In the world of industrial control and automation, the term I/O, which stands for input/output, is used frequently. To navigate the industry successfully, it’s important to have a strong comprehension of what I/O means. Currently, there are two primary types of input/output: local I/O and remote I/O. When it comes to an industrial setting, the latter is the more regularly used. To gain a better understanding of the basics of remote I/O so that you can determine if this system will benefit your specific application, consult this helpful guide.
In order to understand what remote I/O is, you must first be familiar with I/O in general. Essentially, I/O simply refers to a device that transfers data to or from a computer. Such devices ultimately play a critical role in the monitoring and utilization of many critical devices and instruments in the industrial control and automation industry.
Now that you know what I/O means, let's talk about one of its primary subsets—remote I/O. Also known as distributed I/O, remote I/O refers to an input/output device that operates in a remote location at a distance from a programmable logic controller (PLC). For example, the PLC may be located in a safe space closer to a control room, while the I/O system is placed closer to the actual instrumentation being monitored and controlled, such as sensors and actuators. Ultimately, remote I/O allows I/O technology to be utilized more flexibly.
For a remote I/O system to work, it relies on a programmable logic controller. The PLC can process multiple points of data, including inputs and outputs. As such, these devices are capable of reading and receiving data to and from the I/O sections of the PLC device.
Such sections may range from separate I/O cards like digital input cards to fully integrated sections that have been joined with the PLC’s hardware. In any case, the PLC is the device that reads and interprets the data, not the remote I/O system. In addition, the PLC is also responsible for sending commands out while the remote I/O simply acts as a pathway for the data to get to the PLC. Thus, in many I/O setups, the PLC will be located in a completely separate location from the I/O cards or hardware.
Regardless of location, the PLC must be able to receive the data collected by the I/O hardware. To allow the remote I/O portion of the hardware and the PLC to send and receive substantial quantities of information, some type of Ethernet protocol or other specialized transmission technology is typically used. Essentially, the remote I/O device will use an adapter module connected to the backplane in the PLC rack to speak to the PLC.
When it comes to understanding the basics of remote I/O, it’s important to address the benefits that such systems can provide. Here are some of the key advantages of remote I/O:
● Less Wiring: One of the main advantages of a remote I/O system is the ability to substantially reduce the amount of wiring that needs to be run. As you can imagine, locating an I/O close to a PLC in a situation where the PLC cannot be close to the devices and instruments being monitored and controlled, a significant amount of wiring is necessary. By having the option to locate the remote I/O away from the PLC and close to the devices and instruments, however, far less wiring is used.
● More Diverse Hardware Confirguration Options:Due to the flexibility that a remote I/O setup provides, a greater variety of hardware configurations can be created. As such, you will have more freedom to choose the setup that works most advantageously for your specific network.
● Facilitating Commmunication in Harsh Environments: In some cases, a PLC simply can’t be placed next to the field devices that need to be monitored and controlled due to harsh environmental conditions.
In instances of extreme temperatures, severe weather, strong vibration, or other harsh conditions, the PLC can be placed in a safe space—often in or near a control room—while the I/O system is located more closely to the instrumentation. Ultimately, the remote I/O will help facilitate communication with the PLC from a safe distance.
Potential Disadvantages of Remote I/O Systems: While remote I/O systems have the potential to save you a considerable amount of money by reducing wiring needs, certain hidden costs can emerge if you aren’t careful. For example, users should know that when implementing remote I/O in a system, they will need to configure all of the additional I/O modules. This can take a considerable amount of time in a large system when even one module in the system is changed. Thus, you must reconfigure all of the other modules as well. By being mindful of how you plan on implementing the remote I/O modules in your system, you can avoid any unexpected headaches and costs.
While remote I/O systems offer many noteworthy benefits, they also pose a few disadvantages that you should be aware of. Some of the key drawbacks of I/O systems include:
● Vulnerability: All of the signals being communicated in a remote I/O system rely on a single point of communication between the PLC and the remote I/O portion of the hardware. If this point becomes interrupted or lost, all of the data being received could get lost. In addition, you would likely also lose the ability to control the hardware. To minimize this risk, it is important to implement redundant communication channels within a remote I/O system.
● Potential Hidden Costs:While remote I/O systems have the potential to save you a considerable amount of money by reducing wiring needs, certain hidden costs can emerge if you aren’t careful. For example, users should know that when implementing remote I/O in a system, they will need to configure all of the additional I/O modules. This can take a considerable amount of time in a large system when even one module in the system is changed. Thus, you must reconfigure all of the other modules as well. By being mindful of how you plan on implementing the remote I/O modules in your system, you can avoid any unexpected headaches and costs
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Input/output (I/O) devices have advanced rapidly in recent years; the differentiators are blurring the lines that separated them. This feature originally appeared in InTech Focus: Control Systems 2022, the InTech Focus ebook for September 2022.
Process and factory automation controllers connect to sensors, instruments, valves and other equipment through input/output (I/O) cards or racks that are either collocated within the same cabinet (local) at the controller/CPU or installed farther away (remote). Defining the difference between local and remote I/O is straightforward, but the defining differences between remote I/O and distributed I/O can have their nuances and are further confounded by each vendor’s definitions or marketing collateral, just as many automation vendors prefer a name like process automation controller (PAC) over programmable logic controller (PLC).
The most used I/O type is local I/O. It is most often from the same vendor as the controller/CPU, since it is ordinarily directly connected to the controller/CPU by integrated racks or cages that hold 4-, 8-, 16- or 32-point I/O cards. Some local I/O expansion racks, or bricks, as they are often referred to, can be separate from the main CPU and connected over a digital bus or highway via twisted pair wires or Ethernet cables, albeit installed within the same physical cabinet. Since local I/O is typically intended to be installed within the same enclosure as the controller/CPU, environmental operational characteristics and hazardous area approvals are not as robust as remotely installed I/O.
Next comes the challenge of outlining the different characteristics between remote I/O and distributed I/O, especially since many vendors refer to them interchangeably. If we use history and release to market timelines as a basis, remote I/O was first and thus less flexible, capable and smart as compared to later released distributed I/O. Initially, remote I/O was nothing more than local I/O reconfigured and housed to be remotely installed from its corresponding controller/CPU (Figure 1).
Figure 1: Remote I/O was local I/O reconfigured and housed to be remotely installed from its corresponding controller/CPU.
Communication was no longer along a backplane but now designed to convert its connected analog I/O signals to a digital format that was transmitted back to the host controller over proprietary buses or highways. Remote I/O is limited in scope and does not contain a complex or advanced CPU or processor to handle math, complex control, and peer-to-peer communication with other remote I/O modules, or allow the connection of additional I/O modules. While operational characteristics and hazardous approvals exceed that of most Local I/O products, it is still somewhat more limited than its advanced cousin, distributed I/O.
Distributed I/O characteristics Distributed I/O harnesses all the capabilities of remote I/O and more. It is likely, but impossible to determine, that distributed I/O derived its name from “distributed control.” As the name suggests, distributed I/O is a more complex piece of equipment that can be distributed throughout a process plant or automation facility without the concern of continuous communication with its host controller/CPU. This is because most distributed I/O products contain an advanced CPU and often real-time operation system that allows localized control and monitoring, along with several other capabilities.
Since distributed I/O (Figure 2) is designed to exist on its own, it can be a preferred choice of remote I/O due to its ability to be redundant or fault tolerant should it lose communication with a primary controller/CPU. In addition, distributed I/O and its advanced capabilities can share signals between other peer distributed I/O systems from the same vendor or alternative vendors using industry open or standardized protocols such as MODBUS RTU. Designed to be used for remote installations, distributed I/O typically allows for installation in more harsh operating environments and often has a minimum of Class I, Div. 2/Zone 2 approvals and sometimes carry Zone 0/1 approvals.
Figure 2: Distributed I/O can share signals between other peer distributed I/O systems from the same vendor or alternative vendors using industry open or standardized protocols.
With its diverse capabilities, distributed I/O can be well suited to not only be a local controller and I/O device, but it can also have additional built-in capabilities to support peer-to-peer communications, gateway functions such as HART to MODBUS RTU or MODBUS/ TCP conversion, embedded webservers for ease of viewing real time process data with an off the shelf web browser, data logging, and even Industrial Internet of Things (IIoT) or Industry 4.0 capabilities employing message queuing telemetry transport (MQTT) protocol for seamless connections to cloud services like Amazon Web Services (AWS) or Microsoft Azure.
Recent advances in secure spread spectrum, long range, and mesh wireless telemetry have further enabled I/O products to provide solutions once thought impossible. WirelessHART, ISA 100 and many proprietary short- and long-range unlicensed solutions are now optionally embedded directly within the I/O product itself, spawning an entirely new category of remote or distributed type of I/O solutions.
Regardless of type, I/O products have advanced incredibly fast during the last decade. So much so that several of the abovementioned differentiators have blurred the once defined lines that separated them. For the user, it is imperative that each vendor’s solution and technology offering be thoroughly examined to ensure functional, operational, and design compliance.
This feature originally appeared in InTech Focus: Control Systems 2022, the InTech Focus ebook for September 2022.
Paul Harris is the sales and operations manager of Moore Industries Europe Inc. Before being named SAOM in 2019, Harris held various roles within the company, rising from junior test engineer through to the position of export sales manager. Joining Moore Industries in 1983, he quickly became a progressor who could span a breadth of process control instrumentation technologies and is now accountable for implementing the company’s policies while directing local strategies to enhance the overall growth of the EMEA regions.
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