Chapter Goal: To understand the concept and powerful benefits of decentralized I/O. You will learn what a remote I/O station is, how it communicates with the PLC over PROFINET, and why it is the modern standard for wiring large and complex machinery.
For many years, the standard design practice was centralized I/O. This meant a single, large control cabinet contained the PLC and every single I/O module for the entire machine.
The Wiring Reality:
Every sensor, switch, and button on the machine, no matter how far away, required its own multi-core cable to be pulled all the way back to this one central cabinet.
On a 20-meter-long packaging line, a photoelectric sensor at the far end would need a 20-meter cable. A pneumatic valve next to it would need another 20-meter cable. This resulted in huge, heavy bundles of hundreds of cables (often called "homeruns") that had to be run through expensive cable trays along the entire length of the machine.
The Major Drawbacks:
Astronomical Cost: The sheer quantity of copper wire and the immense amount of labor required to pull, terminate, and label hundreds of long cables is a massive project expense.
Difficult Commissioning & Troubleshooting: Fault-finding is a slow, two-person job. One technician stands at the sensor 20 meters away while another is at the cabinet trying to identify the correct wire. It's inefficient and leads to extended downtime. 😩
Increased Potential for Failure: Every termination point is a potential loose connection. Long cable runs are also more susceptible to physical damage and electrical noise from nearby power cables.
The modern solution is simple and elegant: instead of bringing all the wires to the PLC, bring the PLC's I/O to the wires. This is the principle of decentralized or distributed I/O.
What is Remote I/O?
A Remote I/O (RIO) station is a compact assembly of I/O modules that is installed directly on the machine, right next to the group of sensors and actuators it needs to serve.
It functions as a local hub for inputs and outputs. It has no processing power of its own; it is simply an extension of the main PLC's brain.
The Siemens ET 200SP is an industry-leading example of a modular RIO system, allowing you to build the exact combination of I/O needed at any point on the machine.
The magic of remote I/O is enabled by the industrial network.
The Communication Link: The RIO station is a PROFINET IO Device. It connects back to the main PLC (the IO Controller) using a single green PROFINET cable.
How it Works in Practice:
The local sensors (e.g., proximity and photoelectric) are wired to the input modules on the RIO station using short, simple cables.
The RIO station continuously scans these inputs and bundles their statuses (ON/OFF) into a digital data packet.
This packet is sent over the single PROFINET cable to the main PLC.
The PLC executes its logic and sends a return data packet containing the required output statuses for that station.
The RIO station receives this packet and energizes its output modules, controlling the local actuators (e.g., solenoid valves).
This entire two-way communication happens in less than a millisecond, making the process completely seamless. From the programmer's perspective, the remote I/O behaves exactly as if it were plugged directly into the main PLC rack.
1. Massive Wiring Reduction: This is the most obvious and impactful benefit.
Before: 50 devices at the end of a machine required 50 long, expensive homerun cables.
Now: Those same 50 devices require 50 short, inexpensive local cables and one long PROFINET network cable. This can reduce wiring costs by over 60%.
2. Faster Commissioning and Troubleshooting: A technician can stand at one spot on the machine and see both the physical device and the corresponding status LED on the RIO module. This makes wiring checks and fault-finding incredibly fast and efficient. ✅
3. Improved Reliability: Short sensor cables are much less prone to picking up electrical noise from nearby motor cables, which leads to a more stable and reliable system with fewer false signals.
4. "Plug and Play" Modularity: Machines can be built and tested in smaller, self-contained sections, each with its own RIO station. During final assembly, the sections are simply bolted together, and the PROFINET cables are linked. This "modular manufacturing" approach drastically speeds up production and assembly time for machine builders.
In summary, using Remote I/O over an industrial network is the definitive modern standard for creating automation systems that are more cost-effective, reliable, and easier to maintain.