150  Real-Time Requirements and ISA-95 Automation Levels

150.1 Learning Objectives

After completing this chapter, you will be able to:

  • Differentiate between ISA-95 automation levels and their timing constraints
  • Understand hard real-time vs soft real-time requirements
  • Design real-time industrial control systems with appropriate latency requirements
  • Map technologies to appropriate automation levels
  • Apply the ISA-95 model to industrial IoT architectures

150.2 Prerequisites

Before diving into this chapter, you should be familiar with:

150.3 Introduction

Industrial systems have strict timing requirements that vary by automation level. A motor control loop that misses its 1ms deadline can cause equipment damage, while an enterprise report delivered a few seconds late has no operational impact. Understanding these timing requirements is fundamental to designing effective industrial IoT systems.

150.4 ISA-95 Automation Pyramid

Time: ~10 min | Difficulty: Intermediate | Unit: P03.C06.U05

The ISA-95 standard (also known as IEC 62264) defines the interface between enterprise and control systems:

Graph diagram

Graph diagram
Figure 150.1: ISA-95 automation pyramid showing five levels with timing requirements: Level 4 (business planning with hours-to-days response time in gray), Level…

150.5 Timing Requirements by Level

150.5.1 Level 0: Field Devices (Sub-millisecond)

The physical interface with the manufacturing process:

  • Sensors sample physical processes
  • Actuators respond to control signals
  • Examples: Thermocouples (1-100ms), high-speed encoders (50μs), servo drives (62.5μs)

Characteristics:

  • Direct connection to physical process
  • Continuous or very fast periodic operation
  • Simple devices with minimal processing
  • Often intrinsically safe for hazardous areas

150.5.2 Level 1: Basic Control (1-10ms)

Automated control of manufacturing processes:

  • PLCs execute control logic
  • PID loops maintain setpoints
  • Safety systems must guarantee response times
  • Examples: Motion control (1ms), discrete I/O (10ms), process control (100ms)

Characteristics:

  • Deterministic execution cycles
  • Real-time operating systems
  • Redundancy for critical functions
  • Direct I/O to Level 0 devices

150.5.3 Level 2: Supervisory (100ms-1s)

Monitoring and supervision of production processes:

  • SCADA systems collect data from multiple PLCs
  • HMIs display process status to operators
  • Alarm systems notify of abnormal conditions
  • Examples: Data logging (1s), trend displays (5s), alarm response (500ms)

Characteristics:

  • Soft real-time requirements
  • Human interaction interfaces
  • Historical data storage (historians)
  • Recipe and batch management

150.5.4 Level 3: Operations (Seconds to Minutes)

Manufacturing operations and workflow management:

  • MES manages production schedules
  • Tracks work orders, genealogy, and quality
  • Batch control coordinates recipes
  • Examples: Batch phase transitions (10s), reporting (1min), scheduling (hours)

Characteristics:

  • Integration with enterprise systems
  • Production workflow coordination
  • Quality management and traceability
  • Resource allocation and tracking

150.5.5 Level 4: Enterprise (Hours to Days)

Business planning and logistics:

  • ERP handles business planning
  • Supply chain management coordinates materials
  • Customer relationship management
  • Examples: Daily production planning, weekly demand forecasting, monthly financial closing

Characteristics:

  • Business process integration
  • Long-term planning horizons
  • Financial and customer data
  • Corporate-wide visibility

150.6 Determinism vs. Throughput

Industrial systems distinguish between different real-time guarantees:

150.6.1 Hard Real-Time (Deterministic)

Definition: Must respond within guaranteed time; missing deadline is system failure.

Characteristics:

  • Worst-case execution time (WCET) must be bounded
  • Jitter must be minimal (<1μs for synchronized motion)
  • Preemptive, priority-based scheduling
  • Often requires specialized hardware

Examples:

  • Safety systems (emergency stop)
  • Motion control (coordinated axes)
  • Process control (exothermic reactions)

Implementation approaches:

  • Dedicated real-time networks (EtherCAT, PROFINET IRT)
  • Real-time operating systems (VxWorks, QNX, RTAI)
  • FPGA-based control
  • Time-triggered architectures

150.6.2 Soft Real-Time

Definition: Should respond quickly but occasional delays acceptable; results in degraded performance, not failure.

Characteristics:

  • Average response time matters more than worst-case
  • Some deadline misses tolerable
  • Standard operating systems acceptable
  • Statistical quality of service

Examples:

  • HMI updates
  • Data logging
  • Trend analysis
  • Operator notifications

150.6.3 Best Effort

Definition: No timing guarantees; response when resources available.

Examples:

  • Historical data analysis
  • Business reporting
  • Email notifications
  • Non-critical analytics

150.7 Jitter and Synchronization

For coordinated motion and distributed control, jitter (timing variation) is often more critical than absolute latency:

Jitter requirements by application:

Application Cycle Time Max Jitter Nodes
Simple I/O 10ms 1ms 10-100
Process control 100ms 10ms 100-1000
Packaging machinery 1ms 100μs 10-50
Printing press 125μs 1μs 20-100
Semiconductor handling 62.5μs 100ns 10-30

Synchronization mechanisms:

  1. IEEE 1588 (PTP): Precision Time Protocol for sub-microsecond sync
  2. Distributed clocks: EtherCAT’s hardware-based synchronization
  3. Time-triggered protocols: Deterministic message scheduling
  4. GPS timing: Absolute time reference for wide-area systems

150.8 Technology Mapping

150.8.1 Protocol Selection by Level

Level Typical Protocols Latency Determinism
0-1 EtherCAT, PROFINET IRT <100μs Hard real-time
1-2 PROFINET, EtherNet/IP 1-10ms Soft real-time
2-3 OPC-UA, Modbus TCP 10-100ms Best effort
3-4 REST APIs, MQTT 100ms-1s Best effort

150.8.2 Computing Platform by Level

Level Platform OS Processing
0-1 PLC, PAC, IPC RTOS, bare metal Deterministic scan cycle
2 Industrial PC Windows, Linux Standard scheduling
3 Server Windows Server, Linux Virtualization OK
4 Cloud/Enterprise Any Containerization, serverless

150.8.3 Network Architecture by Level

Level Network Redundancy Segmentation
0-1 Dedicated industrial Ring, dual-port Air-gapped from IT
2 Industrial Ethernet RSTP, PRP/HSR VLAN separated
3 Converged IT/OT Standard HA DMZ between zones
4 Corporate/cloud Internet standards Firewall protected

150.9 Case Study: Automotive Assembly Line Design

You are designing the control system for an automotive assembly line with:

  • 100 robotic arms: Each with 6 axes (600 servo motors total)
  • 1,000 quality inspection sensors: Vision systems, force sensors, laser scanners
  • 50 AGVs: Automated guided vehicles delivering parts
  • 10 operator stations: HMIs for monitoring and manual intervention

Requirements:

  • All robots must be synchronized within 1ms
  • Quality data must be logged for 10-year traceability
  • AGVs must avoid collisions with <100ms response time
  • Operators need real-time production status
  • Enterprise ERP system needs hourly production counts
  • Predictive maintenance for all critical assets

150.9.1 Solution: Latency Requirements

Robotic motion control: <1ms hard real-time, <1μs jitter for synchronization

  • Justification: 6-axis coordinated motion requires deterministic timing
  • ISA-95 Level 1 (Basic Control)

Quality sensors: <100ms soft real-time

  • Vision processing and data logging can tolerate slight delays
  • ISA-95 Level 2 (Supervisory)

AGV collision avoidance: <100ms hard real-time

  • Safety-critical, must respond to obstacles deterministically
  • ISA-95 Level 1 with safety rating

Operator HMIs: <1s soft real-time

  • Human perception doesn’t require sub-second updates
  • ISA-95 Level 2 (Supervisory)

ERP production counts: Hourly batch updates

  • No real-time requirement
  • ISA-95 Level 4 (Enterprise)

150.9.2 Solution: Protocol Selection

For robot control: EtherCAT

  • <100μs cycle time supports 1ms requirement with margin
  • <1μs jitter enables precise synchronization of 100 robots
  • Proven in automotive manufacturing (BMW, Tesla, VW all use EtherCAT)

For quality sensors: PROFINET or EtherNet/IP

  • Less stringent timing requirements
  • Standard industrial Ethernet sufficient

For AGV coordination: Wireless OPC-UA over Wi-Fi 6 or 5G

  • Wi-Fi 6 provides <10ms latency with QoS
  • OPC-UA provides standardized data model

For IT/OT integration: OPC-UA

  • Bridges PLCs to MES/ERP
  • Provides semantic data model for quality traceability

150.9.3 Solution: Data Integration Strategy

Field Devices (Level 0-1)
    ↓ EtherCAT (real-time control)
Robot Controllers / PLCs (Level 1)
    ↓ PROFINET or EtherNet/IP
SCADA / Edge Gateway (Level 2)
    ↓ OPC-UA (IT/OT bridge)
MES / Historian (Level 3)
    ↓ REST APIs / Message Queues
ERP / Cloud Analytics (Level 4)

Key architectural decisions:

  • Edge processing: FFT and feature extraction at Level 2 reduces cloud bandwidth by 10,000x
  • Time-series database: Dedicated historian (OSIsoft PI, Ignition) handles 50K data points/second
  • OPC-UA server: Provides unified namespace for all 51,000 data points
  • Data lake: Raw data retained for 10 years in cloud object storage
  • Security: Network segmentation, OPC-UA encryption, VLANs separate control and IT networks

Bandwidth calculation:

  • 600 motors x 10 signals x 1ms sampling = 6 million samples/second
  • At 4 bytes/sample = 24 MB/second raw data
  • After edge processing: 1 Hz features = 6 KB/second to cloud
  • Reduction factor: 4,000x

150.10 Summary

Real-time requirements and ISA-95 levels provide a framework for industrial system design:

ISA-95 pyramid: Five levels from field devices (sub-millisecond) to enterprise (hours-days) with distinct timing requirements and technologies appropriate to each level.

Determinism matters: Hard real-time systems require guaranteed worst-case timing; soft real-time tolerates occasional delays; best-effort has no timing guarantees.

Jitter vs latency: For synchronized motion control, consistent timing (low jitter) is often more critical than absolute speed.

Technology mapping: Protocol, computing platform, and network architecture choices must align with the timing requirements of each ISA-95 level.

Design principle: Never use higher-level (slower) technologies for lower-level (faster) requirements. Cloud cannot control motors; PLCs should not run ERP.

150.11 What’s Next

Continue your learning journey: