What is a PLC and Why Retrofit?
A PLC (Programmable Logic Controller), known in German as SPS (Speicherprogrammierbare Steuerung), is the "brain" of a production plant or machine. It controls and monitors all processes – from simple switching operations to complex manufacturing sequences.
Retrofit refers to the modernization and refurbishment of outdated control systems. Instead of replacing the entire plant, only the control components are renewed – while retaining the mechanical structure. This saves enormous costs and downtime.
💡 Important to know:
A retrofit typically costs only 30–60% of a new purchase and makes your system technologically current, maintainable, and future-proof – with minimal production downtime.
When is a retrofit necessary?
- ⚠️EOL components (End of Life): Manufacturer discontinues production and support – no spare parts available
- ⚠️Outdated software: No more updates, security vulnerabilities, incompatibility with new components
- ⚠️Frequent faults: Old components fail more and more often, repair costs increase
- ℹ️Feature expansion: New requirements (Industry 4.0, data acquisition, remote maintenance) cannot be implemented
- 📊Missing know-how: Original programmer no longer available, no documentation exists
Technical Standards and Programming Languages
PLC programming follows international standards that ensure portability and maintainability of programs.
IEC 61131-3: The International Standard
The IEC 61131-3 standard defines five standardized programming languages for PLCs:
Text-based languages:
- • ST (Structured Text): High-level language similar to Pascal, for complex algorithms
- • IL (Instruction List): Assembly-like, hardware-level (deprecated)
Graphical languages:
- • LAD (Ladder Diagram): Circuit diagram representation, intuitive
- • FBD (Function Block Diagram): Function blocks, modular
- • SFC (Sequential Function Chart): Sequence control, process steps
Manufacturer-Specific Systems:
🔷 Siemens (TIA Portal, Step 7)
Market leader in Europe. S7-300, S7-400, S7-1200, S7-1500. TIA Portal as modern development environment.
🟢 Schneider Electric (Unity Pro)
Modicon series, widely used in process industry. Programming per IEC 61131-3.
🟠 B&R (Automation Studio)
Specialist for motion control and high-speed applications.
🔵 Allen-Bradley/Rockwell
ControlLogix, CompactLogix. Widely used in the USA, increasingly also in Europe.
💡 AMA Systems Expertise:
Our programmers are proficient in all common PLC systems and programming languages. We work with Siemens (S7, TIA Portal), Schneider, B&R, Allen-Bradley, Beckhoff and other systems.
Retrofit: Modernization Instead of New Purchase
Retrofit is often the most economical solution for outdated control systems. The mechanical plant is preserved; only the control technology is renewed.
Cost Comparison: Retrofit vs. New Purchase
| Item | Retrofit | New Purchase |
|---|---|---|
| Control hardware (PLC, I/O, HMI) | Moderate | Included |
| Mechanics and drives | No cost (retained) | Very high |
| Programming/commissioning | Moderate | High |
| Downtime | 1–4 weeks | 3–12 months |
| Installation/conversion | Low | High |
| TOTAL COST | Significantly lower | Very high |
💰 Savings through retrofit:
- ✓ 60–85% cost savings compared to new purchase
- ✓ 75% shorter downtime
- ✓ Proven mechanics are preserved
- ✓ Faster return on investment
Typical Retrofit Scenarios:
1. EOL Components (End of Life)
Problem: Siemens S5, old S7-300 without spare parts
Solution: Migration to S7-1500, program conversion, new I/O modules
2. Missing Features
Problem: Plant needs to be integrated into MES/ERP system
Solution: Addition of data interfaces (OPC UA, MQTT), control system retrofit
3. Safety Requirements
Problem: New CE directives, missing safety functions
Solution: Integration of fail-safe PLC, safety circuit per DIN EN ISO 13849
Fault Analysis and Troubleshooting
PLC faults can have a wide range of causes – from program errors and hardware defects to communication problems. A systematic approach is the key to a quick resolution.
Most Common Fault Sources:
🖥️ Software Errors
- • Logic errors in the program
- • Timing problems (watchdog)
- • Memory overflow
- • Unhandled exceptions
⚙️ Hardware Defects
- • Defective CPU or I/O modules
- • Power supply problems
- • Battery empty (program lost)
- • Corrosion, moisture
📡 Communication Errors
- • Profibus/Profinet faults
- • Network timeouts
- • Address conflicts
- • Defective communication modules
🔌 Sensor/Actuator Problems
- • Defective sensors (incorrect values)
- • Stuck valves
- • Cable breaks
- • Short circuits
Systematic Troubleshooting:
Symptom Recording
Read error messages from HMI/PLC, analyze process sequence, determine time of error, check reproducibility.
Hardware Check
Check diagnostic LEDs, measure power supply, verify I/O status, test modules for defects.
Program Analysis
Online connection to PLC, variable monitoring, step-by-step program testing, identify logic errors.
Communication Test
Network/fieldbus diagnostics, check connections to peripherals, verify timeout settings.
Repair/Adjustment
Program correction, hardware replacement, parameterization, testing under production conditions.
⏱️ Typical analysis times:
- • Simple program errors: 1–3 hours
- • Hardware defects: 2–6 hours (plus spare parts procurement)
- • Complex logic errors: 4–16 hours
- • With expert support: Often 50–70% faster
Documentation and Knowledge Management
One of the biggest challenges in PLC projects is missing or incomplete knowledge about existing programs. Documentation is therefore critical for long-term maintainability.
The Problem of Missing Documentation:
Typical situation in many companies:
- • Original programmer no longer with the company
- • No or outdated documentation available
- • Uncommented program code ("spaghetti code")
- • No program backups available
- • Changes over years without documentation
→ Result: Modifications and troubleshooting take 3–5x longer and are more expensive!
What constitutes good PLC documentation?
- 📋Functional description: What does the system do? What process steps are involved?
- 🗺️Program structure: Overview of blocks, functions, data blocks
- 💬Code comments: Explanation of critical program sections, variable meanings
- 🔌I/O list: Mapping of inputs/outputs to sensors/actuators
- ⚙️Parameter lists: Settings, setpoints, machine data
- 📊Change history: Who changed what, when, and why?
- 💾Backup strategy: Regular backup of all programs and configurations
✓ AMA Systems Documentation Standard:
For all our PLC projects (new programming, retrofit, modifications) we provide:
- ✓ Fully commented program code
- ✓ Functional description in German
- ✓ I/O lists with descriptions
- ✓ Network documentation (IP addresses, participants)
- ✓ Backup of all programs and configurations
- ✓ Change documentation with version numbers
Backup Strategy for PLC Programs:
Recommended backup intervals:
- • After every change: Immediate backup with version number
- • Monthly: Automatic backup even without changes
- • Before major modifications: Complete system backup
- • Retention: Keep multiple versions (at least 10 years)
→ A missing backup can lead to total loss of the program if the PLC fails!
Case Studies: Retrofit and Troubleshooting
The following real-world cases illustrate typical PLC problems and their cost-effective solutions:
Case 1: Siemens S5 Retrofit (EOL)
Initial situation:
- • Siemens S5 (over 30 years old)
- • CPU defective, no spare parts
- • Production shutdown imminent
- • New machine purchase: Very high investment
Retrofit solution:
- • Migration to S7-1500
- • Program conversion and optimization
- • Retrofit cost: Fraction of new purchase
- • Savings: Over 85%!
- • Additionally: Remote maintenance, OPC UA integrated
Case 2: Program Lost After Battery Failure
Problem:
- • PLC battery empty, program deleted
- • No backup available
- • Original programmer deceased
- • Production stop: 3 weeks
Solution:
- • Reprogramming based on plant knowledge
- • Step-by-step testing and adjustments
- • Significant programming costs
- • Massive downtime costs
→ With regular backups, recovery would have been possible in 4 hours!
Case 3: Program Extension for Industry 4.0
Requirement:
- • Connection to MES system desired
- • Data acquisition (OEE, piece counts)
- • Remote maintenance access
- • Existing plant should continue running
Implementation:
- • OPC UA interface programmed
- • Data acquisition blocks integrated
- • VPN remote maintenance set up
- • Manageable investment
- • ROI: < 18 months through efficiency gains
Long-term Maintainability and Future-Proofing
PLC projects should focus not only on current functionality but also on long-term maintainability.
Best Practices for Maintainable PLC Programs:
- ✓Structured programming: Modular design with clear function blocks
- ✓Meaningful naming: Understandable variable and block names (not "M0.0", but "Motor_MainDrive_Start")
- ✓Commenting: Every complex block sufficiently documented
- ✓Standard function blocks: Use reusable, tested blocks
- ✓Error handling: Systematic error handling with meaningful messages
- ✓Version control: Traceable changes with date and description
Future-Proofing Through Modern Technologies:
📡 Industry 4.0 Ready
- ✓ OPC UA for data exchange
- ✓ MQTT for cloud connectivity
- ✓ REST APIs for web services
- ✓ Database connectors
🔒 Security
- ✓ Fail-safe programming
- ✓ Access protection (passwords)
- ✓ Secure remote maintenance (VPN)
- ✓ Audit trails
💡 Knowledge Transfer:
For retrofit and programming projects, we offer:
- ✓ Training of your staff on the finished program
- ✓ Handover workshop: Explanation of the program structure
- ✓ Documentation in German (not just program comments)
- ✓ Post-commissioning support: We remain your contact partner
Financial Benefits Through Professional PLC Support
Professional PLC programming and retrofit pay off multiple times – through cost savings, shorter downtime, and higher productivity.
Cost Savings Overview:
| Area | Without Experts | With Experts | Savings |
|---|---|---|---|
| Retrofit costs | New purchase: Very high | Retrofit: Significantly lower | 60–85% |
| Troubleshooting | 16 hours diagnosis | 5 hours | Significantly less downtime |
| Documentation | Missing, every change takes 3x longer | Complete | Long-term 30–50% time savings |
| Future-proofing | Outdated technology | Industry 4.0-capable | Competitive advantage |
💰 ROI Calculation for Retrofit:
Retrofit offers significant advantages over a new purchase:
- ✓ Significant investment savings (60–85%)
- ✓ Shorter conversion time = less downtime costs
- ✓ New features: Remote maintenance, data acquisition
- ✓ Maintainability: Future modifications 50% faster
→ Total savings: Significant – contact us for an individual calculation!
Conclusion: PLC Expertise as Competitive Advantage
Modern PLC programming and professional retrofit are key factors for economic success. They enable the modernization of existing plants at a fraction of the cost of new equipment.
Key Benefits:
💰 Financial Benefits
- ✓ Retrofit 60–85% cheaper than new purchase
- ✓ Fast troubleshooting
- ✓ Minimal downtime
- ✓ Predictable costs
🔧 Technical Benefits
- ✓ Modern, maintainable programs
- ✓ Industry 4.0 features
- ✓ Expandable and flexible
- ✓ Remote maintenance possible
📚 Knowledge Management
- ✓ Complete documentation
- ✓ Commented code
- ✓ Staff training
- ✓ Regular backups
🚀 Future-Proofing
- ✓ Current hardware (available)
- ✓ Expandable for new requirements
- ✓ Long-term support
- ✓ Stay competitive
Frequently Asked Questions (FAQ)
Is a retrofit worthwhile or should I buy new?
Retrofit is almost always worthwhile when the mechanics are still in good condition. You save 60–85% of costs and have 75% shorter downtime. New purchase only makes sense with fundamental mechanical problems or when completely new functions are needed.
Can you work with third-party PLC programs?
Yes! We analyze existing programs even without documentation and can adapt, extend, or completely restructure them. We often recommend documentation and optimization alongside.
How long does a retrofit take?
That depends on the complexity. Simple systems: 1–2 weeks.Complex plants: 4–8 weeks. Most of the programming is done at our facility; your production interruption is typically only 3–7 days for the conversion.
What does PLC programming/retrofit cost?
Costs depend heavily on the scope. Retrofit projects are typically60–85% cheaper than a new purchase. Contact us for a free feasibility analysis and an individual quote.
Modernize your plants – stay competitive!
PLC retrofit and professional programming are investments in the future of your company. Save costs, gain flexibility, and make your plants Industry 4.0-capable.
Contact our PLC experts for a non-binding consultation and feasibility analysis.
Standards & Regulatory References
Planning, programming and safety assessment of PLC systems and retrofit projects are governed by vendor-neutral standards and the European Machinery Directive:
- Siemens TIA Portal – Engineering framework for Simatic S7 controllers (S7-300, S7-1500) and backbone of many retrofit projects.
- Machinery Directive 2006/42/EC – EU directive on safety requirements for machinery; triggered by substantial modifications during retrofit.
- EN ISO 13849-1 – Safety-related parts of control systems; performance-level assessment for retrofit and new safety logic.
Note: The content of this article does not replace a binding engineering or conformity assessment.