Skip to content
WelcoYour alerts, by text.

What is a PLC in industrial control?

A PLC (programmable logic controller) is a small, rugged industrial computer that runs a machine, pump or process. It reads sensors, follows a simple program and switches motors, valves and lights. When a reading goes out of range, the PLC flags an alarm.

Start free trialBook a demo

7-day free trial, 15 message creditsCard required for verificationNo charge for 7 daysEmail reminder before billing

Published:

Glossary

PLC

In plain English, with what it means for alert texts.

Illustration of PLC.

What a PLC does all day

A PLC reads its inputs, such as a float switch or a pressure sensor. It runs its program, then sets its outputs, such as starting a pump or closing a valve.

It repeats that loop many times a second, day and night. PLCs are built for heat, dust and shaking, so you find them in pump stations, mixers and packing lines.

How a PLC alarm becomes a text

A PLC usually passes its alarms up to SCADA software or an HMI, the operator screen. That system turns the alarm into an email.

Welco takes that email and texts the person who needs it. Nothing is installed on the PLC itself. Some sites send the email from an HMI panel on the floor, others from a SCADA server in the office.

A PLC fault sent as a text

A motor overload trip on a batch mixer, after the HMI emails it to Welco.

  • Mixer motor overload: Welco: [Mixer 4 PLC] Motor overload FAULT, batch on hold at 7:26 PM. Reply STOP to opt out.

A phone showing these alert texts: Mixer motor overload, "Welco: [Mixer 4 PLC] Motor overload FAULT, batch on hold at 7:26 PM. Reply STOP to opt out."

PLC vs RTU vs HMI

A PLC runs equipment on site, fast and close to the machine. An RTU (remote terminal unit) reports readings from a distant site over a slow link. An HMI is the screen people use to watch both.

Many newer devices blur the line, so a station may call its controller either name. Plant and texting words like these fill our glossary of alert terms.

Tips for clear PLC alarm texts

Give each alarm tag a plain name, so the text says which machine and what failed. A tag like M4_OL_FLT means little to a tired tech at night.

Send only the faults someone must act on, so one trip doesn't start an alarm flood. The guide to short alert texts shows how to keep names brief, and PLC alarm texts covers setup step by step.

Frequently asked questions

Can a PLC send a text message by itself?

Some can, with an added cellular modem or SMS module, a SIM card and a data plan. Most sites find it simpler to let the HMI or SCADA send an email instead. No modem or SIM is needed for that route.

What does PLC stand for?

Programmable logic controller. Programmable because its logic is a program you can change, rather than fixed wiring. It replaced the panels of relays that ran machines before the 1970s.

Who makes PLCs?

Common makers include Rockwell Automation (Allen-Bradley), Siemens, Schneider Electric and Mitsubishi. The maker doesn't matter for alarm texts, because the email comes from the HMI or SCADA above the PLC.

Does texting PLC alarms change anything on the plant network?

No. The alarm email goes out the same way it does today, and Welco never connects to the PLC. Nothing new runs on the control network.

Turn PLC faults into texts your techs can act on

If your HMI or SCADA can send an email, Welco can text it.

Text me my PLC alarmsBook a demo

7-day free trial, 15 message creditsCard required for verificationNo charge for 7 daysEmail reminder before billing