Design Log DEC 081 retrospective rotor ttd no series resistor policy in addition to dec 016 - thepinkmile/Enigma-NG GitHub Wiki

DEC-081 - Retrospective: Rotor TTD No-Series-Resistor Policy (In Addition to DEC-016)

  • Status: Decided
  • Date: 2026-05-18
  • Category: Electrical / Signal Integrity
  • Area: Rotor Board (all variants) — TTD chain
  • Author: Izzyonstage & GitHub Copilot

Summary

Formally ratify that the Rotor Board TTD path carries no 33 Ω series resistor at each board-to-board hop. The Rotor was originally omitted from DEC-016's scope; this entry records the no-series-resistor policy as a deliberate architectural decision and explicitly exempts the Rotor from DEC-016's 33 Ω BtB driving rule.

Context

DEC-016 mandates 33 Ω series resistors at every intra-board or BtB-driving JTAG output to match 50 Ω PCB trace impedance. Its Area list covers Controller, Stator, Encoder, Reflector, Extension, and JDB. The Rotor Board was not included in DEC-016's scope — this was an omission in the original DEC, not a deliberate exclusion. This entry is raised retrospectively to close that gap and place the Rotor's TTD policy on the same formal footing as all other boards.

Decision

The Rotor Board TTD path carries no series resistor at each rotor-hop BtB interface (TTD exits the CPLD directly to J4 pin 6 with no intervening 33 Ω component). The Rotor Board is explicitly exempt from DEC-016's 33 Ω BtB driving rule.

End-of-chain damping is provided exclusively by the Reflector Board's single 22 Ω R1 on TTD_RETURN, and cable-driving damping is reserved for the Stator and Encoder ribbon-port interfaces as per DEC-016.

Rationale

A full stack contains up to 30 Rotor boards in a direct-mated ERM8/ERF8 BtB chain. Applying DEC-016's 33 Ω per-hop rule to this chain would introduce up to 30 × 33 Ω ≈ 990 Ω of cumulative series impedance on the TTD signal path. This would severely degrade signal levels at the chain terminus and render the JTAG path non-functional. End-of-chain damping at the Reflector is the architecturally correct approach for a stacked direct-mated topology.

In Addition To

  • DEC-016 — this entry extends DEC-016's coverage to explicitly include the Rotor Board with an exempt status. All other DEC-016 decisions and rules remain unaffected.