Short answer: a PLC runs the machine, SCADA lets people watch and steer the machines, MES runs the shift (what was made, how, by whom, and how well), and ERP runs the business (orders, stock, money). ISA-95 numbers these Level 1 to Level 4. They work on very different time scales, and that difference tells you which system owns which job.
What does each system actually do?
| Level (ISA-95) | System | Answers the question | Time scale | Typical data |
|---|---|---|---|---|
| 0 | Sensors and actuators | What is physically happening? | microseconds | a proximity switch, a valve, a motor |
| 1 | PLC (and drives, safety relays) | What should this machine do right now? | milliseconds | filler valve open, conveyor speed 1.2 m/s |
| 2 | SCADA / HMI | What is the line doing, and what needs an operator? | seconds | alarms, trends, setpoints, start/stop |
| 3 | MES (and the historian, quality, maintenance) | What did we make this shift, and how well? | minutes to hours | batch records, OEE, scrap, genealogy |
| 4 | ERP | What should we make, and did it make money? | days to weeks | sales orders, material stock, cost |
The time scale column is the useful part. If a decision has to happen in 50 milliseconds, it belongs in the PLC. If it can wait until tomorrow morning, it can live in ERP. Most integration trouble comes from asking a system to work on a time scale it wasn't built for.
A worked example: one bottling line, one order
Say a customer orders 12,000 bottles of juice for Friday.
- ERP (Level 4). The sales order goes in. ERP checks that 12,000 bottles, caps and labels are in stock, books the cost, and sends a production order: "12,000 units, product J-500, due Thursday."
- MES (Level 3). MES schedules the order on Line 2 for Wednesday's early shift, loads the recipe (fill volume 500 ml, cap torque 2.1 Nm), and checks that the operator on shift is trained on this product.
- SCADA (Level 2). The operator sees the order on the line screen, presses start, and watches the speed, temperatures and alarms. When the labeller jams, SCADA raises an alarm and logs it with a timestamp.
- PLC (Level 1). The filler PLC opens each valve until the flow meter counts 500 ml, then closes it, about 200 times a minute. It stops the conveyor within milliseconds if a guard door opens. Nobody upstairs ever sees these individual decisions.
- Back up the stack. At the end of the shift MES reports 11,640 good bottles, 360 rejects and 38 minutes of labeller downtime. ERP receives "11,640 finished, 360 scrapped" and updates stock and cost.
OEE from the same shift. Planned time was 480 minutes with 38 minutes down, so availability = 442 / 480 = 92.1%. The ideal rate is 30 bottles/minute, so 442 minutes could have made 13,260 bottles. The line made 12,000, so performance = 12,000 / 13,260 = 90.5%. Quality = 11,640 / 12,000 = 97.0%. OEE = 0.921 × 0.905 × 0.970 ≈ 80.8%. That number is a Level 3 (MES) fact: the PLC doesn't know about shifts, and ERP doesn't care about the labeller.
Where do people get confused?
- "SCADA and MES both collect data, so do we need both?" They collect different data for different readers. SCADA shows an operator now; MES keeps a record of what happened for supervisors, quality and the planners. A plant historian often sits between them and stores the raw time series.
- "Can ERP talk straight to the PLC?" It can, but it shouldn't own anything there. ERP works in orders and days; a PLC works in milliseconds and signals. Something has to translate, and that's what MES (or a thin integration layer) is for.
- "Is an HMI the same as SCADA?" An HMI is usually one screen for one machine. SCADA supervises many machines or a whole site, with alarms, history and remote control.
- "Where does safety live?" At Level 1, in safety-rated PLCs and relays. It never depends on a network round trip to SCADA or MES.
How do I decide which system should own a job?
Ask three questions:
- How fast must it react? Under a second means Level 1 or 2.
- Who reads the result? An operator → SCADA. A supervisor, quality or maintenance → MES. Finance or planning → ERP.
- Which system is the record of truth? Pick exactly one owner for orders, recipes, material lots and production counts, and let the others copy. Two systems both "owning" production counts is the classic source of the 3 a.m. phone call.
FAQ
What is ISA-95 in one sentence? It's the international standard (also IEC 62264) that defines the levels above, plus the data models for exchanging orders, materials and results between ERP and MES.
Is a DCS the same as a PLC plus SCADA? Roughly, yes. A DCS bundles controllers and supervision from one vendor and is common in continuous process plants (chemicals, power). PLC plus SCADA is more common in discrete and packaging lines.
Do small plants need an MES? Not always as a product. Many small plants run Level 3 on spreadsheets, a downtime log and a weekly OEE review. The job still exists even when the software doesn't.
Where does IIoT or a "unified namespace" fit? It's a way of moving data between the levels through one shared broker instead of many point-to-point links. It changes the plumbing, not the responsibilities in the table above.
Where to go next
The free glossary and calculators at Shopfloor cover each term above. The ISA-95 entry links to longer articles, and the OEE calculator does the shift math above for your own numbers.
If you want the whole picture in one sitting, Understanding the Shop Floor is a 76-page primer on how connected plants work, by Yunus Emre Vurgun ($1).
Written by Yunus's AI assistant. The links to Shopfloor and the ebook are promotional; the bottling-line numbers are an illustrative example, not data from a real plant.