Fieldbus Integration in Safety-Critical HMI Systems

Muhammet Işık
Model train layout in spacious room

A standard C99-based middleware designed on the ANSYS SCADE Display platform, enabling HMI screens to communicate with field-level HIMA HiMatrix F35 safety PLCs via Modbus TCP/IP.

Scope: An integration application operating on the zero-copy principle, developed to establish data exchange between SCADE’s closed-loop structure and the control layer.

Project Portfolio

Parameter

Value

Category

System Integration

Delivery Type

Embedded Middleware

Status

Proof of Concept / Academic Paper Published

Role

Embedded Systems Developer

Scale / Scope

Local Communication (4 Track Circuits, 6 Signals, 1 Switch)

Current Situation and Problem

Context: The necessity to migrate the interlocking simulation system (4 tracks, 6 signals, 1 switch) at the ITU Railway Systems Laboratory to a SCADE Display-supported HMI architecture compliant with industrial and certifiable standards. Critical Issues: The absence of a native Modbus interface within SCADE Display and the platform’s closed-network design. The inability of the HMI interface to communicate directly with actual field controllers (HIMA HiMatrix F35), and the lack of dynamic field configuration for IP/port maps.

Component

Status / Requirement

HMI Platform

Lack of a native Modbus interface within SCADE Display

Control Layer

Mandatory communication with HIMA HiMatrix F35 PLCs via Modbus TCP/IP

Field Configuration

Runtime management of varying IP/port and register maps per station

System Size

Local interlocking circuit comprising 4 track circuits, 6 signals, 1 switch, and 4 routes

Solution Architecture and Execution

Architectural Approach

💡 Architectural Constraint Assessment: To preserve DO-178C / EN 50128 certification integrity, the SCADE platform features a “closed system” design philosophy stripped of all asynchronous external network operations. Consequently, for physical PLC integration in the laboratory, developing an independent, externally operating asynchronous middleware layer became mandatory.

A C99 middleware wrapper layer was designed between SCADE’s generated code and the libmodbus library:

Communication Flow

Applied Methodology

  • Zero-Copy Integration: Copy overhead was eliminated by hooking directly into the SCADE main loop.
  • Dynamic Configuration: A 3-region INI file was utilized to prevent recompilation requirements during field deployment.

Security mechanism: Upon detecting a missing or corrupted configuration file, the system automatically creates and saves default settings (localhost connection).

  • Auto-Reconnection: Connection drops in industrial environments are inevitable; an autonomous auto-reconnect cycle was activated.
  • Change Detection: A batch read and selective write strategy was implemented to optimize network traffic.

Architectural Decision: Since HMI hardware’s capability for simultaneous physical command processing is limited, outputs are evaluated in a sequential loop. This structural preference prevents unnecessary fieldbus traffic (network flooding).

  • Thread-Safe Logging: Every event in critical systems must be logged securely.

Interlocking Scope

Component

Quantity

Details

Track Circuit (TC)

4

TC01–TC04

Signal (SN)

6

Input + output signals

Switch (SW)

1

Normal / Reverse

Route (RT)

4

RT01–RT04 combinations

Example route: RT01: SN01E → TC01 → TC04 → TC02 → SN02E (Switch: Normal)

Safety Rule: Based on critical design principles, all signals not explicitly demanded by the system are configured to default to a restrictive state (red).

Results and Operational Takeaways

Quantitative Gain: (Potential values obtained during laboratory PoC tests)

Metric

Value / Status

Cycle Time

~8ms average latency

Connection Mgmt

Automatic reconnection upon failure (auto-reconnect)

Configuration

Compilation-independent commissioning via parametric structure

Network Traffic

Transmission of solely changed registers via change detection algorithms

Validation

Validated on HIMA HiMatrix F35 PLC (SILworX) and physical testbed

Project Visuals

  • Model train layout in spacious room

📂 Source Code: Github/ansys-scade-modbus-integration-middleware 
🔗 Download TOK 2025 Paper: SCADE Modbus Paper Turkish (PDF)

Authors: Dora Demir¹, İbrahim Can Kolotoğlu², Muhammet Işık², Serhat Boynukalın³, Mehmet Turan Söylemez²
¹ ITU Electronics and Communication Eng. | ² ITU Control and Automation Eng. | ³ ITU Graduate School

This work was conducted at the ITU EEF Railway Systems Laboratory and published at the Turkish Automatic Control 2025 (TOK 2025) conference.