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ARRI Cameras Require Solid 24V PSU for Sync and Trigger to Work

CONFIRMED BOTH

Summary

ARRI cameras must be powered from a rated 24V AC/DC power supply for the sync output to be strong enough to lock, and for the trigger to operate reliably. Running on camera battery produces sync output that may be too weak for Flair or the MRMC sync cable circuit board to detect. The first diagnostic is to check that the sync LED on the MRMC cable box is flashing at the correct frame rate. One Shot Sync is a useful tool for cameras with sync jitter.

Battery power = unreliable sync and trigger

An ARRI camera running on battery (which may be below 24V) can produce sync that appears to work but is too weak to reliably lock. Both Sync and Triggering through Flair may fail entirely. Always use a solid 24V rated PSU for ARRI camera sync setups.

Symptoms

  • Flair sync does not lock before the move starts, triggering a pop-up error
  • Sync LED on the MRMC cable box is not flashing, or flashing at the wrong rate
  • Flair shows sync jitter or oscillating frequency (e.g., alternating between 12 fps and 25 fps)
  • Run/Stop from Flair (Turnover) doesn't reliably start or stop the camera
  • Camera appears to sync briefly but loses lock during the move
  • Flair warning: sync lost during run

Systems Affected

BOTH
  • Flair Classic and Flair 7
  • All ARRI cameras using the MRMC sync cable (RS connector or 8-pin Extension socket)
  • Confirmed on: ARRI Alexa Mini, Alexa Mini LF, Amira, and others in the ARRI range

How ARRI Camera Sync Works

ARRI cameras use a 3-pin RS connector (or 8-pin Extension socket on newer bodies) for sync and trigger:

  • Pin 1 — 24V (camera-supplied)
  • Pin 2 — 0V (ground)
  • Pin 3 — Sync output / Trigger input (shared pin)

When the MRMC sync cable momentarily shorts Pin 3 to ground, the camera starts. When running, the camera outputs a sync pulse on Pin 3 at the recording frame rate. Shorting Pin 3 again stops the camera.

Some smaller ARRI cameras supply 12V instead of 24V — check the specific model.

The MRMC sync cable has a circuit board that interfaces with this signal. If the camera supply voltage is low (battery below rated voltage), the sync pulse output is too weak for the circuit board to detect reliably.

"VERY IMPORTANT — in order to work correctly with Flair and the MRMC Sync Cable ARRI cameras must be powered with a steady 24v Power Supply. A battery with lower voltage will fail to Trigger the camera and the Sync output may be too weak to be detected by Flair or the circuit board on the Sync Cable." — Tim Cerchio (TimHC), 2020-05-04

"If an Alexa Mini (with EXT-RS adapter) is not powered with a 'solid' 24V Power Supply both Sync and Triggering through Flair might not be at all possible! A weak Sync output from camera might still be detected as a Sync Pulse but it will not be enough to get the Sync LED to Flash — this should be the first indicator of low Voltage from Camera." — TimHC, 2021-06-07

First Diagnostic — Check the Sync LED

The MRMC sync cable circuit box has a flashing LED that confirms camera sync output.

  1. Power the camera from a rated 24V AC/DC PSU.
  2. Set the camera running (recording).
  3. Connect the MRMC sync cable.
  4. Observe the LED on the MRMC cable box — it must flash at the camera's frame rate (e.g., 25 flashes per second at 25fps).
  5. If the LED does not flash, or flashes slowly / irregularly: sync is too weak — check PSU voltage.
  6. If the LED flashes correctly: sync is present. Proceed to Flair setup checks below.

Flair External Camera Setup Checklist

If the sync LED flashes correctly but Flair still won't lock:

  1. Sync Source — set to Network; Sync Node set to the correct node (Ultibox on Bolt arm, RT14 on Milo).
  2. Test Sync — use the Test button to confirm sync is stable with little or no jitter.
  3. Incoming frequency — confirm it matches your move FPS setting.
  4. Sync Frame — set to 1 or 2 (or more if needed).
  5. Ramp Frames — add ramp frames (typically 24 or 25 for one full second) to allow the camera to be at speed / positively recording before locking sync. Some cameras have a particularly long delay.

"It is always wise to use a bloop as all cameras have variable start times, I have often had to put 'Ramp Frames' in External Camera setup so I know the camera has actually started recording before the move starts." — Peter (MRMC)

Using Turnover to Start the Camera

If Flair's normal Shoot-to-Start fails because the start pulse is too long and sync is lost briefly:

  • Use the Turnover button in Flair to start the camera. This starts the camera first, and the bloop (first frame marker) tells you when recording began.
  • Alternatively, open Setup Outputs → Test Outputs and start the camera from there before running the move.

One Shot Sync

For cameras with sync jitter (unstable sync that occasionally causes Flair to lose lock during a move):

  • Enable One Shot Sync in the Camera Setup.

What One Shot Sync does: Flair locks to the camera's frame rate for the first couple of frames, then allows both to run independently on the same time base — similar to jamming free-run timecode between cameras at the start of a session. This prevents sync dropouts during the move from triggering a warning or abort.

"To avoid this you can 'tick' the One Shot Sync box in the Camera Setup. What this function does is to Lock Flair to Camera Frame Rate for the first couple of frames and then lets them both run 'in parallel' on the same time base." — Tim Cerchio, 2020-05-04

One Shot Sync is particularly useful for digital cameras that exhibit slight shutter timing jitter.

Monitoring Sync Status

Sync status and incoming frequency can be displayed on the Run Control Bar:

  • Go to View → Adjust Numeric Controls → Customise Control Bar and add Sync Status and Frequency.

ARRI Cable Notes

  • ARRI cameras use the RS connector (3-pin Lemo) or the 8-pin Extension socket (newer bodies — an ARRI adapter cable is needed).
  • The Amira uses the same 3-pin RS connector, not the 8-pin Extension.
  • The ARRI cable is different from the Phantom/RED cable — signals and control differ.
  • Some recent ARRI cameras vary the Mark/Space ratio of the sync pulse with shutter angle, which can cause issues with older MRMC circuit boards. Contact MRMC if you suspect this is affecting your camera.

Source: talk.mrmoco.com — Some data on ARRI cameras & Sync — Posts by Peter (MRMC), Tim Cerchio (TimHC), Gordon (MRMC), stafford, federico, marcin, 2020-04-28 through 2021-06-07

Key posts: - Peter (2020-04-28): RS connector wiring, how ARRI sync works, cable notes - TimHC (2020-05-04): 24V PSU requirement, sync LED diagnostic, checklist, One Shot Sync - TimHC (2021-06-07): Confirmed with Alexa Mini — weak sync from battery, LED diagnostic

Official Documentation

Camera Sync and Trigger V2 — ARRI Camera Wiring and Sync Setup

Flair 7 Manual — Ch.4 Camera Setup: External Camera Sync

Camera Triggering and Synchronisation

Watch on YouTube · 08:01

Revision History

Date Change Editor
2026-05-29 Initial extraction from forum topic 258 Tom D / Claude Code