Connect I/O systems with the IIoT

Traditional wired input/output (I/O) connections are crucial for most automation systems. The latest generation of I/O systems bridges the gap to bring these connections into the Industrial Internet of Things (IIoT).

By Josh Eastburn October 3, 2019

Industrial automation applications are benefitting from the growing availability of smart devices. These intelligent components, also known as Industrial Internet of Things (IIoT) devices, range in capabilities and sizes from small, individual sensors to larger, packaged control systems. One thing they have in common is the ability to communicate extensive information via digital communication links.

Based on the conversation around IIoT, one might get the impression these smart devices will all be wireless. The reality, however, is traditional wired inputs and outputs (I/O) will continue to be in demand as a fundamental automation need. Many devices such as relays, solenoids, switches and transmitters still make sense as basic components with simple functionality. This is especially the case for retrofit situations, but it also applies to new installations.

Automation I/O systems take on new importance, however, when they consolidate basic signals to make many existing hard-wired I/O points look like fewer IIoT devices. This consolidation is crucial for reining in the volume of data the IIoT produces and use classic wired signals in a modern networking architecture for IIoT applications. Organizations can use these I/O systems to attain their digital transformation and IIoT goals.

I/O system basics

Traditional wired I/O connections are an established technology going back many decades.

I/O points are usually referred to as discrete (on/off signals) or analog (signals varying over a range), whether they are an input to an automation controller or an output commanded by the controller. The terms discrete input (DI), discrete output (DO), analog input (AI) and analog output (AO) describe traditional I/O.

In the early years, one I/O module contained a single input or output channel, and several single-channel inputs or outputs could be installed together on a chassis. Later, manufacturers increased the density of I/O modules, so up to 32 channels may be contained in one module (Figure 1).

Each DI/DO/AI/AO channel on an I/O system module is rated for certain voltage and current characteristics. Modules can be distributed or standalone, expandable or rack-based. Field wiring connections were traditionally made using screw-clamp terminals, although many users are transitioning to spring-clamp terminals for easier wiring and vibration resistance. Most I/O modules must be installed within a protected environment, but some are connectorized for use in classified and other demanding areas.

Flexibility is key

With many options to choose from, users often mix and match I/O types to best fit their application. Some specific features make it easier than ever for users to connect, configure and power their I/O systems.

Form factor

From a physical standpoint, I/O systems with compact form factors are easier to integrate into a system. Compact size can be achieved through higher I/O density, or by designing narrow-width modules. Some trade-offs need to be considered, however, since reduced size constrains permissible wire gauge and may make diagnostics more difficult. Features such as removable terminal block and wiring arm systems also allow for quicker assembly and replacement of modules (Figure 2).

Signal options

Most I/O systems offer signal levels at 24 VDC or 120 VAC for discrete points, and 4-20 mA or 0-10 VDC for analog points. Relay contact DOs also are common. Thermocouple (TC), resistance temperature detector (RTD) and integrated circuit temperature detector (ICTD) are specialized versions of AIs. They’re often used to provide high input density and to avoid the need for separate temperature transmitters. Some manufacturers offer more specialized I/O, including rate (Hz), resistance (ohms), or millivolts (mVs).

Mixed and multifunction I/O

Typically, all the channels in one module are alike in basic format—all DI, all AO, etc. Some newer systems, however, offer modules containing discrete inputs and discrete outputs, or a mix of all four basic types. In the past, the variety of typical signal levels has required designers to plan for I/O allocations to match field instruments, made difficult because field device design details may not be defined until later in a project.

To address this issue, many I/O system suppliers offer multifunction I/O modules that accept related types of signals on the same terminal points, using software-based configuration to configure specific characteristics for each. These modules may be more expensive, but they also simplify initial design efforts and provide flexibility for future changes.

Power distribution

Related to the discussion of I/O signal levels is how those signals are powered. Usually, loop power is either sourced from the I/O location or sinking from the field. However, designers need to evaluate the I/O module isolation characteristics to ensure there are no constraints on how to wire I/O points sourced from various locations.

Today, many I/O systems use open Ethernet protocols instead of traditional and proprietary industrial fieldbuses. This means some of these I/O systems can leverage commercial power over Ethernet (PoE) technology to operate remote I/O, and even to power loops. Although the available power is limited, PoE can allow I/O systems to be installed without dedicated power supplies in some applications.

Software-based configuration

A final key I/O system feature is software-based configuration of I/O modules and points. This attribute lets users adjust, view and document I/O ranges and features. A well-designed user interface also provides useful diagnostics. If the system includes a web-based interface, then this information can even be accessed on a user’s PC or mobile device.

Going the distance

Determining the communications link from the I/O module up to a monitoring or control system is just as important. A communications adapter is sometimes needed to enable the I/O modules to communicate with a supervisory system.

Regardless, I/O supervisory communications is another area where technology improvements are making things easier for designers.

Two concepts are important. The first is knowing the difference between local and remote I/O; the second is understanding the scope of communication possible for each I/O channel. Local I/O is directly connected with a controller, or quite close, often using a proprietary bus or serial connection. In contrast, remote I/O can be located anywhere in relation to the rest of the system and is connected using one of many networking or fieldbus technologies.

The earliest iterations of I/O systems consisted of I/O adapters and modules that could only be interfaced with the controller (or PC) that mastered them, whether they were local or remote.

Standard Ethernet networking was introduced for industrial I/O in the late 1990s; today it is common. While it has some distance limitations compared with traditional industrial I/O fieldbuses, Ethernet using industrial communications protocols such as Modbus/TCP, EtherNet/IP, or Profinet has proven to be a capable and reliable I/O communications method.

Modern network methods offer other benefits for I/O as well. First, Ethernet is a well-understood, high-speed standard. It’s also the basis for corporate computer systems, facilitating interoperability. I/O adapters on Ethernet also can act as peers, publishing their data to any number of devices.

I/O gets upgraded

Because modern I/O systems can be networked using standard Ethernet and are not limited to master-slave communications, some new architectural possibilities are available that bridge the gap between traditional wired and smart wireless, and between I/O and IIoT.

One of the biggest costs in a medium- to large-scale project is field wiring, especially when wireless options aren’t feasible. The latest generation of remote I/O systems reduces the cost of integrating far-flung devices, replacing wiring with a single Ethernet cable between controller and remote node (Figure 3).

These systems also pair I/O control with embedded IT technologies to transform remote slaves into distributed data nodes. Independent of the master, remote I/O modules can, for example, communicate with a messaging queuing telemetry transport (MQTT) broker to enable peer-to-peer connectivity within the field. Pairing this technology means field data can be sent to cloud services or databases, integrating the furthest edge of the network into the IIoT.

I/O systems in the age of IIoT

Even with the proliferation of IIoT devices and intelligent field equipment, there is still demand in new and legacy installations for monitoring and controlling conventional wired I/O points. Traditionally, they would be connected to an I/O system mastered by a controller, and newer I/O systems offer flexible features to ease design, installation, and maintenance, saving time and money.

The latest generation of I/O systems goes even further, offering greater connectivity through Ethernet networks to peers, other devices and software systems, instead of being bound to only one master. This new I/O will make it possible to create fully IIoT-capable automation systems.

Josh Eastburn, director of technical marketing, Opto 22. Edited by Chris Vavra, production editor, Control Engineering, CFE Media,


Keywords: I/O systems, I/O modules, Industrial Internet of Things (IIoT)

Input/output (I/O) applications benefit from the rise of the Industrial Internet of Things (IIoT).

Users have many options to choose from and can mix and match I/O types to best fit their applications.

The latest generation of I/O systems offers greater connectivity through Ethernet networks.

Consider this

What other benefits can users gain from the IIoT connecting to I/O devices?

Author Bio: Josh Eastburn is director of technical marketing, Opto 22. After 12 years as an automation engineer working in the semiconductor, petrochemical, food and beverage, and life sciences industries, Eastburn works with the engineers at Opto 22 to understand the needs of tomorrow's customers. He is a contributing writer at