1968 – When logic left the circuit diagram Birth of PLC programming
In 1968, Dick Morley invented the first PLC – a turning point in automation.
Goal: replace relay circuit diagrams with programmable logic.
First language: Ladder Diagram (LD) – a reflection of relay logic.
Later: IL, FBD, SFC, ST → standardized in 1993 in IEC 61131-3.
But:
Manufacturer dialects remained,
programs are hard to read,
documentation is inconsistent.
👉 The clarity of circuit-diagram logic was sacrificed in favor of flexibility.
Before 1968, logic was visible in the circuit diagram – any electrician could understand processes. After the PLC was introduced, the logic moved into software programs.
Flexibility and modifiability increased.
But the documentation lost clarity.
Today, documentation often means: “Call the programmer who wrote it.”
👉 1968 marks the break: Logic was decoupled from the standard language of electrical engineering.
Historical development of machine logic
1. Before the PLC (until approx. 1968) – logic in the circuit diagram
Machine control via relay and contactor technology
Every logical function (AND, OR, NOT, interlock, self-hold) → physically wired
Documentation in the circuit diagram → unambiguous transparency:
contacts = conditions
coils = results
signal flow → immediately readable for any electrician
Advantage: Logic was clear, traceable, documented
Language: circuit diagram = electrics + logic
2. The invention of the PLC (from 1968) – flexibility instead of clarity
First programmable logic controller (PLC) by Dick Morley (Modicon 084)
Goal: replace relay wiring with flexible software
First language: Ladder Diagram (LD) → direct reproduction of the relay circuit diagram
Advantages:
Changes without rewiring
Space and material savings
Disadvantages:
Logic disappeared from the circuit diagram
Documentation only in the program – manufacturer-dependent
3. Standardization and language diversity (1980s–1990s)
Introduction of new programming languages:
IL (Instruction List) → text-based
FBD (Function Block Diagram) → graphical
SFC (Sequential Function Chart, GRAFCET) → stepwise logic
ST (Structured Text) → high-level language
IEC 61131-3 (1993): worldwide standardization of these five languages
Problem: manufacturer dialects, lack of formal unambiguity
4. Today (2000s–2020s) – complexity without a clear language
PLC programs are powerful, but:
hard to read for outsiders
Documentation often incomplete or not standardized
Logic is distributed across LD, FBD, ST, SFC, data blocks, etc.
Forgotten: That logic used to be clearly documented – in the circuit diagram itself
Consequence: Clarity and competence were lost in favor of flexibility
5. Future – formal language for machine behavior
What is needed is a new, formal, deterministic language for machine logic, comparable to:
technical drawing (mechanics)
circuit diagram (electrics)
Objective: Unambiguous, globally understandable, standardized description of machine behavior
→ Approach: model-based methods (e.g., Selmo)
Overview: Three languages of engineering
mechanics
Technical drawing
Technical drawing
clear, standardized
Electrics
circuit diagram (including logic)
Circuit diagram
clear, standardized
Logic
circuit-diagram logic (relays)
PLC programming
flexible but not unambiguous
👉 This makes it clear:
Earlier logic used to be part of the clear electrical language (circuit diagram).
Today now it is flexible but not unambiguously documented.
Tomorrow we will again need a formal, standardized logic language – as the third pillar alongside drawing and circuit diagram.
Historical course PLC
1968: Dick Morley develops the first PLC (Modicon 084)
1970s: Ladder Diagram (LD) – graphical like relay logic
1980s: IL, FBD, SFC → more expressiveness
1993: IEC 61131-3 defines five languages (LD, FBD, IL, ST, SFC)
Today: languages standardized, but dialects and lack of transparency persist
Historical course logic
Before 1968: relay circuits → logic documented in the circuit diagram
1968: PLC replaces relays → flexibility rises, clarity decreases
1990s: standardization IEC 61131-3, yet no real “language”
Today: dependence on programmers, lack of formal traceability
Logic before the PLC
1. Logic in relay technology
Before the PLC existed, machines were controlled with relay and contactor circuits controlled.
Every logical linkage (AND, OR, NOT, self-hold, interlock) was physically wired.
The logic was directly documented in the circuit diagram:
Relay contacts = logical conditions
Coils = logical outputs
Linkage in the diagram → immediate logical function
Advantage: 100% transparency. Any professional could immediately see from the circuit diagram how the machine works logically.
2. Logic as part of the circuit diagram
Until the 1960s, the circuit diagram was the document for mechanics and control logic.
One could understand how a machine works from the circuit diagram alone.
Thus, logic has always been a describable and documented language – as unambiguous as the technical drawing for mechanics.
3. The break with the PLC
With the introduction of the PLC (from 1968), logic was extracted from the circuit diagram and shifted into software.
Advantage: high flexibility (changes without rewiring, modular programs).
Disadvantage: loss of the Documentation transparency.
The logic is no longer visible in the circuit diagram.
Different programming languages (LD, FBD, ST) and manufacturer dialects make readability more difficult.
The clear competence of electrical engineers, who used to be able to trace everything in the diagram, was displaced.
4. Consequences to this day
Many today see logic only as part of a PLC program – forgotten is that logic has always had a clear, documented description.
Thus, the break between electrics and software arose.
Where previously drawing (mechanics) + circuit diagram (electrics + logic) represented the whole, today the unambiguous, formal language for control logic.
The arc: three languages of engineering
mechanics
Technical drawing
Technical drawing
stable, clear, standardized
Electrics
circuit diagram (including logic)
Circuit diagram
stable, clear, standardized
Logic
circuit-diagram logic (relays, contacts)
PLC programming
Flexible, but no longer unambiguously documented
Conclusion
The Logic has always been part of the language of engineering – visible in the circuit diagram, unambiguous and documented. With the PLC it became more flexible, but at the same time invisible and inconsistent. Today it is largely forgotten that historically logic was described just as clearly as geometry and electrics.
👉 The task for the future is to restore formal clarity – with a universal, model-based language for the behavior of machines, comparable to the drawing in mechanics and the circuit diagram in electrics.
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