1.1 Functional Hierarchy (ISA-95)

In complex manufacturing environments, blurring the lines between business planning and machine control invites disaster. The ISA-95 standard is not academic theory; it is the structural firewall that prevents your ERP finance system from accidentally crashing a robotic arm. Adhere to this hierarchy to ensure latency-sensitive processes remain isolated from high-level transactional logic.

The Five Levels of Control

Respect the separation of concerns. Each level operates on a different time scale and data granularity.

Level 4: Business Planning (ERP)

Role: The "Brain". Handles Finance, Order Entry, Purchasing, and HR.
Time Scale: Days/Months.
Rule: Level 4 asks "What do we need to build?" It never asks "What is the temperature of the oven right now?"

Level 3: Manufacturing Operations (MES)

Role: The "Coordinator". Manages Workflow, Quality, WIP Tracking, and Genealogy.
Time Scale: Minutes/Seconds.
Rule: Level 3 converts the ERP's "Order" into a specific "Job" for the floor. It acts as the bridge between the dollar and the sensor.

Level 2: Monitoring & Supervisory (SCADA / HMI)

Role: The "Watchtower". Real-time visualization and control of a specific line or area.
Time Scale: Seconds/Sub-seconds.
Rule: Aggregates machine data into actionable dashboards for operators.

Level 1: Sensing & Manipulation (PLC / CNC)

Role: The "Muscle". Logic controllers that drive motors, valves, and actuators.
Time Scale: Milliseconds.
Rule: Critical safety logic lives here. Never rely on the cloud to stop a conveyor.

Level 0: Physical Process

Role: The "Reality". The actual sensor, motor, or chemical reaction.

The "Demilitarized Zone" (DMZ) Logic

Direct communication between non-adjacent levels creates security holes and dependency hell.

Communication Rules

L4 (ERP) ↔ L1 (PLC): Forbidden. The ERP should never talk directly to a machine. If the network lags, the machine crashes.
L4 (ERP) ↔ L3 (MES): Permitted. Via transactional APIs (REST/SOAP).
L3 (MES) ↔ L1 (PLC): Restricted. Use an OPC-UA server or Edge Gateway (Level 2) as a buffer. Do not let the MES query the PLC 100 times per second directly.

Pro-Tip: If your ERP goes down, the factory (L1-L3) must continue to run. If your architecture requires the ERP to be online to print a label, you have violated the hierarchy.

Data Granularity & Summarization

Data gains context as it moves up the stack, but loses granularity.

The Aggregation Flow

Level 1 (PLC): Reads temperature at 100Hz (100 samples/sec).
Level 2 (SCADA): Calculates the 1-second average.
Level 3 (MES): Records the Min/Max/Avg for the specific "Unit Serial Number".
Level 4 (ERP): Records "Process Pass/Fail" for the Production Order.

Data Storage Logic

If you need to debug a motor stall → Query L1/L2 Historian.
If you need to prove regulatory compliance for a specific unit → Query L3 Database.
If you need to calculate Cost of Goods Sold (COGS) → Query L4 Ledger.

Convergence & Edge Computing

Modern IIoT (Industrial Internet of Things) devices blur these lines. However, the logical hierarchy remains valid even if the physical hardware changes.

Smart Device Handling

Even if a smart screwdriver connects via Wi-Fi (physically skipping L1/L2 wiring), logically treat it as an L1 device managed by an L3 driver.

Rule: Edge devices must buffer data locally. If Wi-Fi drops, the torque value must be saved and pushed later.

Final Checklist

Category	Metric / Control	Threshold / Rule
Architecture	L4 ↔ L1 Link	Strictly Forbidden (Air Gap logic)
Resilience	Decoupling	L3 runs 24h+ without L4 connection
Safety	Logic Hosting	Critical Safety Stops must reside in L1 (PLC)
Data	Granularity	ERP stores Financials; MES stores Genealogy
Latency	Control Loop	< 10ms loops stay in L1; > 1s loops go to L3
Network	Segmentation	Isolate Shop Floor (OT) from Office (IT) VLAN