Tier 0 wiring (Duo Standard)

⚠️ Read SAFETY.md first. Everything below is ≤ 24 V DC. Mains only goes into certified plug-in power supplies. This plan has not been built yet; please report what you find (build report issue).

The pin map is the source of truth in firmware/opod-pico2/src/board.rs. If you change pins, change them there and here.

Power and TEC path (per zone)

flowchart LR
  B24["24 V IEC-inlet supply<br/>(tec_psu)"] --> DPS["DPS-class buck<br/>CC mode, Modbus addr 1/2"]
  DPS --> F["DC fuse 20 A"] --> ZR["Zone relay contacts<br/>(NO)"] --> PR["DPDT polarity relay"] --> TECS["4× TEC in parallel<br/>on cold plates"]
  B12["12 V IEC-inlet supply<br/>(aux_psu)"] --> CUT["Bimetal cutoffs in series:<br/>one 50 °C per cold plate<br/>+ one 80 °C per heatsink"] --> COIL["Zone relay coil"]
  COIL --> DRV["Low-side driver<br/>(NPN/MOSFET, pull-down)"] --> GND((GND))
  PICO["Pico 2 GP10 / GP16"] --> DRV

The 20 A fuse sits after the DPS, where the current is highest (the DPS can deliver up to 20 A; the 24 V side is limited to ~11.7 A by the supply’s own protection). Solo Starter: 3 TECs, 3 cold plates, 3 heatsinks, zone 0 only.
The zone relay sits between the TEC driver and the TECs. If any cutoff opens, the coil drops and the TECs lose power, whatever the firmware or the DPS module does.
Every relay, pump and fan driver input has an external 4.7 kΩ pull-down to GND, so a Pico that is in reset, unflashed or crashed leaves everything off. Don’t rely on the RP2350’s internal pull-downs: erratum E9 can latch a pad near mid-rail with a weak pull-down, so the external one must be ≤ 8.2 kΩ (10 kΩ is not enough).
The firmware closes the zone relay only when the DPS output state and setpoint are confirmed over Modbus. The polarity relay (GP11 / GP17) switches only after the DPS output is off and reads back < 0.1 A.
Ground: connect the 24 V negative, the 12 V negative and the Pico GND at one star point near the DPS modules.

Signals

Pico 2 pin	Function	Connects to
GP0 / GP1	UART0 TX / RX, OPL 460 800 Bd	ESP32-S3 hub RX / TX (cross over), common GND
GP4 / GP5	UART1 TX / RX, Modbus 9 600 Bd	RS-485 transceiver DI / RO (3.3 V type)
GP6	RS-485 DE + /RE	transceiver direction (high = transmit)
GP8 / GP9	I2C0 SDA / SCL	ADS1115 @ 0x48 (3.3 V)
GP10 / GP16	zone relay, zone 0 / 1	relay driver input (via the cutoff chain, see above)
GP11 / GP17	polarity relay, zone 0 / 1	relay driver input; low = cool, high = heat
GP12 / GP18	pump on, zone 0 / 1	MOSFET / pump enable (D5 pumps: PWM input, later)
GP13 / GP19	fans on, zone 0 / 1	MOSFET for the heatsink fans
GP14 / GP20	flow sensor pulse, zone 0 / 1	open-collector output, internal pull-up
GP21	leak sensor	LOW = dry; HIGH = leak or broken wire. External 10 kΩ pull-up to 3.3 V
GP22	water level	LOW = level OK; HIGH = low or broken wire. External 10 kΩ pull-up to 3.3 V

Both inputs are wired so that a broken wire reads as a fault. Choose sensor modules (or add an inverter) whose output is LOW in the good state.

The DPS modules have a TTL UART. Put an RS-485 adapter on each module and run one RS-485 bus (A/B, 120 Ω termination at both ends) to the Pico’s transceiver. Give the modules Modbus addresses 1 (zone 0) and 2 (zone 1).

Temperature sensors (ADS1115 @ 0x48)

ADS channel	Sensor
A0	zone 0 water (inline G1/4 NTC, return line)
A1	zone 0 TEC hot side (NTC on the heatsink base)
A2	zone 1 water
A3	zone 1 TEC hot side

Each NTC sits in a divider: 3.3 V → 10 kΩ (0.1 %) → node (ADS input) → NTC → GND. Calibrate every NTC and enter r25/beta in board.rs (NTC_CAL). Open and shorted sensors are detected and trip SENSOR_INVALID.

Bring-up order (do not skip)

Flash the firmware with nothing connected to the outputs. Check the defmt log over the debug probe.
Connect the relays without TEC power. Send SET_TARGET from the hub and watch the relays click in the right order (polarity changes only at zero current).
Bench-test each DPS module on a 1 Ω power resistor, not on TECs. Check the Modbus register map and the current read-back (DPS_MAP in board.rs).
Plumbing leak test (SAFETY.md checklist step 1).
Then the full SAFETY.md commissioning checklist.

Rendered from hardware/WIRING.md in the repository. View or edit the source.