The following PLC program preforms the function of an alarm annunciator, where a discrete input signal from an alarm switch (e.g. high temperature alarm) first causes a warning light to blink and a siren to audibly pulse until a human operator presses an acknowledge pushbutton. If the alarm switch signal is still activated, the light will remain on (steady) instead of blink and the siren will go silent. The light turns off as soon as the alarm signal goes back to its "safe" state. A timing diagram shows how this should work: Alarm switch Warning light Warning siren Acknowledge pushbutton www w Alarm_input Blink Latch Latch Blink Latch H/H Acknowledge_input Alarm_input Light Siren Latch pushbutton Alarm_input Blink Latch Latch Blink Latch Acknowledge_input Alarm_input Light Siren Latch Take this "generic" PLC program and enter it into your own PLC, assigning appropriate addresses to all instructions, and demonstrating its operation. Suggestions for Socratic discussion Does the PLC program (as written) "expect" a closed alarm switch contact to trigger the alarm, or an open alarm switch contact? If the real-world alarm switch contact was a pressure switch wired NC (normally-closed), would this circuit function as a low pressure alarm or as a high pressure alarm? If the real-world alarm switch contact was a temperature switch wired NO (normally-open), would this circuit function as a low temperature alarm or as a high temperature alarm? file i02342