1.1 Functional Hierarchy (ISA-95)

In complex manufacturing, blurring the lines between business planning and machine control is not an "agile" feature; it is a structural vulnerability. The ISA-95 standard acts as the firewall preventing your ERP finance system from accidentally crashing a robotic arm. You must adhere to this hierarchy to ensure latency-sensitive processes (Machine Control) remain mathematically isolated from high-level transactional logic (Business Planning).

The Five Levels of Control

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

Level 4: Business Planning (ERP)

• Role: The "Brain". Manages Order Entry, Purchasing, HR, and Ledger.
• Time Scale: Days / Weeks.
• Mandate: Level 4 defines what to build. It must never ask, "What is the nozzle pressure right now?"

Level 3: Manufacturing Operations (MES)

• Role: The "Coordinator". Manages Workflow, Quality, WIP Tracking, and Genealogy.
• Time Scale: Minutes / Seconds.
• Mandate: Level 3 converts the ERP "Order" into a specific "Job". It is the bridge between the dollar and the sensor.

Level 2: Monitoring & Supervisory (SCADA / HMI)

• Role: The "Watchtower". Visualization and line-level control.
• Time Scale: Seconds / Sub-seconds.
• Mandate: Aggregates raw signals into actionable operator dashboards.

Level 1: Sensing & Manipulation (PLC / CNC)

• Role: The "Muscle". Logic controllers driving motors, valves, and actuators.
• Time Scale: Milliseconds (< 10ms).
• Mandate: Critical safety logic lives here. Never rely on a cloud server to stop a conveyor.

Level 0: Physical Process

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

Decision Logic: The "Golden Record" Rules

Do not duplicate data ownership. Use this logic to assign the "Master" status for critical data objects.

Rule 1: Product Data (BOM & Routing)

• IF data defines Cost, Vendor, or Top-Level Structure → THEN ERP is the Golden Record.
• IF data defines Recipe Parameters, Feeder Setup, or Screw Torque → THEN MES is the Golden Record.

Rule 2: Work Orders (WO)

• IF the object represents Financial Demand or Customer Commitment → THEN ERP owns the Header.
• IF the object represents a specific Batch, Serial Number, or Split-Lot → THEN MES owns the WIP State.

Rule 3: Inventory & Genealogy

• IF the query is "What is the total value of stock?" (COGS) → THEN Query ERP.
• IF the query is "Which specific capacitor batch is in this PCB?" (Compliance) → THEN Query MES.

ISA-95 Mapping Worksheet

Use this table to map objects across your specific facility.

Data Object	Level 4: ERP Object (Planning)	Level 3: MES Object (Execution)	Level 0-2: Machine/SCADA Object (Control)
Time Horizon	Shifts / Days	Minutes / Hours	Milliseconds / Seconds
Product Definition	Item Master: SKU, Bill of Materials (BOM), Std Cost.	Process Recipe: Reflow Profile (245˚C), SMT Feeder List, AOI Inspection Criteria.	Machine Program: G-Code, PLC Tag, Setpoint Variable (SP).
Production Command	Production Order: "Make 500 units of SKU-A due Friday."	Dispatch List / WIP: "Line 1, Run Job #101. Sequence: Solder → Place → Reflow."	State Logic: Start / Stop / Hold / E-Stop.
Quality Result	Lot Disposition: Pass/Fail status for the entire order (100 units).	Unit History: "Serial #12345 passed AOI but failed ICT at Test Point 4."	Telemetry: Voltage read (5.1V), Camera Image, Torque value (2.5 Nm).
Maintenance	Asset Ledger: Depreciation schedule, Capital Expenditure (CapEx).	Maintenance Log: Cycle counts, Calibration expiry dates, Tool usage.	Alarms: Motor Over-current, Temp High Limit, Vibration Alert.

The "Demilitarized Zone" (DMZ) Architecture

Direct communication between non-adjacent levels creates security holes and dependency chains that cause downtime.

Communication Rules

• L4 (ERP) → L1 (PLC): Strictly Forbidden. If the office network lags, the machine must not crash.
• L4 (ERP) → L3 (MES): Permitted. Use transactional APIs (REST/SOAP) for order exchange.
• L3 (MES) → L1 (PLC): Restricted. Use Level 2 (Edge Gateway/OPC-UA) as a buffer. Do not allow MES to query a PLC 100 times per second directly.

Pro-Tip: Design for "Headless" operation. If the ERP (L4) goes offline, the factory (L1-L3) must continue to produce, label, and pack goods for at least 24 hours. Sync data when L4 returns.

Data Aggregation Flow

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

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

Final Checklist

Category	Metric / Control	Mandatory State	Engineering Consequence
Architecture	L4 → L1 Link	Forbidden	Air-gap logic prevents business traffic from flooding control networks.
Resilience	Decoupling	24h+ Buffer	Production must not stop during ERP maintenance windows.
Safety	Logic Hosting	L1 (PLC)	Critical stops (E-Stop, Light Curtain) must be hardwired/local.
Data	Granularity	Separated	ERP stores Financials; MES stores Genealogy.
Latency	Control Loop	< 10ms	High-speed loops stay in L1; Loops > 1s can move to L3.
Network	Segmentation	VLAN Separated	Isolate Shop Floor (OT) from Office (IT) traffic.