MES sits at Level 3 of the ISA-95 hierarchy — above the PLCs and SCADA running the floor, below the ERP system planning the business. What MES actually does between those two layers, and why the integration is harder than the software demo suggests.
ERP knows what should be produced this month. A PLC knows what a
single machine is doing right now. Neither one, on its own, can answer
“what’s actually happening on the shop floor this shift” — that’s
specifically the gap a Manufacturing Execution System exists to fill.
The ANSI/ISA-95 standard organizes plant systems into a functional
hierarchy, and MES sits at Level 3 — between ERP at Level 4 (business
planning, order management, scheduling at a high level) and process
control at Levels 0-2 (the actual PLCs, SCADA, and sensors running
equipment). MES is the layer that translates a high-level production
order from ERP into detailed, shift-level execution — tracking what’s
actually being produced, by whom, on which equipment, right now.
| Area | What it covers |
|---|---|
| Production | Work order execution, tracking, and dispatch to the floor |
| Quality | In-process quality data collection and disposition |
| Logistics | Material tracking and inventory movement tied to production |
| Maintenance | Equipment status and maintenance activity relevant to production execution |
Without an MES layer, the gap between “ERP says produce 500 units
this week” and “here’s exactly what happened on the floor this shift”
often gets filled manually — paper travelers, spreadsheets, verbal
handoffs — which introduces delay and error into exactly the data a
plant needs most in real time. MES closes that gap by capturing
execution data directly from the floor and feeding it both upward to
ERP and outward to metrics like OEE.
See our related guide on Overall
Equipment Effectiveness for the metric itself — MES is frequently
the system that actually captures the raw downtime, cycle time, and
quality data an OEE calculation depends on. Manually tracked OEE data,
reconstructed after the fact from paper logs, is far less reliable than
data captured automatically as production happens.
Modern MES implementations increasingly connect to plant-floor
equipment through standardized protocols like OPC-UA and MQTT rather
than proprietary, point-to-point integrations — a shift that makes it
meaningfully easier to add or swap equipment without rebuilding the
entire integration layer each time. That’s a direct consequence of the
same push toward standardized industrial communication covered in our
related guide on industrial
Ethernet protocols.
MES selection and implementation isn’t a standalone course in
SCMEP’s current catalog — it connects to the digital readiness work
covered in our Digital Transformation
— Are You Ready? course. As a NIST
Manufacturing Extension Partnership affiliate serving South Carolina
manufacturers since 1989, our focus is helping plants understand
where MES fits in their automation stack before evaluating specific
software.
If your plant is evaluating an MES implementation, you can
browse the Industry 4.0 and Digital
training catalog or email the training team.
MES sits at Level 3 of the ISA-95 hierarchy; a related system sits directly above it, planning at the business level. See our related guide on ERP and ISA-95 for where enterprise resource planning fits and why the integration between the two is the hard part.
An MES sits at Level 3 of the ISA-95 hierarchy, between ERP (Level 4) and process control (Levels 0-2), translating high-level production orders into detailed, shift-level execution tracking on the actual shop floor.
Production (work order execution and tracking), quality (in-process data collection), logistics (material tracking), and maintenance (equipment status relevant to production).
MES is frequently the system that captures the raw downtime, cycle time, and quality data an OEE calculation depends on, producing far more reliable data than manually reconstructed paper logs.
Increasingly through standardized protocols like OPC-UA and MQTT rather than proprietary point-to-point integrations, making it easier to add or swap equipment without rebuilding the integration layer.
