A Stronger Link for I/O

Summary

In the world of product and service delivery, there is a concept known as the “last mile problem.” This refers to the difficulty and inefficiency associated with the last link in a process, such as delivering a package to an end customer’s door, or running internet cables from a street junction box the final distance to a home. Industrial automation systems experience their own last mile issues, involving connectivity from localized controllers and remote input/output (I/O) modules to the many instruments, sensors, and other field devices.
 
Depending on the field device and the environment, users may need to design and install a significant amount of conduit, wiring, cabling, and other provisions to make these connections, often in a challenging environment. And each signal of interest has traditionally required its own wiring connection.
 
For some advanced field devices, such as intelligent transmitters and variable frequency drives (VFDs), there are more sophisticated options in the form of digital serial communications, industrial fieldbuses, and even Ethernet. However, for the multitude of basic field sensing and actuating devices, this has not been the case until recent years. Today, the IO-Link protocol is making it easy and economical to establish advanced connectivity with a wide variety of field devices, so users can maximize the available information, resulting performance, and maintainability of their systems.
 

A two-way information highway

Hardwired I/O signals come in four basic types: discrete input, discrete output, analog input, and analog output. Discrete signals are on/off, while analog signals vary over a range. These are uni-directional (either inputs from the field sensor to the I/O module, or outputs from the I/O module to the commanded field device). While they can be wired and installed to run great distances, there is of course a cost for conduit and wire, so users try to design I/O modules as close to the field devices as practical.
 
Digital signaling methods, on the other hand, are attractive because they can convey much more information—in the form of data packets—over a single cable. However, this type of networking requires more design effort, relatively complex installations, distance constraints, specialized field devices, and consideration of issues like electrical noise. For many years, it just did not make sense to use digital networks for basic signals and devices.
 
This has changed in recent years with the development of IO-Link, as specified in IEC 61131-9. IO-Link provides a cost-effective yet smart standard, able to provide point-to-point bi-directional communications over reasonable distances of up to 20 meters using standard unshielded three-wire cable at normal 24V DC levels. In many ways, IO-Link acts in much the same way that standardized USB connectivity for PCs made it much easier to connect a multitude of devices like keyboards, printers, and cameras.
 
Although IO-Link is not ultimately as capable as other fieldbuses, it is a fit-for-purpose technology that standardizes wiring—and can even re-use existing wiring—providing a reliable and rapid way to transmit up to 32 bytes of data with a field device. The device data is typically some combination of:

• Process value and status: discrete or analog, input or output.
• Configuration: the settings of the field device.
• Diagnostics: extended information to help users monitor and troubleshoot devices.

Process data is delivered cyclically on a defined schedule, while other data is transmitted asynchronously as needed. Although IO-Link provides a wealth of capability, suppliers can economically build this protocol into a wide variety of field devices. Next, we will look at some device characteristics and supporting architectures.

IO-Link architecture

Because IO-Link is a three-wire technology, the cable can supply power to each field device and handle signaling. This is important because many sensor types—such as proximity and photoeye sensors—require power to operate, provide local indication, and perform the communications aspect at the sensor. While some sensors are offered with dedicated IO-Link connectivity, many come as standard three-wire discrete or analog devices, which can operate in IO-Link mode if desired. Note that these sensors operate either in traditional hardwired mode, or in IO-Link mode, but typically not both at the same time. IO-Link sensors can provide additional functionality, such as local indication/displays and detection counting, and even discrete devices can provide analog-style signaling.
 
A typical architecture uses “master modules” and may use “hubs” (Figure 1). Both devices are available in IP67-rated potted housings, with key-coded M12 connectors, so users can install them openly on machines wherever they fit, without requiring an enclosure (Figure 2). Field devices will detect if they are attached to an IO-Link master or not, and operate in SIO (standard input/output) or IO-Link mode accordingly.

• Eight I/O connectors which can support up to 16 discrete inputs or outputs, 8 IO-Link devices, or a combination.
• Power in and out connectors, so 24V DC can be easily daisy-chained through multiple modules.
• Network in and out connectors, for establishing the 100Mbps Ethernet connection between the host system and one or more master modules.

For simple field I/O, a hub is an economical way to convert an IO-Link connection from a master module to standard I/O signals. Because IO-Link is a standard, it is possible to mix-and-match modules and field sensors, so users can choose the devices that work best for their application.
 
Using IO-Link architecture, OEMs and automation designers have the freedom to place a primary programmable logic controller (PLC) in a convenient location, and then route Ethernet and 24V DC cabling to master modules and hubs located near end devices. The final connection is a plug-and-cord, resulting in simple initial installation and future servicing.
 

Easy integration

Networking an IO-Link system to a main PLC is easy using the standard EtherNet/IP industrial communications messaging protocol. More advanced “premium” modules support additional protocols, including OPC UA and MQTT for industrial internet of things (IIoT) applications, which enable direct connectivity between field sensors and the cloud without even requiring a local PLC. This architecture is a good choice for data monitoring systems.
 
Although IO-Link is a standard, users should still investigate to ensure products are supplied with thorough electronic data sheet (EDS) files and documentation, and consider choosing PLC platforms with corresponding provision. When a modern PLC is used to integrate IO-Link devices supplied with proper EDS files, the tasks of configurating and monitoring are greatly simplified (Figure 3).

IO-Link expands options

IO-Link makes new types of field devices possible. For example, while most users are familiar with standard pushbuttons and indicator lights, there is a new style of IO-Link device with an integrated display consisting of a touch button with a central 4-digit display, surrounded by 16 multi-color LED segments. A controller can command all the display elements while monitoring the button. This device can be used at a warehouse picking station, where a worker is prompted by the colors and display to pick the right parts, and pushes the button when complete. Once small device takes the place of many and avoids the need for a full-size human-machine interface.
 
Connection and configuration flexibility is another benefit. A brewery expanded operations with four new fermenters, installing temperature and pressure sensors on the new tanks and interfacing them to the existing control system while only needing to install an IO-Link master and run two M12 cables for each new fermenter. Installation and startup were executed rapidly, and in the future any failed sensor can be replaced and it will automatically restore all configuration parameters to the new device. The brewery can maintain fewer spares, and self-perform maintenance without engaging an integrator or even needing a PC.
 

A smarter I/O system

Traditional I/O wiring and connectivity methods are well understood, and OEMs and systems designers will continue to carry these in their automation toolkits. However, IO-Link has rapidly become the right-sized approach for providing advanced communications options, even among field devices that have historically been simple on/off affairs.
 
Standardization empowers users to select the best products for their needs. With IO-Link, they can gather valuable diagnostics and extended information from field sensors, and easily create IIoT applications. The result is streamlined installations, improved operational performance and reduced downtime.