OAK 4 D Pro — Pin 7 (DET_PIN) termination and external FSYNC drive

PietervanRooyen

Subject: OAK 4 D Pro — Pin 7 (DET_PIN) termination and external FSYNC drive for 4-camera star distribution

Hi - I'm installing 4× OAK 4 D Pro Fixed Focus (SKU OAK-4-PRO-FFN/A) on a vehicle as a hardware-synchronized capture array. The cameras are mounted at separated locations (front, rear, port, starboard), so the Y-Adapter daisy-chain topology isn't practical — I'm planning a star-distributed external FSYNC drive instead.

Planned drive topology:

- Single 5 V FSYNC signal source: Phoenix Contact PLC-OPT-24DC/5DC/100KHZ-G optocoupler output, driven on the primary side by an Sfera Labs Iono Pi Max v2 open-collector at 24 V.

- Pulse spec: 20–50 µs wide pulses, at sync-event rate (occasional, on demand) plus optional continuous low-rate genlock during shoots (≤60 Hz).

- Cable run: short M8 female pigtail at each camera → shielded twisted pair (~6–8 m max) routed through the vehicle bulkhead → common breakout terminal block in equipment bay → Phoenix opto output.

- All 4 cameras' Pin 2 (FSYNC) tied in parallel to the opto output; all Pin 8 (GND) tied to common signal ground. Other M8 conductors not used (no USB, no UART, no accessory power, no STROBE).

Two questions before I commit to the harness:

1) Pin 7 (DET_PIN) termination on RVC4.

Your M8/M12 connector documentation (https://docs.luxonis.com/hardware/platform/deploy/m8_m12_connectors/) describes Pin 7 on RVC4 devices as "Used to detect what device is connected, can't be used for any other purpose," but doesn't state what termination it requires when FSYNC is driven from an external source (i.e., when no Luxonis Y-Adapter or other Luxonis M8 accessory is plugged in). On older M8 devices you explicitly required the isolated FSYNC ground (Pin 3 in that generation) to be tied to signal ground or the first camera in a chain wouldn't sync. Is there an analogous requirement on RVC4 Pin 7?

Specifically: when externally driving FSYNC on an OAK 4 D Pro with nothing else plugged into the M8, does Pin 7 need to be:

(a) tied directly to GND (Pin 8),

(b) tied to GND through a specific resistance,

(d) left open-circuit,

(e) or something else?

Can this termination be done locally at the M8 pigtail backshell at the camera end (since our pulled cable is 2-conductor twisted pair and won't carry Pin 7 across the run)?

2) External star distribution to 4 cameras.

Has Luxonis tested or validated parallel fan-out of a single external 5 V FSYNC source to four OAK 4 D Pro cameras, each on its own cable run from a common drive point? Any guidance on:

Cable-length limits per drop (~6–8 m max in our installation),
Impedance or termination recommendations at the camera end or source end,
Whether the 4× FSYNC inputs in parallel present any source-loading issue we should account for (e.g., pull-up sizing on the opto secondary),
Any other gotchas you've seen with multi-camera external FSYNC in this configuration.

For context: the source-side pull-up is straightforward to spec once we know the input characteristics of the camera-side FSYNC pin (input impedance, input capacitance, any internal pull-up/down). If those values are documented somewhere I haven't found, a pointer would be great.

Thanks for any guidance — happy to share schematics or photos if it helps.

Best,

Pieter van Rooyen

jakaskerl

Hi PietervanRooyen

For OAK 4 D Pro, pin 7 (DET_PIN) needs to be pulled low if you want the device to operate as an external-FSYNC slave, meaning pin 2 listens for your external sync source. Pin 7 is pulled high by default, which makes the device behave as master by default. In practice, we usually pull pin 7 low through a resistor in the 30-300 ohm range. A direct pull-low may also work, but the resistor pull-down is our normal recommendation. If your harness only carries a 2-conductor pair over the long run, that pull-down can be done locally at the camera-end pigtail / backshell.

For the sync signal itself, your basic wiring approach is correct: pin 2 as FSYNC, pin 8 as ground, with all four cameras tied back to the same source and return. We have not validated this exact 4-camera star topology internally, so we cannot give a hard approval on cable length, source sizing, or termination values for that exact layout. As a rough electrical note, each device should draw about 300 uA on the detect pin.

From a cabling standpoint, shielded twisted pair would be the default choice here, since you want the sync signal and its return to stay tightly coupled over the vehicle run. Coax can also work well for a single-ended pulse signal, but it is usually less convenient for multi-drop field wiring. For a star layout, we would keep the drops as short and symmetrical as practical, keep the ground return low-impedance, and avoid unnecessary stubs or loose branching at the breakout.

Thanks,
Jaka