FIBER DIG-IN ALARM - CABINET BOARD, 2-CHANNEL BENCH PROTOTYPE (Rev P1)
Schematic package for engineering review

CONTENTS
  fiberalarm.kicad_pro / fiberalarm.kicad_sch   KiCad 7 project, top sheet
  power / mcu / optical_tx / optical_rx / alarm_io .kicad_sch   sub-sheets
  schematic PDF and BOM.csv (sent alongside)

CHECKS ALREADY RUN
  - Netlist exported by KiCad and compared pin-by-pin with the design intent: 433 pins, 0 mismatches,
    no single-pin nets, no unintended merged nets.
  - Power-pin and output-conflict check: 0 issues (every power input has a source or a PWR_FLAG).
  - Full KiCad ERC was not run (kicad-cli 7 has no ERC); run Inspect > ERC in KiCad 7/8 as part of review.

ITEMS FOR THE REVIEWER TO CONFIRM BEFORE LAYOUT
  1. SFP cage + 20-pin connector: choose TE part numbers and create the footprint (not in KiCad library).
  2. Photodiode: choose the InGaAs PIN part (LC receptacle) and create its footprint.
  3. AP63203 / AP63205 inductor values (4.7 uH / 6.8 uH) against the Diodes Inc datasheet table.
  4. Omron G5LE-1 contact numbering (1 COM, 3 NC, 4 NO) against the datasheet.
  5. STM32G474 alternate functions (PA0/PA1 ADC, PB6/PB7 I2C1, PA9/PA10 USART1, PA4-PA7 SPI1) in STM32CubeMX.
  6. TLV1702 pinout is the standard LM393 pinout (LM393 symbol used, value TLV1702).
  7. Polyfuse exact Littelfuse suffix (1.5 A hold, >= 30 V).
  8. Chosen SFP module keeps its laser on with TD+/TD- idle (else feed J5 with a 125 MHz clock).

DESIGN INTENT IN ONE PARAGRAPH
  Each received strand drives an InGaAs photodiode into a transimpedance amplifier (V_SIG = 0.25 V + I x 4.99k).
  A comparator with ~33 mV hysteresis compares V_SIG with a per-channel threshold from an MCP4728 DAC (stored
  in DAC EEPROM, set at commissioning to -3 dB). Any channel below threshold pulls ALARM_N low through a diode
  OR, which asynchronously sets a 74AUP1G74 latch that drives the siren relay with no firmware in the path.
  The MCU clears the latch only after the keyed reset. SFP lasers have a 1k TX_DISABLE pull-down so they stay
  on even if the MCU is held in reset.

BOARD (fiberalarm.kicad_pcb) - FIRST-PASS PLACEMENT, Rev P1
  - 190 x 125 mm, 4 copper layers, 4 x M3 mounting holes. All 134 parts placed by function; NO traces routed.
  - Zones: optical edge on the left (2 SFP cages, 2 photodiodes) | analog receive front end beside the
    photodiodes | MCU centre | 12 V input and regulators top right | latch, relays and terminal blocks on the
    right edge | key switch, CAL button, LEDs and service headers along the bottom.
  - KiCad DRC: 0 courtyard overlaps, 0 footprint errors; 331 unconnected items = the connections still to route;
    a few silkscreen-overlap warnings to tidy during layout.
  - FiberAlarm.pretty holds PLACEHOLDER footprints for the SFP cage/connector and the LC photodiode.
    Their pad numbering matches the schematic, but the geometry is approximate: replace with the real
    TE / photodiode drawings before routing.
  - The board is linked to the schematic: Tools > Update PCB from Schematic (F8) keeps the placement.