In today's era of rapid development of industrial automation technology, intelligent devices such as PLC
(Programmable Logic Controller) and DCS (Distributed Control System) have become mainstream. However,
when we talk about the foundation of these advanced systems, a classic and powerful concept can never be
bypassed - hardwired logic. It is like the cornerstone of the industrial automation building buried deep in
the ground, although it is not often mentioned, but carries the most fundamental reliability and certainty.
What is hardwired logic?
Hardwired logic, also known as relay logic or fixed logic, is the most primitive and direct form of industrial
control. It does not rely on a software program or microprocessor, but rather on actual, tangible electrical
connections (wires) between physical components (e.g., relays, contactors, timers, pushbuttons, switches) to
build control loops. Each logical function - e.g. “and”, ‘or’, “not”, “self-locking” - is realized by the specific wiring
of the components. When a certain condition is met, current flows along a pre-determined physical path,
ultimately driving actuators (e.g., motors, indicator lights) to action.
In short, the control logic of hardwired logic is determined by the structure of the hardware itself. Modifying the
logic means that physical wiring and components must be reconnected or replaced.
The core advantage of hard-wired logic: Unsurpassed reliability
With so much emphasis on flexible production and smart manufacturing, why do we still need to pay attention
to hardwired logic? The answer lies in its unrivaled reliability and simplicity.
Determinism and transparency: The behavior of a hard-wired system is completely deterministic. As long as the circuit
is connected, the result is inevitable, without the risk of uncertainty that can occur in software programs such as “crash”,
“program runaway” or “software bugs”. For operators and maintenance personnel, the circuit diagram is clear, and fault
points can be directly measured and traced by tools such as multimeters, making the troubleshooting process intuitive
and efficient.
Coping with extreme environments: Hard-wired logic is often the first choice in situations that are sensitive to
electromagnetic interference, harsh environments, or have extremely high safety requirements (e.g., emergency stop
systems, safety interlock circuits). It does not rely on fragile CPUs and operating systems, is more resistant to interference,
and ensures that critical safety functions can still be executed independently and reliably in the event of failure of the main
system control unit.
Extremely Fast Response: For simple switching controls, the response time of hardwired logic is on the physical level,
often much faster than the PLC's scan cycle. This speed advantage is critical in critical protection circuits that require
millisecond or even microsecond responses.
Hard-wired logic vs. modern automation systems
It is a misconception that hard-wired logic is obsolete. In fact, its relationship with modern automation systems is
not one of replacement, but rather one of complementarity and integration.
Foundation and Extension: Modern complex automation systems are often designed in layers. At the top level, complex
process control, data acquisition and human-machine interaction are accomplished by PLC/DCS, while at the bottom
level, emergency control circuits for personal and equipment safety are often hard-wired independently. This architecture
ensures that the underlying safety barrier remains solid even if the upper level of the intelligent system collapses.
Integration and Interfacing: In practice, hardwired logic loops are often used as input signal sources or output actuators
for PLCs. For example, a field emergency stop button hardwired to directly cut off power to a motor (safety circuit) also
sends a signal to the PLC that the system has triggered an emergency stop for host computer alarms and process logging.
The practical significance of mastering hard-wired logic
For engineers and technicians in the field of automation, an in-depth understanding of hard-wired logic has long-term value.
Understanding the nature of automation: it is the key to learning about electrical control. Through the design and construction
of hard-wired circuits, you can deeply understand the “self-locking”, “interlocking” and other basic control ideas of the physical
implementation, which is the basis for understanding the higher-order PLC programming language (such as ladder diagrams,
whose graphical symbols are derived from relay logic).
Strong troubleshooting skills: When an automation system fails, the problem may be at the software level or at the hardware
level. An engineer who is well versed in the principles of hardwiring can quickly determine the scope of the fault, whether it is
a program logic error, or a physical fault in the peripheral sensors, actuators, or the wiring itself, so as to quickly locate and
solve the problem.
Optimized system design: In the system design phase, knowing how to rationally allocate control tasks between hardwiring
and soft logic can find the optimal balance between cost, reliability and flexibility, and design an automation solution that
is both advanced and robust.
Conclusion
Under the wave of Industry 4.0 and Smart Manufacturing, there has been a dizzying iteration of technology. However, the
more we pursue the frontiers of intelligence and connectivity, the more we must not forget the underlying principles that
have stood the test of time. Hard-wired logic, as a classic, reliable and direct control method, remains the solid foundation
for building a safe and stable industrial automation system. It reminds us that true automation begins with the plainest and
firmest control of the physical world. Understanding and mastering it means mastering the soul and root of industrial automation.