Design Log DEC 023 jdb gnd chassis omitted mounting holes tied to gnd - thepinkmile/Enigma-NG GitHub Wiki

DEC-023 - JDB GND_CHASSIS Omitted; Mounting Holes Tied to GND

  • Status: Decided
  • Date: 2026-04-06
  • Category: Electrical
  • Area: JTAG Daughterboard - Grounding & EMC

Decision

The JTAG Daughterboard (JDB) does not implement a GND_CHASSIS net. Mounting holes are included for mechanical stability via conductive standoffs to the Controller board; the copper pours around those mounting holes are tied to GND (circuit return). No dedicated chassis bond pad, stitching via, or GND_CHASSIS net is present on the JDB.

Cross-reference: DEC-039 keeps the single Power Module bond rule but now makes this JDB treatment the generic exception for non-chassis-connected daughterboards.

Rationale

GND_CHASSIS exists to bond a PCB to the metal enclosure for EMC shielding, ESD protection, and safety earth. None of those functions apply to the JDB because:

  1. No chassis surface - the JDB is an internal daughterboard that floats above the Controller board on hat-header pins and conductive standoffs. It has no metal enclosure surface it can directly bond to.
  2. EMC envelope already provided - the Controller board beneath it carries a correctly implemented GND_CHASSIS net bonded to the system enclosure. The JDB sits entirely within that envelope.
  3. No external connections - the JDB has no cables or connectors that exit the enclosure, so there are no ESD entry points that would require chassis bonding at the JDB level.
  4. Low-EMI device - the FT232H operates at USB 2.0 speeds on an internal point-to-point link. It generates no significant conducted or radiated emissions that would require additional chassis shielding.
  5. Daisy-chaining adds no value - connecting JDB GND_CHASSIS via standoffs to Controller GND_CHASSIS would create an additional path to chassis earth but no new bond point, adding PCB complexity for zero measurable EMC benefit.

Implementation

  • Include mounting holes on the JDB for mechanical stability.
  • Standoffs are conductive (metal); this incidentally bonds the JDB mounting hole copper pours to the Controller board mechanically.
  • Mounting hole copper pours connect to GND (circuit return) - not to a separate GND_CHASSIS net.
  • This is consistent with standard practice for small internal sub-modules (e.g. Arduino shields, FPGA daughter cards).

Alternatives Considered

  • Standoff bonding to Controller GND_CHASSIS: Technically implementable but adds a separate net, a copper pour, and design rules for no measurable EMC gain on a low-EMI internal board. Rejected.
  • No mounting holes at all: Rejected - mechanical retention via standoffs is required to prevent the board from flexing on the hat-headers during connector insertion/removal.