Entering the Age of
Smart Distributed I/O

Photo 1. Smart I/O devices have an ever-present role in industrial monitoring and control applications. Incorporating intelligent functions-such as diagnostics, onboard data processing, control algorithms, and self-identification-smart I/O devices can also benefit users with a network interface for high-level communications with other devices and a PC.

here's a good chance you've heard that I/O devices are getting smarter. But more than likely, this information leaves you with some unanswered questions. For instance, what exactly is smart I/O? What are its real benefits? And is smarter always better? In this article, I answer these questions and help you determine whether smart I/O is the right choice for your future I/O and data acquisition (DA) systems (see Photo 1). Defining Smart I/O
Generally speaking, an I/O device can be considered smart if it performs such functions as diagnostics, onboard data processing, control algorithms, and self-identification. But the key functions of these devices is acquiring input signals and generating output signals to and from sensors, transducers, processes, and control elements (see Figure 1).

Figure 1. Smart I/O devices typically use standardized industrial networks to distribute analog and digital I/O functionality closer to sensors, actuators, and signal sources.

The I/O component of these devices can monitor sensor outputs and often includes signal conditioning for direct connection to sensing devices, such as thermocouples, RTDs, and strain gauges. Smart I/O devices can monitor digital or discrete inputs and generate analog and digital output signals for control. Given their broad functionality, these devices can perform DA, data logging, process monitoring and control, factory automation, production testing, and laboratory measurement and automation.

The core of the device is an onboard microprocessor or microcontroller, which provides the brains for its operation. The device must, therefore, have the necessary hardware to support the microprocessor, including RAM for program execution and data storage, flash memory for program storage, and EEPROM for storing configuration information. At least two of these forms of memory are required: RAM and either flash memory or EEPROM for program/firmware/ configuration data storage. Most devices, though, include all three because of technical tradeoffs. Flash memory is more inexpensive than EEPROM and good for storing programs that don't change. On the other hand, EEPROMs cost more but are good for storing things that change and frequently must be rewritten and stored (e.g., configuration data).

As shown in Figure 2, smart I/O devices usually include a network interface, visual display or LED indicators, and field signal I/O interfaces. Each I/O interface, or module, includes signal conditioning; I/O or DA circuitry, with appropriate A/D, D/A, or discrete I/O components; and an engine that manages and controls the local I/O operation.

Figure 2. The brains of the smart I/O device is an onboard microprocessor that handles network and communications tasks, as well as I/O processing tasks associated with the I/O module. In the modular system shown here, a local bus supplies efficient communications between the I/O modules and the microprocessor-based network module.

Higher Reliability, Less Downtime
One of the most important benefits of smart I/O is increased diagnostics at the sensor and device level. By identifying and reporting problems promptly, smart devices can minimize downtime and increase the overall reliability of a system. The devices gather and evaluate diagnostic information on system components, including the network, the device itself, and even the sensor or process.

For example, the device should monitor the communications interface for data integrity and network problems and routinely perform self-tests to identify malfunctioning components. Some smart devices can even sense and report problems with sensors and field wiring. For example, broken leads on thermocouples and RTDs can be detected and reported to a PC for immediate attention and service.

In addition to finding and reporting problems, many smart I/O modules can react autonomously to potential system failures. A smart device may use a watchdog timer to detect a loss of communications with the host PC. In this scenario, the device automatically sets its output into a known, safe state until communications are restored with the PC or controller.

Distributed Data Processing and Control
Microprocessors in smart I/O modules can process collected input data, relieving the PC of the computational tasks and delivering improved performance. A smart I/O device can digitize a thermocouple input; linearize and perform

cold junction compensation; average multiple readings to reduce measurement noise; scale the result to the desired engineering unit; make calibration corrections; compare the result to alarm limits; and use the result to calculate energy transfer. With networked smart I/O devices, this processing can be executed simultaneously in multiple smart I/O devices, improving the performance of the system (see Table 1).

The next step in the evolution of smart I/O devices is the execution of control and logic algorithms. Whether this involves a simple logic function, a single-loop PID, or a more complicated multivariable control algorithm, the distribution of the control to the I/O-device level can result in more reliable and deterministic performance. With this capability built into a modular I/O device, you can combine the inputs and outputs of different I/O modules in one control algorithm for more flexible configurations.

Delivering Plug and Play
Talk about ease of use of I/O, and you are talking about quick and easy configuration. In general, smart I/O should ease configuration considerably by implementing plug-and-play autoconfiguration. Some smart I/O devices, including smart sensors, implement electronic data sheets (EDS) in local, nonvolatile memory (e.g., EEPROM). The EDS describes the device with information necessary to configure and use the device, such as input ranges, calibration information, and data type (see Table 2). When powered on, the device transmits the EDS to the system controller, which uses the information to configure the device and its software or create a catalog for maintenance.

Network Connectivity
The trend toward smarter I/O and the development of digital industrial networks are closely intertwined. Devices that gather, generate, and report more information require an efficient means of communicating the information. Along these lines, the automation market is experiencing a major technological shift from point-to-point wiring and 4-20 mA communications to high-performance digital networks that reduce wiring costs and support intelligent devices. As the performance-price ratio of these network options improves, more DA applications are moving to a networked topology.

Which networks are best for smart I/O devices? There are many networks in use today targeted at different levels of applications, ranging from low-level systems for simple discrete sensors to sophisticated networks for distributing control with highly intelligent devices. Although many smart I/O devices employ a proprietary network scheme, more and more systems are moving toward one of the open, standardized networks now available.

One example of an industrial network designed for intelligent I/O devices is Foundation Fieldbus, a network based on existing standards, including ISA/IEC, PROFIBUS, FIP, and HART (a widely used open protocol in industrial instrumentation). Foundation Fieldbus targets distributed control in process automation applications and defines a mechanism for distributing the execution of control algorithms and data processing among I/O devices, sensors, and instruments. For example, you can configure a PID function block to use the input from a flowmeter and send the output to a control valve, then have the PID execute in the flowmeter, the control valve, or another networked device.

One network in particular is gaining momentum in industrial automation and DA applications. Ethernet is an economical, high-performance communications bus. More importantly, it has a huge installed base, is well integrated into the PC, and is familiar to most users. Although issues still remain for its widespread usage—including determinism (i.e., guaranteed predefined performance) and lack of a higher level standard protocol—Ethernet will be used more and more with smart I/O devices.

Is Smart I/O Right for You?
While smart I/O delivers many benefits, you should carefully evaluate your application and alternative technologies. If your application doesn't require networked or remote I/O, consider using PC-based virtual instruments and DA systems.

If smart I/O does fit your application, you have to consider many factors to make the right choice, including type and quality of signal I/O, modularity and expandability, network compatibility, control capabilities, packaging and size, and environmental specifications. However, your choices are sure to continue to expand as rapid advances in microprocessor and software technology make smart I/O devices more capable and even easier to use.

David Potter is Signal Conditioning Product Manager at National Instruments, 6504 Bridge Point Pkwy., Austin, TX 78730; 512-794-5489, fax 512-794-5569, david.potter@natinst.com

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