No internet. No router. No VPN. Just a SIM card and a number that calls you when the machine stops. For a certain class of industrial sites — remote, rural, or simply cheap — SMS is the most reliable alarm channel that exists, and the GSM-capable xLogic CPUs have it built in.
The manual dedicates whole sections to it: SMS error codes, phonebook management, parameter modification by text message. This is not an afterthought feature. It is a working remote-maintenance system.
What You Can Do by Text
The list reads like a remote-control panel:
- Modify F, Q, AQ, AF, AM, REG and RTC values by SMS
- Change phonebook numbers remotely
- Adjust GPRS parameters and settings
- Modify email parameters and settings
- Check GSM parameters
In plain language: the machine can text you "tank level low, pump 1 tripped" at 3 AM, and you can text back "raise low-level setpoint to 2.1m, reset pump 1." The site never sees a technician untill the problem actually needs hands.
Why SMS Beats the Cloud for Some Sites
Cloud monitoring needs a data conection that works everywhere. In a steel-frame warehouse, a basement plant room, or a valley site with patchy LTE, that assumption fails. SMS rides the same network but with a far lower bar: a text message gets through where a TCP session times out.
There is also a cost story. A data plan for continuous polling on a remote site adds up. SMS alarms are event-driven — a few messages a month, cents per message, no idle-time data charge. For alarm-only monitoring, the bill is almost nothing.
Real Case: Night-Shift Cold Storage
A cold store with a single refrigeration controller, no site network, and a night manager who is not technical. The CPU sends an SMS when the temperature exceeds the alarm threshold and every fifteen minutes afterward until someone acknowledges. The owner's phone is the HMI. The owner's reply text — "reset, lower setpoint" — is the control action.
The unit ran two years like that. The only service calls were the scheduled ones.
Setup Reality
You need a GSM/GPRS-capable CPU model, a SIM with SMS enabled, and the phonebook configured with the allowed numbers. The manual includes the SMS command formats and error codes — keep that page bookmarked, because the first configuration always has one typo.
Security note, because I have to say it: the phone number list is your firewall. Configure it strictly. A PLC that accepts commands from any number is a PLC that accepts commands from anyone. Lock the phonebook down, use a dedicated SIM with no roaming, and treat the number like a key.
The SMS Command Grammar
Learning the command formats is the difference between a demo and a working system. The manual documents the SMS syntax for modifying F, Q, AQ, AF, AM, REG and RTC values — each with its own format, each with an error code for when it fails. The phonebook supports multiple numbers, and the CPU distinguishes who is allowed to command from who is only allowed to be alerted.
Build a one-page command cheat sheet for the site: "to raise the low-level setpoint, send X; to reset pump 1, send Y." Laminate it next to the phone the duty engineer carries. The people who actually use SMS control are not PLC programmers — they are operators with a phone and a procedure, and the cheat sheet is the procedure.
Test every command format during commissioning, not during the storm. A typo in the format produces an error code on the phone and a mystery on the machine; a tested format produces a working remote control. Fifteen minutes of commissioning saves a 2 AM phone call.
When SMS Is Not Enough
SMS is event-driven and human-directed; it is not a historian. If the site needs trends, a SCADA history, or multi-user dashboards, the Ethernet route with EasySCADA or a cloud platform carries the data. The honest architecture runs both: SMS for the alarm and the quick command, Modbus TCP or MQTT for the record. The same CPU supports both channels, and the two never conflict — the GSM module handles the texts while the Ethernet port handles the data.
The failure mode to design against is the one where the alarm text goes out and nobody acts. Configure the acknowledgment: the operator replies, the CPU logs the acknowledgment, and the escalation logic knows who has not answered. That turns SMS from a notification into a process with accountability.
And keep the SIM topped up and tested monthly. A dead SIM is a silent alarm system, and silent alarm systems are discovered at the worst possible moment.
The Acknowledgment Loop
An alarm that nobody acknowledges is a rumor. Configure the acknowledgment path: the SMS goes out, the operator replies, the CPU logs the reply and escalates if nobody answers within the window. The manual's command set covers the acknowledgment formats; the design decision is yours. Two numbers — the acknowledgment timeout and the escalation target — turn a notification system into a process with accountability, and they are worth more than any feature in the brochure.
Internet-dependent alarm systems are fashionable. SMS alarms are indestructible. Know which one your site actually needs.
The SIM's monthly test is the unsung ritual: send a test alarm, watch the phone ring, confirm the reply path. A silent SIM is a silent alarm system, and silent systems are discovered at the worst possible moment. Put the test on the calendar — first Monday, every month — and the alarm chain stays alive.
