Industrial I/O (input/output) modules are key industrial network components that transmit input signals from field devices (such as sensors or actuators) to the controller. From the controller, output commands are routed back to the device via these modules. Previously, I/O was only connected to the programmable logic controller (PLC) component; that is, the CPU, or by adding expansion modules to the CPU. As time went by and industrial automation changed dramatically, there were cases where some I/O was located in a very remote location from the PLC; and expansion modules were constantly added due to business and operational needs.
This greatly increased the burden of wires and made the overall system design more complex. At this time, there was a need for some method that could help us connect the I/O to a specific location and transmit the data to the PLC through some remote method. Therefore, remote I/O and distributed I/O came into being. Although this seems simple and straightforward, the decision of how to deploy I/O modules on an industrial network becomes more complicated when we consider the available remote and local I/O options.
If local I/O is like a person in a cabin, then with remote I/O, our technicians are now distributed on the other side of the room. Separate, independent, but still in control. Compared with local I/O, remote I/O solutions are known for their distance flexibility.
1. Types of I/O
To avoid confusion, here I will talk about the commonly used I/O types in PLC systems. There are basically three types of I/O in PLC systems – local I/O, remote I/O, and distributed I/O. I will describe the principles of the three types of I/O so that you can understand the difference.
In simple terms, local I/O refers to I/O modules that are located in the same rack or chassis as the controller and, due to their proximity to the controller, typically do not have any onboard computing capabilities.
Distributed or remote I/O is typically deployed in a different location from the main controller. Therefore, distributed I/O usually has some degree of onboard computing power to perform data processing, as well as the ability to turn outputs on and off independently of the main processor.
Local versus remote I/O can be distinguished easily. However, defining the difference between remote I/O and distributed I/O can be more subtle. Vendor definitions or marketing materials can further confuse them, just as many automation vendors prefer to use names such as process automation controller (PAC) rather than PLC.
1). Local I/O
Local I/O is the most commonly used type of I/O. It refers to input and output devices that share a direct connection with the main controller over a short distance, usually within the same control cabinet or very close to the main controller. It usually comes from the same supplier as the controller/CPU, because it is usually connected directly to the controller/CPU through an integrated rack or chassis that can accommodate 4, 8, 16, or 32-point I/O cards. Some local I/O expansion racks (commonly called “bricks”) can be separated from the main CPU and connected through a digital bus or high-speed channel via a twisted pair of Ethernet cables, although they are installed in the same physical cabinet.
Since local I/O is usually intended to be installed in the same cabinet as the controller/CPU, the environmental operating characteristics and hazardous area approvals are not as reliable as those of remotely mounted I/O. There are I/Os built into the CPU assembly, and if these I/Os are missing, expansion modules are connected to the CPU. This method is good if the I/O we already have is limited to locations near the electrical panel and the distance does not need to exceed a long distance. Of course, if they are out of range, then you can also connect them to the PLC. However, there is a high probability that the signal will be degraded or lost. And if we connect more long-distance I/O, it might mess up the system wiring we already have.
2). Remote I/O
Now, remote I/O can solve this problem. Imagine a scenario where N I/O clusters exist in specific locations far away from the main PLC (such as key factories in headquarters and branches). Here, these I/O groups are connected to remote racks. They will communicate with the main PLC through a network adapter. A network adapter is nothing more than a communication module that can use any available communication protocol (Ethernet IP, Modbus TCP IP, Profibus, Profinet, etc.).
This will reduce the overall wiring pressure and make the system more efficient. Data will continuously communicate with the main CPU in the central panel through this adapter. In a remote I/O system, the I/O network is the backbone that supports communication between the CPU and field devices. In essence, the network is the path for data transmission. The quality and type of the network directly affect the efficiency and reliability of the system. The I/O network ensures the safe and reliable transmission of data packets between different components. The network should be strong enough to handle high traffic loads and adverse operating conditions. It must guarantee low error rates and high reliability to prevent communication errors and data loss.
3). Distributed I/O
Now, let’s move on to a more advanced system – Distributed I/O. Basically, it is the same as remote I/O, the only difference is the CPU. It can be used if you want to write logic for that location/section and reduce the programming burden in the centralized PLC.
Although many people claim that remote and distributed I/O are extremely similar, distributor I/O is a more advanced system where the DSC has multiple “brains”, or what we call multiple distributed autonomous controllers. Each rack is a control system responsible for operating each designated parameter and is connected together through a central supervisory controller. There is a CPU inside each remote rack to prevent the pressure caused by the increase in total memory in the main PLC and to prevent lengthy programs. In actual application scenarios, many vendors use them interchangeably.
2. Local vs. Remote I/O
To get closer to our actual needs, let’s go back to the differences between local I/O and remote I/O. Understanding the differences between local and remote I/O modules will help us make a good comparison and selection based on the feasibility of the situation.
1) Distance
Local I/O devices are located near the main controller, limiting installation flexibility. Remote I/O, on the other hand, can be located away from the main controller, providing greater flexibility for large or complex facilities.
2) Wiring
Wiring local I/O directly to the controller can result in a confusing and complex wiring scheme, especially as the system grows. Remote I/O can be connected to the host controller using network cables or Fieldbus systems, significantly reducing the amount of wiring required over long distances and simplifying the overall system layout.
3) Reliability
The development of modern network communication technology ensures the high reliability of signal delay between the two.
4) Maintenance
Local I/O is easier to maintain because all devices are located in a centralized area, but as the system scales, maintenance can become cumbersome. The modularity of remote I/O makes troubleshooting and expansion easier.
5) Cost
Initial costs are lower when using local I/O. These savings diminish as the system grows in size and cabling requirements become more complex. Remote I/O provides the greatest value for large or distributed systems by reducing the need for long cable runs and simplifying future expansion.
3. Role of I/O modules
Let’s go back a few decades to the early days of computer control. The predecessor to DCS and PLC was the direct digital control or DDC system, which can only be seen in museums today. Increasing the rate and speed of data transmission and reducing the data to an extremely small size are now important goals. Today, many vendors have DDC systems centered around a microcomputer connected to multiple proprietary I/O devices located in remote locations around the factory.
The I/O module is the core of the remote I/O system. The main function of the module is to act as an intermediary between the CPU and peripheral devices. It supports the exchange of digital or analog input signals between the CPU and field devices.
Input and output operations can both be handled by a typical I/O module. This modular device converts digital signals from the CPU into electrical signals for output devices, and converts electrical signals from input modules into digital signals for the central processing unit. These conversions are necessary because field devices such as sensors and actuators typically process electrical signals, while the CPU communicates in digital signals.
The I/O module also helps manage the data traffic in the communication network. In this module, data from multiple peripheral devices is buffered before being sent to the CPU. This buffering operation helps manage the data flow and prevents data congestion in the system.
The design and complexity of I/O modules can vary depending on the specific needs of your system. Some modules may only process digital signals, while others can process analog signals. Some modules may have built-in data processing capabilities to pre-process the data before it reaches the CPU, while others simply let the data pass through. The choice of module depends on the specifications of the field devices and the requirements of the system.
4. Conclusion
Using local or remote I/O has its own advantages and disadvantages. Remote I/O can bring benefits such as security, less wiring, and standardization, which is quite cost-effective. Local I/O still plays an important role in fast response time and minimum signal delay. After all, there are also quite a few engineers who are usually skeptical about wireless reliability. Therefore, before choosing the appropriate I/O mode, you may want to refer to the above differences to determine the final solution.
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