Cable tester

Cable tester PCB (KiCad 9)

design.py is the single source of truth: parts, footprints, pin-to-net map and positions. Everything else is generated from it.

Regenerate

python gen_schematic.py
"C:/Program Files/KiCad/9.0/bin/kicad-cli.exe" sch erc cable_tester.kicad_sch
"C:/Program Files/KiCad/9.0/bin/kicad-cli.exe" sch export netlist --format kicadsexpr -o cable_tester.net cable_tester.kicad_sch
python check_design.py
"C:/Program Files/KiCad/9.0/bin/python.exe" gen_pcb.py

check_design.py checks the exported netlist against firmware/hwmap.py:

• each of the 58 test lines runs connector pin → 220 Ω → the MCP23017 pin the firmware expects
• the address straps are correct
• no DUT pin touches a power net

The board is routed, so don't run gen_pcb.py again, because it overwrites the board. After that, make changes in the schematic (or design.py followed by gen_schematic.py), then use Tools → Update PCB from Schematic (F8).

Circuit summary

• A1: Raspberry Pi Pico or a WeAct RP2040 (USB-C, same pinout).
  • USB supplies VBUS (+5 V), which powers the LEDs and buzzer.
  • The module's 3V3 output powers the MCP23017s.
  • Pins used: GP4 SDA, GP5 SCL, GP14 buzzer, GP15 button, GP16 LED data.
• U1–U4: MCP23017 at I²C addresses 0x20–0x23. Every DUT contact goes to its own expander pin through one resistor in RN1–RN15 (4×220 Ω arrays).
  • U1: H1–16. U2: H17–32. U3: GPA = H33–40, GPB0–5 = TELEM1.
  • U4: GPA0–5 = TELEM2, GPB0–5 = TELEM3.
  • Spare lines: U3 GPB6–7, U4 GPA6–7, U4 GPB6–7.
• J1: the 40-pin header. None of its pins touch GND, 3V3 or 5V. Each one is an isolated test line, which is why any future pinout can be tested with a config change only.
• D1–D10: WS2812B, driven through U5 (74AHCT1G125) to shift the data line from 3.3 V to 5 V.
  • D1–D9 are the wire LEDs: TX / RX / GND for each TELEM connector.
  • D10 is the PASS/FAIL status LED.
• BZ1: 5 V active buzzer, switched by Q1 with flyback diode D11.
• SW1: START / LEARN button.
• J5: optional SSD1306 OLED header. The firmware doesn't drive it yet.

Before ordering

1. Route it. Freerouting works well:
   • export with File → Export → Specctra DSN, route in Freerouting, then import the .ses file
   • press B to fill the GND pours
   • run DRC
2. Check JST orientation in the 3D viewer (Alt+3). The TELEM connectors must open toward the bottom board edge.
3. Check the header orientation. It copies a Pi seen from above: pin 1 is bottom-left on the silkscreen, and the even-pin row is along the board edge. The cable must plug in exactly as it does on the Pi.
4. Check the module footprint. If you use a Pico clone, confirm it matches RaspberryPi_Pico_SMD_HandSolder.
5. JST-GH receptacles are consumables. They are rated for only ~50 mating cycles. Order spares and replace them with hot air when mating gets loose.