RED DIGITAL CINEMA DSMC2 BRAIN with HELIUM 8K S35 Sensor (2018 Unified DSMC2 Lineup) - Manual (Page 230)

SYNC TRIGGER BUTTON CIRCUIT
The SYNC connector has a 3.3 V Schmitt trigger (5 V tolerant). The trigger is not a switch closure circuit, so it requires
that the trigger source supplies current. Both edges of the input signal may be used as a trigger.
NOTE: The SYNC trigger button circuit only applies to the DSMC2 Base Expander.
For example, this is the behavior of the circuit when the GPI trigger is used for record start/stop:
Start Record : On ground to 3.3 V transition
During Record : Hold at 3.3 V
Stop Record : On 3.3 V to ground transition
During Stop : Hold at ground
Figure: Trigger Button Circuit Diagram
NOTE: In the diagram above, values are approximate. Use standard values.
COMPATIBLE CABLES
790-0154, 790-0643 : 3BNC-to-00 Sync Cable
790-0187, 790-0648 : 4-Pin 00B-to-Flying Lead
White : Ground
Yellow : RS232 RX
Blue : Shutter/sync, general purpose output
Red : RS232 TX
Black : Shield
790-0415 : RED Start/Stop Cable (14-Pin 1B to SYNC, CTRL, BNC)
790-0428 : RED Start/Stop Cable (14-Pin to 00B SYNC)
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DSMC2 HELIUM OPERATION GUIDE