1.1 Functional hierarchy: ISA-95

In complex manufacturing environments, blurring the lines between business planning and machine control does not provide agility; instead, it creates a point of structural weakness. The ISA-95 standard acts as a crucial framework, preventing high-level business systems like an ERP from inadvertently issuing commands that could disrupt low-level machine operations, such as controlling a robotic arm. Adhering to this functional hierarchy ensures that time-sensitive control processes remain isolated from the slower, transactional logic of business planning, which is essential for stability and safety.

The five levels of control

This separation of concerns is fundamental. Each level operates on a specific time scale and handles data at an appropriate level of granularity.

Level 4: Business Planning (ERP)

• Role: The strategic “Brain” of the operation. It manages enterprise-wide functions like order entry, purchasing, human resources, and financial ledgers.
• Time Scale: Days and weeks.
• Function: Level 4 defines what to build and the business context. It should not be concerned with real-time machine states, such as asking for instantaneous nozzle pressure.

Level 3: Manufacturing Operations (MES)

• Role: The on-the-floor “Coordinator.” It manages production workflow, quality tracking, work-in-progress (WIP) status, and product genealogy.
• Time Scale: Minutes and seconds.
• Function: Level 3 translates a business order from the ERP into a specific, executable production job. It serves as the essential bridge between business intent and physical execution.

Level 2: Monitoring & Supervisory (SCADA / HMI)

• Role: The operational “Watchtower.” It provides visualization, historical data logging, and line-level supervisory control through Human-Machine Interfaces (HMIs).
• Time Scale: Seconds and sub-seconds.
• Function: It aggregates raw data from machines and sensors into coherent, actionable dashboards for operators.

Level 1: Sensing & Manipulation (PLC / CNC)

• Role: The “Muscle” that performs the work. Programmable Logic Controllers (PLCs) and Computer Numerical Control (CNC) systems directly drive motors, valves, actuators, and read sensors.
• Time Scale: Milliseconds (typically under 10ms).
• Function: Critical, real-time safety and control logic resides here. This level must be autonomous; you should never rely on a cloud server or business system to execute an emergency stop on a conveyor.

Level 0: Physical Process

• Role: The “Reality” of manufacturing. This is the physical world itself—the sensor measuring temperature, the motor turning, or the chemical reaction occurring.

Decision logic: the “golden record” rules

To maintain data integrity, ownership of critical data objects should not be duplicated. Use the following logic to determine which system holds the master, or “Golden Record,” status for different types of data.

Rule 1: Product data (BOM & routing)

• The ERP acts as the Golden Record when the data dictates cost, specifies a vendor, or defines the top-level product structure.
• The Manufacturing Execution System (MES) serves as the Golden Record when the data defines machine-specific recipe parameters, feeder setup instructions, or torque settings.

Rule 2: Work orders (WO)

• The ERP owns the header data for a work order when it represents financial demand or a firm customer commitment.
• The MES owns the detailed state data when the object represents a specific production batch, serial number, or split-lot of work in progress (WIP).

Rule 3: Inventory & genealogy

• Query the ERP when you need to determine the total financial value of stock or calculate the Cost of Goods Sold.
• Query the MES when you need to determine traceability details, such as which specific capacitor batch was placed on a particular printed circuit board (PCB) for compliance purposes.

ISA-95 mapping worksheet

Use this table as a guide to map common data objects and their characteristics across the functional levels within 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 Paste Printing → Pick & Place → Reflow Soldering.”	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

Allowing direct communication between non-adjacent levels, such as from business planning directly to a machine controller, creates security vulnerabilities and fragile dependency chains that can lead to unplanned downtime.

Communication rules

• L4 (ERP) → L1 (PLC): Not Recommended. Machine control must remain functionally independent. If the office network experiences latency or an outage, the production line must continue to operate safely without crashing.
• L4 (ERP) → L3 (MES): Standard Practice. This is the primary integration point, typically using transactional APIs (like REST or SOAP) to exchange order information.
• L3 (MES) → L1 (PLC): Managed Connection. Use Level 2 (an Edge Gateway or OPC-UA server) as an intermediary buffer. This architecture prevents the MES from directly querying a PLC at a high frequency, which could overload the sensitive control network.

Field Experience

Always design for “headless” operation. If the ERP (Level 4) goes offline, the factory floor (Levels 1-3) should be able to continue producing, labeling, and packing goods autonomously for a period—ideally at least 24 hours. Data can be synchronized back to the ERP once it is back online.

Data aggregation flow

As data moves up the hierarchy from machines to the enterprise, it loses fine-grained detail but gains broader business context.

1. Level 1 (PLC): A sensor reads temperature 100 times per second (100 Hz).
2. Level 2 (SCADA): The system calculates and logs a one-second rolling average of that temperature.
3. Level 3 (MES): For each specific unit with a serial number, the MES records the minimum, maximum, and average temperature experienced during its processing.
4. Level 4 (ERP): The ERP ultimately records a simple “Process Pass” status for the entire production order.

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Recap: ISA-95 Functional Hierarchy Levels

Level	Role	Time Scale	Key Mandate / Rule
4 (ERP)	Business Planning	Days / Weeks	Defines what to build. Golden record for BOM, cost, customer order.
3 (MES)	Manufacturing Operations	Minutes / Seconds	Converts order into work. Golden record for recipe, batch, genealogy.
2 (SCADA/HMI)	Monitoring & Control	Seconds / Sub-seconds	Aggregates signals into control panels. Not for direct, real-time execution of safety-critical logic.
1 (PLC/CNC)	Sensing & Actuation	Milliseconds (<10 ms)	Controls motors, valves. Safety-critical logic.
0	Physical Process	Real-time	Sensors, actuators, physical process.