A factory is five layers stacked from the spinning, heating, flowing reality at the bottom to the boardroom at the top. The magic isn't any one layer — it's how a single order flows down and turns into motion, while a single measurement flows up and turns into a business decision. This lesson lets you watch both journeys happen, hop by hop, and see why the time-scale stretches from milliseconds to months as you climb.
The ISA-95 model (and its cousin the Purdue model) stack a plant into levels. Each speaks to its neighbours, never end-to-end — and each thinks on a wildly different time-scale. Here's the cast, bottom to top of the simulator below.
⬇️ Going down, an order decomposes. "Make 600 units" becomes a dispatch, becomes setpoints, becomes I/O bits, becomes a heater turning on. Each level translates the one above into something the one below can act on.
⬆️ Going up, data aggregates. A photo-eye pulse becomes a count, becomes a rate, becomes an OEE figure, becomes "we'll ship on time." Each level summarises the raw reality below into something the one above cares about.
Send an order down and watch it transform at every hop into something more concrete — or send a signal up and watch raw reality distil into a business KPI. The travelling token carries the message; each level lights up and shows what the value looks like there, with the latency it adds. Flip the time-horizon view to see the milliseconds-to-months stretch.
Everything in the pyramid is one of these two flows. Get this and the whole model clicks:
⬇️ Decomposition (top → bottom)
A vague, slow, high-level intent ("fulfil this order by Friday") is broken into ever more specific, ever faster instructions until — at the bottom — it's a single output bit flipping a contactor. Abstraction falls as you descend.
⬆️ Aggregation (bottom → top)
A flood of fast, raw, low-level signals is summarised into fewer, slower, more meaningful numbers until — at the top — it's a single sentence a manager reads. Abstraction rises as you ascend.
🧭 The golden rule: each level talks only to its neighbours. ERP never pokes a PLC directly; the field never calls the boardroom. Skipping levels is exactly the hole attackers and spaghetti-integrations crawl through — which is why the IT/OT boundary and MES exist.
The single most useful intuition: everything changes scale as you climb. Time horizons lengthen, data volume shrinks, and the decision-maker shifts from a microchip to a manager.
| Level | Lives here | Time horizon | Data & decision |
|---|---|---|---|
| L4 · ERP | SAP, finance, planning | Days → months | Low volume, high meaning · humans & planning |
| L3 · MES | Execution, OEE, genealogy | Minutes → shift | Orders & KPIs · software + supervisors |
| L2 · SCADA/HMI | Screens, alarms, trends | Seconds | Live tags · operators |
| L1 · Control | PLCs, the logic | Milliseconds | I/O & logic · the controller |
| L0 · Field | Sensors, motors, valves | Now (physics) | Raw signals · the physical world |
This is why you don't run motion logic in the ERP and don't do monthly planning in a PLC: each tool is built for its own time-scale. Put work at the wrong level and it's either dangerously slow or hopelessly over-complex.
You'll hear both names. They're the same stack seen by different crowds:
📊 ISA-95 is the functional / data view — beloved by the MES and IT crowd. It's about how levels exchange information (orders, schedules, results) and underpins MES↔ERP integration (B2MML).
🛡️ Purdue is the network / security view — beloved by the OT-security crowd. Same levels, but it adds the IT/OT boundary and the DMZ, and is the backbone of segmentation and IEC 62443.
🧩 Same five levels. ISA-95 asks "what data moves between them?"; Purdue asks "where are the network walls?" Learn one and you understand the other.