Differential SPI signal extender

Paired HOST and REMOTE boards carry one transparent SPI connection on four fixed-direction balanced data pairs. A separate two-wire link transports external 12 V power. This document reconciles the retained prototype contract; it does not certify the existing CAD or a physical assembly.

Development status

Baseline documentation snapshot: 2026-10-11, before the requested connector/protection/usability revision program. New revision features require their own reviewed source and qualification evidence.

This page documents an unqualified engineering study, not a released or tested product. Saved CAD, renders and automated checks do not establish a safe assembly, compatible cable, firmware support or a usable current rating.

Board-specific qualification limits: Native0 retained;ambiguous capacitor MPNs,12V-only assessed,timing/fault/physical open.

Interfaces and wiring

Inputs

  • HOST S Gadgeteer connector: canonical pin 1 = 3.3 V input, pin 2 = no connection, pins 6–9 = SPI, pin 10 = ground.
  • HOST external 12 V DC nominal supply, with 10.8–13.2 V defined operating range.
  • Shielded straight-through Cat5e for four data pairs, and a separate two-wire 12 V/return cable of at least 24 AWG (about 0.205 mm²; use 0.25 mm² metric wire or larger).

Outputs

  • REMOTE S Gadgeteer connector: pin 1 = locally sourced 3.3 V, pin 2 = protected locally sourced 5 V, pins 6–9 = matching SPI, pin 10 = ground.
  • HOST MISO and optional active-low STATUS_N on pin 3; pin 3 is no connection at REMOTE. Pins 4 and 5 are unused at both ends.

Power off before connecting or changing jumpers. Verify pin 1, signal direction, supply voltage and ground at both ends of every cable. A mechanically fitting connector is not proof of electrical compatibility. If a host pin mapping or current allocation remains unresolved in the design record, do not connect the board until it is resolved.

Electrical design requirements

The following are specification requirements or study assumptions; they are not measured operating ratings. A newer selected-revision note takes precedence over historical draft options.

  • Retained physical layer: THVD1451DR-class 3.3 V fixed-direction differential channels, one pair per SPI signal, 100 Ω/1%/1206 termination at each receiving end. Receiver common mode must remain within the device’s ±15 V operating limit. The power return supplies the shared reference; the boards are not galvanically isolated.
  • External 12 V is carried outside the RJ45. The RJ45 is a data connector, not Ethernet/PoE. REMOTE load allocation is at most 100 mA on each peripheral 3.3 V and 5 V rail; the actual peripheral and cable loss must be measured. HOST 3.3 V allocation of 150 mA is an assumption pending the named Smoothieboard S-socket limit.
  • The length/rate limits are calculations, not qualified link ratings. Check host CS lead and setup, MAX31855 first-bit and later-bit timing, actual cable delay/skew, receiver eye, and open/short/disconnected behavior before physical approval.
  • The retained contract mentions 24 V sustained fault tolerance; this is unverified and is not an operating range. Sustained wrong-voltage behavior, PTC/TVS coordination, regulator inductor saturation under short, and terminal temperatures remain release gates.

Protection in the design record

The design describes the following protection intent. Check the selected schematic and remaining qualification gates before relying on it:

HOST F1/D1/D2 form the 12 V PTC/reverse diode/TVS path; REMOTE D1/D2 guard the buck input. Four THVD1451 receivers provide the datasheet’s open/short/idle high state, subject to wiring and ground offset. HOST CS is pulled inactive-high when its S pin floats; SCK and MOSI are pulled low. The REMOTE OR gate forces high MISO while deselected, and the HOST buffer leaves S MISO high impedance while deselected. This is not a fault-detection or safety function. Cable open/short/reversal, unpowered endpoints and brownout require physical verification; surge immunity is not claimed.

Selected component architecture

Exhaustive current pair review: v14 HOST/REMOTE retain transparent channels/existing stock annotations,no successor.68 identities/159selected/2058 historical hashes match;actual HOST10k pull-ups supersede100k intent. Ambiguous capacitor MPN fields and12V-only assessed/timing/fault/physical gates remain open.

Configuration and commissioning

Use the configuration and firmware contract for the exact selected revision. Do not copy another module’s socket assignments or assume that an unused pin is available. First-article work must verify source selection, reset/unplug behavior, cable orientation and the actual load or sensor interface before normal operation.

Requested connector and usability upgrades

The following are requirements for the next design iterations; this page does not claim they are already fitted:

  • On electrically suitable three/four-pin low-voltage ports, provide compact alternative 2.54 mm header/Molex-style and keyed STEMMA-family footprints. Populate one connector option per shared position and document its pin order. Use STEMMA QT only for compatible four-wire I²C; connector appearance does not make another signal protocol I²C-compatible.
  • Render each supported connector population separately. Document clearances, cable access and the exact fitted option.
  • Use uppercase titles in a consistent Montserrat-style face, labels on both sides, visible domain boundaries, a revision-specific wiring guide and this page’s URL on the silk where readable.
  • Keep mounting-hole centers on a documented 10 mm lattice; add cable-tie holes where space and electrical clearances permit. Place status LEDs near accessible edges and give every indicator its own disable solder jumper. Provide labeled test pads and a ground observation point.
  • Review applicable DC power entries for reverse-polarity cutoff with a blinking warning, overvoltage cutoff/indication, controlled startup and electronic overcurrent protection. Indicator and protection circuits must work in the fault state without defeating isolation or safe output defaults. Thermal protection belongs only where the power/thermal analysis justifies it.
  • Electronic protection does not automatically replace a required mains/branch safety fuse. Backfeed blocking and inductive-load flyback clamps are separate design functions. Do not add identity EEPROMs, fault memory, calibration storage or paid self-test hardware under this upgrade program.

Design sources

Documentation route: smoothieware.org/extensions/differential-spi-signal-extender.

Additive design studies

The 2026-10-11 upgrade program has saved additional candidates v15, v16, v17. These studies preserve the historical design described above. Their connector, LED, mechanical and protection changes are revision-specific and remain unqualified.

See the per-board changes, failures and remaining work and mandatory family specification. Use the source hash and native check for the exact candidate being reviewed; do not infer hardware ratings from a rendered population.

Upgrade candidate readback

The three additional revisions are v15, v16, v17. They are engineering studies; saved CAD and renders do not release the hardware. The latest candidate is described below. Older baseline notes above remain historical.

host

  • Latest PCB source, SHA-256 6f8ceeed032944219c2e032167a4f3c706df1df5f9601444f1d9a1cf5aa3f04b.
  • Last native check: 21 errors, including 21 open connections. This is a CAD result, not electrical, assembly or firmware qualification.
  • No alternative connector population is recorded for this physical role. Any applicable connector conversion remains an explicit design-review item.
  • normal population/render record: 6 images; source current; image hashes match; 0 unresolved models.
  • Changes, failed trials and remaining work.

remote

  • Latest PCB source, SHA-256 b8794eb336198e1d7016c4b6ef40ddd0136e2c21c0ee4f94b0c4a106302fcaa3.
  • Last native check: 27 errors, including 26 open connections. This is a CAD result, not electrical, assembly or firmware qualification.
  • No alternative connector population is recorded for this physical role. Any applicable connector conversion remains an explicit design-review item.
  • normal population/render record: 6 images; source current; image hashes match; 0 unresolved models.
  • Changes, failed trials and remaining work.

Applicable protection, current-path, thermal, pinout, cable access and physical tests remain open wherever the linked board record has not verified them. A zero native-error count does not close those gates.

Second-round partial repair: v18

The saved host and remote v18 candidates close two connections per role and clear the remote jumper overlap. They remain incomplete engineering candidates, with 19 host and 24 remote open connections. Fresh full-track native checks find no other enabled electrical errors or schematic parity errors, and no schematic-rule errors. Host retains 71 board warnings and one schematic library-link warning; remote retains 95 board warnings and two schematic library-link warnings. Enabled-rule results do not cover physical qualification or ignored rule categories.

Both roles have current revision labels and wrapped documentation URLs. Parent verification checked 127 unchanged baseline files, 151 source/artifact hashes, all model paths and twelve current normal-population views. The dense back-side instruction overlap remains visible. No alternate connector land pattern is declared for either role; connector conversion remains open.

The added power-net copper does not establish a load rating. Current, thermal, mounting, assembly, backfeed, fault-injection and signal-integrity qualification remain open. Manufacturing exports and renders are review artifacts; fabrication release is false. The v17 readback above remains historical.

Trusted-power layout review — 2026-10-13

Both V18 roles already fit a 50 × 50 mm outline. The host J3 input supplies the protected 12 V cable output; Gadgeteer J1 does not provide that field-power rail. The remote J3 user-wired supply feeds its protected buck input, while peripheral 5 V protection remains functional load protection. These circuits are retained. The existing 19 host and 24 remote opens remain unresolved; this classification review does not qualify either board for fabrication.

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