PLC, HMI and SCADA are the three terms you hear most in industrial automation. All three are parts of the same system but do different jobs: the PLC runs the machine, the HMI is the screen where the operator talks to the machine, and SCADA monitors, records and reports on the plant as a whole. In this article we explain all three with field examples and sum up the differences in a table.
What is a PLC?
A PLC (Programmable Logic Controller) is an industrial computer that reads signals from the machine’s sensors and drives its motors, valves and gates. It runs in a continuous cycle: read inputs, execute the program, write outputs, many times per second.
- Inputs: Push buttons, limit switches, sensors, load cells, temperature probes.
- Outputs: Contactors, valves, relays, drives.
- Programming languages: Defined in IEC 61131-3: ladder diagram (LD), function block diagram (FBD), structured text (ST) and sequential function chart (SFC).
PLCs withstand harsh conditions and keep their program through power cuts. Control that needs safety and speed should always stay in the PLC.
What is an HMI?
An HMI (Human Machine Interface) is the screen where the operator monitors the machine and gives commands, usually a touch panel on the machine. It shows values, raises alarms and lets the operator enter recipes and settings. An HMI focuses on one machine and usually does not keep long-term records.
What is SCADA?
SCADA (Supervisory Control and Data Acquisition) is software that collects data from several PLCs and lets you monitor and manage the whole plant from one place. On top of what an HMI does, it:
- Stores production and alarm history in a database.
- Produces reports and trend charts.
- Manages recipes, users and permissions.
- Enables remote monitoring and data exchange with other software (accounting, ERP).
PLC vs HMI vs SCADA
| PLC | HMI | SCADA | |
|---|---|---|---|
| Role | Controls the machine. | Operator interaction with the machine. | Monitors, records and reports on the plant. |
| Hardware | Controller in the panel. | Touch panel. | PC and server. |
| Scope | One machine or section. | Usually one machine. | Several machines or the whole plant. |
| Data history | Almost none. | Limited. | Long-term database. |
How do they communicate?
PLCs talk to HMIs and SCADA over industrial protocols; Modbus TCP, Profinet, EtherNet/IP and OPC UA are the most common. Thanks to its open and simple structure, Modbus TCP is widely used to bring devices of different brands together. See PLC connection with Modbus TCP for details.
Field example: a concrete batching plant
In a concrete batching plant the PLC drives the aggregate gates, cement screws, water valve and mixer, and reads weights from the load cells. The operator screen selects the recipe, starts production and shows the weighing live. DKT’s CPlants goes beyond that screen: it stores production records, prepares delivery notes, produces reports and lets authorised users monitor production remotely. In other words, it combines the jobs of HMI and SCADA in one piece of software, while control that needs safety and timing stays in the PLC.
Summary
- The PLC runs the machine; safety-critical and fast control stays there.
- The HMI handles operator interaction with a single machine.
- SCADA collects, stores and reports data from several machines and enables remote monitoring.
- They communicate over protocols such as Modbus TCP and Profinet.
Need software for your machine or plant?
We develop operator screens, record and report software that connect to your PLC. Send us your PLC model and we will take a look.
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