A watchdog timer is a simple, powerful safety idea: keep a countdown running, and something must keep petting it, or the system assumes a failure and reacts. It is how automation detects a crashed processor, a hung program, or a lost communications link - the absence of a heartbeat. This guide explains how watchdog and deadman timers work, the difference between them, and where they are used in SCADA and control.
Watchdog Timer in one line: A watchdog (or deadman) timer is a countdown that must be periodically reset by a healthy system; if the reset stops arriving before the timer expires, the timer assumes a failure and forces a predefined safe action such as a shutdown or alarm.
The mechanism is deliberately simple: a timer counts down toward zero, and normal operation continually resets it before it gets there. As long as the resets keep coming, the timer never expires and nothing happens. If the resets stop - because the processor froze, the program hung, or the link went dead - the timer runs out and triggers a predefined action: reboot the processor, trip an output to a safe state, or raise an alarm.
A common form is a communications watchdog (or heartbeat). Two systems agree that one will regularly toggle or increment a value the other watches. If that heartbeat stops updating for too long, the receiving side knows the link or the far end has failed and can fail safe - close a valve, shut in a well, or annunciate a comms-fail alarm - rather than blindly holding the last known command forever.
The terms overlap. A watchdog timer usually refers to automatic supervision of a machine or communications link. A deadman (or dead-man's switch) classically refers to a control that requires continuous human presence - release it, or fail to acknowledge it, and the system stops - the idea being that an incapacitated operator should not leave equipment running unattended. Both share the core principle: the safe default is triggered by the absence of an expected signal.
In practice these timers are everywhere in industrial systems. PLC processors run an internal hardware watchdog that reboots the CPU if the scan hangs. SCADA and telemetry links use communications watchdogs so a remote site fails to a safe state if it loses contact with the host. Remote-control functions often layer a deadman-style requirement so a command must be actively confirmed and does not persist indefinitely. The result is a system that treats silence as a fault, which is exactly what you want when a controller or link fails.
It counts down and must be reset regularly by a healthy system. If the resets stop arriving before it expires - due to a crash, hang, or lost link - it triggers a predefined action such as a reboot, a trip to a safe state, or an alarm.
A watchdog typically supervises a machine or communications link automatically, while a deadman requires continuous human presence or acknowledgment and stops the system if that input is absent. Both fail safe when an expected signal disappears.
A heartbeat scheme where one system regularly updates a value the other watches. If the heartbeat stops updating for too long, the receiver treats the link or far end as failed and fails to a safe state instead of holding the last command indefinitely.
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