The Differences Between Local and Remote I/O

To learn more about industrial I/O options and the best way to deploy them for different applications, we connected with Bryan Little of system integrator Avanceon to learn more for a recent episode of the “Automation World Gets Your Questions Answered” podcast.

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We began our discussion with an explanation of the different industrial I/O types. According to Little:

• Local I/O refers to I/O modules located in the same rack or chassis as the controller and typically lack independent computing power due to their proximity to the controller.
• Distributed or remote I/O is generally located separately from the main controller. These systems often include onboard computing power for data processing and can manage outputs independently of the main processor.

Given that remote or distributed I/O modules are typically not located in the main electrical enclosure, readers questioned the feasibility of using remote I/O in the main control enclosure. According to Little, this question is more valid than it might seem.

Reasons to put remote I/O in the main enclosure

Little points out the numerous advantages of placing remote I/O modules in the main control cabinet, citing improved safety, reduced wiring, and standardization as three key benefits.

Summary

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. However, the distinctions between remote I/O and distributed I/O can be more nuanced, often complicated by each vendor’s definitions or marketing. Like how many automation vendors prefer the term "process automation controller (PAC)" over "programmable logic controller (PLC)."

The most commonly used I/O type is local I/O. Typically sourced from the same vendor as the controller/CPU, it connects directly to the controller/CPU via integrated racks or cages housing 4-, 8-, 16-, or 32-point I/O cards. Some local I/O expansion racks, or "bricks," can be separate from the main CPU, connected over a digital bus or Ethernet cables but still within the same physical cabinet. Since local I/O is intended to be in the same enclosure as the controller/CPU, it usually lacks the robust environmental and hazardous area approvals of remotely installed I/O.

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Remote I/O Characteristics

Understanding the characteristics of remote I/O versus distributed I/O can be challenging, especially since many vendors use the terms interchangeably. Historically, remote I/O was the first to appear and was simpler and less capable than distributed I/O, which came later. Initially, remote I/O was merely a reconfigured version of local I/O, designed to be installed away from its corresponding controller/CPU (Figure 1).

Figure 1: Remote I/O was local I/O reconfigured for remote installation from its corresponding controller/CPU.
Communication switched from a backplane setup to converting connected analog I/O signals into a digital format, transmitted back to the controller over proprietary buses or highways. Remote I/O lacks a complex CPU or processor for advanced tasks like math or peer-to-peer communication with other modules, limiting its scope compared to distributed I/O. Although it surpasses local I/O in operational characteristics and hazardous approvals, remote I/O remains more limited than distributed I/O.

Distributed I/O Characteristics

Distributed I/O expands on the capabilities of remote I/O and more. Likely named after “distributed control,” distributed I/O systems are more complex and can function independently within a process plant or automation facility, maintaining localized control without continuous communication with a host controller/CPU.

Figure 2: Distributed I/O shares signals with peer distributed I/O systems from the same or different vendors using standardized protocols.
Due to its advanced design, distributed I/O may be redundant or fault-tolerant if it loses communication with a primary controller. It can share signals between peer systems using industry-standard protocols like MODBUS RTU. Often designed for harsh environments, distributed I/O usually has at least Class I, Div. 2/Zone 2 approvals and sometimes Zone 0/1 approvals.

With its robust capabilities, distributed I/O can serve as a local controller and I/O device, supporting peer-to-peer communications, gateway functions, embedded web servers, data logging, and Industrial Internet of Things (IIoT) capabilities using MQTT protocol for seamless cloud service integration like AWS or Microsoft Azure.

Looking Ahead

Recent advancements in secure spread spectrum, long-range, and mesh wireless telemetry have enabled I/O products to offer solutions once thought impossible. Technologies like WirelessHART and ISA 100, along with proprietary solutions, are now embedded directly within I/O products, creating new categories of remote and distributed I/O solutions.

Regardless of the type, I/O products have advanced rapidly over the last decade, blurring the once-clear lines separating them. For users, it is crucial to thoroughly examine each vendor’s offerings to ensure functionality, operational efficiency, and design compliance.

This feature originally appeared in InTech Focus: Control Systems 2022, the InTech Focus ebook for September 2022.

About The Author

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 to export sales manager. Joining Moore Industries in 1983, he has accumulated extensive expertise in process control instrumentation technologies and currently oversees the company’s policies and regional growth strategies in the EMEA regions.

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