Connecting PLC Alarm Events to an SMS Gateway

Agree the alarm interface

Obtain the approved tag or register list, data types and meanings from the machine owner. Use an authorised read-only interface where possible. Document polling limits and network access. Do not infer alarm semantics from changing values alone; a bit may represent an acknowledgement, latch or historical condition rather than an active fault.

Model transitions explicitly

Create events for activation and recovery, with a stable alarm identifier. Define handling for a fault already active when the gateway starts. Use appropriate persistence or deadband for noisy conditions. Loss of communication should create an unknown state or communications alarm, not falsely report that all machine alarms have cleared.

Separate acknowledgement from reset

Receiving or acknowledging a message does not reset a PLC fault. Keep machine reset and restart under the approved control procedure. The notification system should record who accepted responsibility and when, while the PLC remains the source for machine state. Avoid writing to control registers merely to mark an SMS as handled.

Commission with safe test events

Use simulated tags or an agreed test mode rather than deliberately creating hazardous plant faults. Verify activation, persistent alarms, recovery, gateway restart and network interruption. Compare the PLC event history with the notification log. Document any delay introduced by polling and queueing so operators understand what the timestamps mean.

FAQ