = XY
adapter PCB plane       = XY
DDA PCB plane           = XY
DDA PCB normal          = +Z
DDA socket mating axis  = -Z toward adapter
J2 mating posts         = +Z
J2 six-position axis    = +X
J2 row 1 → row 2        = -Y

The shared DDA transform is the identity basis:

[[1, 0, 0],
 [0, 1, 0],
 [0, 0, 1]]

scripts/design_config.py is the sole parameter source used by the PCB generator, geometry validator, STEP generator, SVG generator, and PNG renderer. Unit tests hard-fail if the DDA becomes upright, the GNSS face points inward, the mating direction reverses, or J2’s columns/rows change axes.

Exact physical-pin mapping

Pins J1.1–J1.10 map one-for-one to J2.1–J2.10. J2.12 additionally joins J1.7/J2.7, so GPS PPS reaches Compute Blade physical pin 7 / GPIO4. J2.11 remains explicitly unconnected. Duplicate rails remain separate: 5V_PIN2, 5V_PIN4, GND_PIN6, and GND_PIN9 are not merged by the adapter.

Dimension provenance and stack calculation

Physically measured Compute Blade dimensions:

• PCB surface to header-plastic top: 2.500 mm.
• PCB surface to pin tip: 9.000 mm.
• Exposed post: 6.500 mm.

Manufacturer HLE requirements:

• Bottom-entry minimum reach: 2.590 mm plus host PCB thickness and the intentional seating gap.
• J1_SEATING_GAP_MM = 0.350 is the single authoritative gap between the Compute Blade header-plastic top and adapter/J1 seating plane.
• With a 1.000 mm adapter and the 0.350 mm gap, required reach is 3.940 mm.
• The measured 6.500 mm post exceeds this requirement by 2.560 mm.
• -PE-BE is open/pass-through, so excess post does not bottom in a closed socket.
• Adapter underside is therefore 2.850 mm above the Compute Blade PCB. The 0.350 mm separation is an intentional assembly datum, not an assumed connector-body height; assembly should use a 0.35 mm seating gauge/fixture and verify the resulting separation on the first article.

The same exact connector Z-chain was evaluated at all requested gaps:

The preferred 0.350 mm value passes every hard gate while retaining more BladeRunner margin than 0.400 mm and giving more tail clearance than 0.300 mm.

Manufacturer MTLW geometry:

OAL                                      8.510 mm
insulator                                1.520 mm
designated -140 mating post              3.556 mm
solder tail                              3.434 mm
DDA insertion                            3.400 mm
free post after insertion                0.156 mm
lower solder-tail tip above blade PCB    0.416 mm

MTLW-106-05-G-D-140 uses the normal manufacturer orientation: the -140 post is the DDA mating end and receives the specified 10 µin gold plating; the opposite 3.434 mm segment is the solder tail. Samtec lists this exact configuration in its Reserve program. See the exact Samtec product page and official MTLW series print.

Measured DDA geometry:

• PCB: 16.5 × 22.5 × 1.6 mm.
• Socket mating face to socket-side PCB surface: 8.3 mm.
• Socket-side PCB surface to GNSS top: 4.0 mm total.
• GNSS projection beyond the opposite PCB surface: 2.4 mm.
• Battery/RTC inward local envelope: 4.75 mm.

The 4.0 mm value already includes the 1.6 mm PCB. It must not be calculated as 1.6 + 4.0.

adapter underside                                  2.850 mm
+ adapter PCB                                      1.000 mm
+ MTLW insulator                                   1.520 mm
+ unused designated post after insertion           0.156 mm
= DDA socket mating face                           5.526 mm
+ mating face to socket-side PCB surface           8.300 mm
+ socket-side surface to GNSS top                  4.000 mm
= total outward stack                             17.826 mm

Placement and BladeRunner clearance

The official Compute Blade STEP anchors J3 at (133.075077, 18.325065, 0.0) mm. The selected complete DDA projection is:

X = 160.000 .. 176.500 mm
Y =  19.500 ..  42.000 mm

The validator extracts the official PCB solid bounds from the STEP and requires the full DDA PCB, socket, GNSS envelope, and inward battery envelope to remain inside its XY footprint with a 0.5 mm configured margin.

BladeRunner clearance uses the physical slit-edge result, not slit centerline pitch:

physical per-blade clearance       19.9317 mm
safety reserve                      1.0000 mm
design maximum                     18.9317 mm
approved outward stack             17.8260 mm
physical nominal clearance          2.1057 mm
margin after 1.0 mm reserve         1.1057 mm

CI authenticates the pinned official half body.stl, rechecks its mesh encoding, triangle count, hash, and outer bounds, and applies the previously slit-edge-derived 19.9317 mm physical clearance. It does not substitute the obsolete (-1.5, 45.0) clearance box or the former 36 mm total-depth rule.

Official CAD and mechanical validation

scripts/fetch_reference_cad.py authenticates upstream CAD at immutable Compute Blade commit 9c7e472f4fb5c74401d17cdc7a766254d78a32c6:

• models/blade/v1.0-mk4/v1.0_mk4_DEV.step
• models/bladerunner/19-inch/half body.stl
• models/bladerunner/19-inch/left bracket.stl
• models/bladerunner/19-inch/right bracket.stl

Pure-Python validation checks the shared transforms, pin-1 mapping, designated J2 mating end, connector reach, 3.40 mm insertion, the 0.30 mm minimum tail clearance, DDA XY keep-in, 17.826 mm stack, 2.00 mm minimum physical clearance, and 1.00 mm minimum post-reserve margin. The CadQuery assembly stage additionally intersects the DDA, J2, and adapter-outside-J1 region against the exact official Compute Blade B-Rep and fails on material overlap.

The confirmed DDA pin-1 orientation remains:

• readable GNSS/component side: bottom-left;
• underside/socket-hole view: bottom-right.

This is encoded independently of electrical net names.

Inspecting the assembly

scripts/generate_assembly_model.py produces:

• mechanical/generated/full_assembly.step
• mechanical/generated/full_assembly_lightweight.step
• mechanical/generated/full_assembly_with_bladerunner.step
• standalone J1/J2 approximation STEP models
• top, side, end, isometric, J1-closeup, and J2/DDA-closeup PNGs

The full assembly imports the exact official Compute Blade B-Rep. The BladeRunner-context assembly adds the validated 18.9317 mm design plane and 19.9317 mm physical-clearance plane. All placement uses the same boxes and transforms as validate_geometry.py; there is no separate visual-only placement.

Open the STEP files in FreeCAD. From a successful GitHub Actions run, download the compute-blade-dda-adapter-reports artifact for the STEP files, fixed renders, manifests, ERC/DRC reports, and board views. The visual model is an inspection aid and does not replace a physical prototype.

Regeneration and CI

python3 scripts/fetch_reference_cad.py
python3 scripts/generate_schematic.py
python3 scripts/generate_or_update_pcb.py
python3 scripts/generate_mechanical_model.py
python3 scripts/verify_connectivity.py
python3 -m unittest discover -s scripts -p 'test_*.py' -v
python3 scripts/validate_geometry.py
python3 -m pip install --requirement requirements-assembly.txt
python3 scripts/generate_assembly_model.py

GitHub Actions repeats deterministic generation, connectivity verification, unit tests, geometry validation, STEP round-trip checks, blank-render checks, KiCad ERC/DRC, Gerber export, and Excellon drill export. Download compute-blade-dda-adapter-manufacturing for compute-blade-dda-adapter-gerbers.zip; the BOM is included in compute-blade-dda-adapter-reports.

Prototype-only risks

• First-article confirmation that the specified 0.350 mm HLE seating gap can be set and retained using the assembly gauge/fixture, plus practical soldering on 1.0 mm FR-4.
• Production-tolerance and solder-fillet confirmation for the nominal 0.416 mm gap between the J2 tail tips and Compute Blade PCB plane.
• Physical connector insertion and extraction forces.
• Residual differences between simplified connector bodies and production parts.
• Final assembled fit in a real 19-inch BladeRunner, including manufacturing and seating tolerances.
• First-article confirmation before ordering multiple boards.

The project must not be described as fabrication-ready until those physical checks are closed.