Decentralized I/O: The Secret to Simpler, Cleaner Machine Designs
The Way You Wire Your Machine Is Just as Important as What You Wire Into It
There is a moment on almost every machine startup that every controls engineer knows well. You are standing in front of a cabinet that looks like the inside of a telephone exchange from 1987. Hundreds of wires. Dozens of terminal blocks. Cable bundles so thick you cannot see where one ends and another begins. And somewhere in that tangle, something is wrong.
It might be a mislabelled terminal. A missed ground. A ferrule that was not quite crimped properly. A wire that was supposed to go to input 14 but ended up on input 41. It does not matter which one it is, because finding it will take hours. And the customer is waiting.
This is not a software problem. It is not a PLC problem. It is an architecture problem. And it is one that the packaging industry has been living with for decades, largely because no one has stopped to ask whether there is a better way.
There is. It is called decentralized I/O, and once you understand how it works, you will find it very difficult to justify going back to the way things were.
What Centralized Architecture Actually Costs You
The traditional approach to machine wiring is to run everything back to a central control panel. Every sensor, every actuator, every drive, every safety device has a cable that travels the length of the machine and terminates at a terminal block in the main cabinet. From there, wires fan out to the I/O modules in the PLC rack. The cabinet becomes the nervous system of the machine, and all signals must pass through it.
This approach made sense when PLCs were expensive, and IP67 remote I/O was scarce. You centralized the intelligence because the hardware was too costly to distribute. But that logic stopped being true a long time ago. Modern IP67 I/O systems are inexpensive, compact, and designed specifically to live on the machine rather than in a cabinet. The centralized approach has persisted largely out of habit, not because it is the best way to do things.
The real cost of centralized wiring shows up in three places: build time, startup time, and maintenance time.
Build time increases because every device on the machine requires a long cable run back to the cabinet. Those cables need to be routed, bundled, labelled, and terminated at the cabinet end. On a machine with 200 I/O points, that’s a minimum of 600 terminations (3 wires per device), and all of them are manual. The more manual terminations, the more opportunities for error, and the more time the build team spends checking and rechecking their work.
Startup time increases because, when something does not work, the fault is likely lurking in a terminal block or a field connector. A sensor that is not triggering might have a wiring fault in a field-attachable connector, a bad connection at the terminal block in the cabinet, or a loose connection at the PLC I/O module. Tracing and locating that fault takes time, and wasted time on a startup is the most expensive and frustrating type.
Maintenance time increases because the same problems that make startups difficult make field troubleshooting difficult. When a fault develops on a live production machine, the maintenance team faces the same challenge: find the fault somewhere in one of the manual termination points, often with the production manager impatiently watching and waiting.
The centralized cabinet is not just a wiring approach. It is a risk multiplier. Every wire that is manually terminated introduces an opportunity for variability and human error, which becomes a potential fault location. Every terminal block has at least two connection points that can be wired wrong or work loose, and every extra wire in a wire duct just adds time and complexity for installers and maintenance teams.
What Decentralized I/O Actually Means
Decentralized I/O turns the traditional architecture inside out. Instead of running all signals back to a central cabinet, you place IP67 I/O modules directly on the machine, close to the devices they serve. Each module connects to the PLC via a power cable and an industrial Ethernet or fieldbus cable. The sensors and actuators in that zone connect to the local IP67 module with short cables, typically less than a metre or two.
The central cabinet still exists, but it shrinks dramatically. Instead of housing hundreds of I/O points, it contains the PLC, power supplies, safety controller, network switch, motor control and VFDs. The cabinet becomes a coordination hub rather than a wiring destination.
The result is a machine where the wiring complexity is distributed and localized rather than centralized and concentrated. Each zone of the machine is self-contained. The sensor for a particular actuator is connected to the I/O module that serves that actuator, which is mounted within arm’s reach of both. The cable between them is short, clearly labelled, and easy to trace.
The Numbers That Make the Case
The business case for decentralized I/O is not theoretical. The reductions in wiring complexity are measurable and translate directly into build time, startup time, and maintenance costs.
Consider a typical packaging machine with 100 I/O points distributed across a 6-meter machine frame. In a centralized architecture, the average cable run from device to cabinet might be 5 meters. That is 500 metres of cable, and 300 terminations at the cabinet end. That’s 700-900 terminations in total, all manual, all subject to human error.
In a decentralized architecture with seven IP67 I/O modules placed at short intervals along the machine, the average cable run from device to I/O module drops to about 2 meters. That is one hundred 2-meter double-ended cables, with only 4 terminations (24VDC power) at the panel end. Plus, a network cable daisy-chained and plugged in at the cabinet. That massively reduces the wiring work, effectively eliminates the termination count, and basically removes all fault opportunities.
There is a 98% reduction in manual termination points when moving from centralized to decentralized I/O on a typical packaging machine.
A Direct Comparison
| Factor | Centralized I/O | Decentralized I/O |
| Cable lengths | Long runs, often 5-15m per device | Significantly shorter cables with connectors |
| Termination count | High – 3-9 wire terminations per device | Basically zero – everything connectorized |
| Cabinet size | Large, complex, expensive | Smaller, simpler, lower cost |
| Fault location | Difficult – fault could be any termination point | Easy – LED indicators, factory-tested cordsets |
| Build time | Higher – more cable routing and termination work | Lower – fewer cables, less routing, basically no termination work |
| Startup time | Longer – many manual connection points to verify and fault-find | Shorter – very few manual connection points, faster verification |
| Maintenance | Complex – long cable runs, dense cabinet wiring | Simple – zone-based, short cables, clear layout |
| Machine modularity | Low – all wiring tied to central cabinet | High – zones can be added or modified easily |
The Modularity Advantage Nobody Talks About
The wiring efficiency of decentralized I/O gets most of the attention, but there is another benefit that matters just as much in the packaging industry: modularity. Packaging machines are rarely static. Customers add stations, change formats, upgrade filling heads, add vision systems. In a centralized architecture, every change requires running new cables back to the cabinet, potentially rerouting existing bundles, and adding new I/O modules. It is disruptive, time-consuming, and expensive.
In a decentralized architecture, adding a new station means adding a new I/O module with power and network cables connected to a nearby I/O module. The new station is self-contained. Its wiring is local. The impact on the rest of the machine is minimal. This is not just a convenience for the machine builder. It is a genuine competitive advantage when selling to customers who know their production requirements will evolve.
The same logic applies to machine reuse. Many OEMs build similar machines repeatedly, with variations for different customers. In a centralized architecture, each variation requires rerouting and re-terminating the wiring that changes. In a decentralized architecture, a modular zone can be added, removed, or reconfigured without touching the rest of the machine. The build team works on the new zone in isolation, connects it to the network, and the PLC sees a new set of I/O points. Clean, fast, and repeatable.
What About the Network?
The most common objection to decentralized I/O is network reliability. If all your I/O is connected via Ethernet/IP, ProfiNet, or EtherCat, what happens when the network has a problem? It is a fair question, and it deserves a straight answer.
Modern industrial Ethernet protocols, including PROFINET, EtherNet/IP, and EtherCAT, are designed for exactly this environment. They run on dedicated networks, separate from office IT infrastructure, with deterministic timing and built-in diagnostics. A properly designed industrial network is more reliable than a properly designed terminal block installation, not less. The difference is that when a network fault occurs, the diagnostics tell you exactly where it is. When a terminal block connection works loose, you find out by chasing signals through a cabinet for two hours.
The key term here is “properly designed.” A decentralized I/O system requires a properly specified network: the right cable type, the right connectors (M12 D-code for 100Mbit, M12 X-code for Gigabit), correct cable routing away from power cables, and proper grounding and shielding. None of this is complicated, but it does need to be specified correctly at the design stage rather than improvised during build.
The network is not the weak point in a decentralized architecture. The weak point is always the connectors. Specify industrial-grade M12 connectors, use factory-made overmolded shielded cables, and your network will outlast the machine it serves.
Getting Started: You Do Not Have to Change Everything at Once
The transition from centralized to decentralized I/O does not have to be a wholesale redesign of how you build machines. The most practical approach is to start with the next new machine design and apply decentralized I/O to the zones that have the highest I/O density or the longest cable runs. Measure the build time and startup time against your previous centralized design. The results will make the case for the next machine more persuasively than any technical argument.
A few practical starting points worth considering. First, map your current machine designs and identify the zones with the most I/O points and the longest cable runs to the cabinet. Those are the zones where decentralized I/O will deliver the most immediate benefit. Second, choose a network protocol that your PLC platform supports natively and that your team already knows. The goal is to reduce complexity, not introduce a new learning curve. Third, specify the I/O modules and connectors correctly from the start. The savings from decentralized I/O evaporate quickly if the network is built with a cheap switch, RJ45 connectors or office-grade network cables without adequate shielding.
The packaging industry is moving in this direction, whether individual OEMs choose to lead or follow. The machines that win on the floor are the ones that start up fast, run reliably, and are easy to maintain. Decentralized I/O delivers all three. The question is not whether to make the change. It is how quickly you want to start seeing the benefits.
The Architecture Decision That Changes Everything
The way a machine is wired is not just a technical detail. It is a fundamental design decision that affects build cost, startup time, field reliability, and the ease with which customers can maintain and expand the machine over its working life. Centralized wiring made sense when the alternatives were limited. Those limitations no longer exist.
Decentralized I/O is not a new idea. It has been available and proven for more than two decades. What is new is the cost and accessibility of the hardware, the maturity of the network protocols, and the growing pressure from customers who expect machines to start up faster and run more reliably than they did ten years ago.
If you are still designing machines around a large central cabinet with long cable runs to every device, you are carrying a cost that your competitors are starting to eliminate. The Connectivity Colin Workshop is a short, on-site session designed to help you understand exactly where that cost is showing up in your current machine designs and what a practical transition to a cleaner architecture would look like for your specific products and customers.
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