EverFocus EDR810H Insruction Manual page 70

EDR810/410 provides a record function which is triggered by external signal
via the 19th pin of the ALARM INPUT / OUTPUT port. When the record
input signal is pulled low constantly, EDR810/410 will start to record. The
system will stop recording when the record input signal is not pulled low.
Note: Please check APPENDIX C to see other available alarm
input/output functions.
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