Design Log DEC 045 rotor samtec erm8erf8 connectors require esd protection - thepinkmile/Enigma-NG GitHub Wiki

DEC-045 - Rotor Samtec ERM8/ERF8 Connectors Require ESD Protection

  • Status: Decided
  • Date: 2026-04-28
  • Category: Electrical / ESD Protection
  • Area: Rotor Board, ERM8/ERF8 BtB connectors
  • Author: Izzyonstage & GitHub Copilot

Summary

The Samtec ERM8/ERF8 board-to-board connectors on the Rotor Boards are designated as requiring TVS/ESD protection, as an exception to Global_Routing_Spec.md §9, which otherwise exempts internal BtB connectors from ESD protection requirements.

Problem

Global_Routing_Spec.md §9 exempts Samtec ERM8/ERF8 BtB connectors from ESD protection on the basis that they are internal board-to-board connectors. However, the Rotor Boards are hot-swappable by design: individual rotors are inserted and removed from the live stack without tools. This makes the ERM8/ERF8 mating faces accessible during servicing, exposing them to ESD events from operator handling.

The Samtec connectors were originally specified for the PM/Stator interconnect (an internal, non-hot-swappable path) - this was incorrect. They are now correctly used for rotor external connections (between individually removable/serviceable rotor modules).

Decision

Add TVS/ESD protection to all Samtec ERM8/ERF8 connector interfaces on the Rotor Board. Update Global_Routing_Spec.md §9 with a servicing-access exception clause covering hot-swappable connector families.

Rationale

  • Rotors are explicitly designed to be hot-swappable (Samtec ERM8/ERF8 rated for high mating cycles - §2.3).
  • ESD events at the mating face during live insertion are a realistic threat to CPLD, FDC2114, and JTAG chain integrity.
  • The relevant boundary for ESD protection is "accessible during servicing" not merely "external to the enclosure at all times."
  • This is consistent with the §9 principle extended to servicing access rather than just permanent physical exposure.

Alternatives Considered

Alternative Reason rejected
No ESD protection (rely on enclosure shielding) Enclosure is open during rotor swap; ESD from operator handling is a real threat
Per-rotor deviation note only (no §9 update) Modifying §9 is cleaner and prevents future designs from missing the same requirement

Impact

  • design/Standards/Global_Routing_Spec.md §9: Added hot-swappable/service-accessible connector clause.
  • design/Electronics/Rotor/Design_Spec.md §6: ESD arrays documented. Selected device: TPD4E05U06QDQARQ1 (U5-U12); U5-U8 protect Board A connectors J1 and J3; U9-U12 protect Board B connectors J4 and J6. 8 per rotor pair x 30 = 240 system total. JTAG signals corrected: J1 Board A = TDI/TMS/TCK (3 lines, 1 spare); J4 Board B = TDO/TMS/TCK (3 lines, 1 spare); nTRST not routed. Encoder signals corrected: J3/J6 carry ENC_IN[5:0] + ENC_OUT[5:0] (12 lines) - phantom signals ENC_ACTIVE_N and ENC_CLK removed.
  • design/Electronics/Consolidated_BOM.md: TPD4E05U06QDQARQ1 row updated - ROT (x1) = 8, ROT Total (x30) = 240, System Total = 244.

Cross-ref

design/Standards/Global_Routing_Spec.md §9, design/Electronics/Rotor/Design_Spec.md §6.