Original "dumb" smoke detector interconnect sounds the siren on all interconnected smoke detectors when a 9-to-12 volt (referenced to neutral/white) direct-current signal is continuously present on the signal wire (red). This still how pretty much all interconnected smoke alarms indicate a fire condition.
However, modern interconnected detectors are capable of detecting carbon monoxide and other conditions. When these conditions are signaled, they must be identified as something other than a smoke alarm by the other detectors. Ideally, any dumb smoke detectors