IEEE 1451.4 Standard Overview

If you have ever thought that using a sensor should be as easy and trouble free as plugging a mouse into your computer, then an IEEE 1451.4 standard may be your answer for self-describing, plug and play sensors. The IEEE 1451.4 standard defines the mechanism for adding self-describing behavior to transducers with an analog signal interface. Rather than defining yet another network or bus for sensors, IEEE 1451.4 defines a low-level transducer interface and information structure that can work across literally any type of measurement interface or network.

Introduction to IEEE 1451.4 Plug and Play Sensors

IEEE 1451.4 is a standard that defines how analog transducers can inherit self-describing capabilities for simplified plug and play operation. The standard defines a mixed-mode interface that retains the traditional analog sensor signal, but adds a low-cost serial digital link to access a transducer electronic data sheet (TEDS) embedded in the sensor for self-identification and self-description.

When you set up and configure a traditional DAQ system, you must manually enter important sensor parameters, such as the range, sensitivity, and scale factors, in order for the software to properly use and scale the sensor data. A system outfitted with IEEE 1451.4 sensors and actuators can automate this configuration step, while also increasing the general integrity and reliability of the system.

While other smart sensor technologies do exist that also deliver plug and play operation, IEEE 1451.4 is unique because it maintains the analog output of the sensor. Therefore, IEEE 1451.4 transducers are compatible with legacy systems that include traditional analog interfaces. And the simplicity of IEEE 1451.4 implementations has the very significant and pragmatic advantages of easy retrofit of legacy sensors, and very low risk of adoption for sensor manufacturers.

The two main components of the IEEE 1451.4 standard are the (TEDS) and the mixed-mode interface.

TEDS for Sensor Self-Identification

IEEE 1451.4 defines a standard format for TEDS data to be embedded into plug and play transducers. At a minimum, an IEEE 1451.4 TEDS contains the manufacturer, model number, and serial number for the transducer. Most TEDS also describe the important attributes of the sensor or actuator, such as measurement range, sensitivity, temperature coefficients, and electrical interface. In other words, the TEDS describes everything you need to know to make a measurement using the sensors. Figure 2 shows one example of a TEDS for an accelerometer.

IEEE 1451.4 Mixed-Mode Interface

The standard defines two types of mixed-mode interfaces, designated as Class 1 two-wire and Class 2 multi-wire interfaces. The Class 1 two-wire interface works with constant-current powered, or Integrated Circuit Piezolelectric (ICP®), transducers, such as accelerometers. ICP is a registered trademark of PCB Piezotronics, Inc. Class 1 transducers include diodes or analog switches with which the multiplexing of the analog signal with the digital TEDS information on the single-pair of wires is possible.

For other types of sensors, the Class 2 interface uses a separate connection for the analog and digital portions of the mixed-mode interface. The analog input/output of the transducer is left unmodified, and the digital TEDS circuit is added in parallel. This enables the implementation of plug and play transducers with virtually any type of sensor or actuator, including thermocouples, RTDs, thermistors, bridge sensors, electrolytic chemical cells, and 4-20 mA current loop sensors. For example, Figure 3 illustrates the implementation of a Class 2 mixed-mode interface with a bridge interface.

The digital portion of the mixed-mode interface (Class 1 or Class 2) is based on the 1-Wire protocol from Maxim/Dallas Semiconductor. This is a very simple, low-cost, master-slave serial communication protocol, requiring that a single master device (e.g. the data acquisition system) supply power and initiate each transaction with each node according to a defined transaction timing sequence, on a single wire and return.

The IEEE 1451 Family of Standards

IEEE 1451.4 is one of the members of the IEEE 1451 family of Smart Transducer Interface Standards. The IEEE Standard 1451.1-1999, Network Capable Application Processor Information (NCAP) Model defines a common object model, with interface specifications, of a networked smart transducer. IEEE Standard 1451.2-1997 defines a digital point-to-point interface to connect a smart transducer module with digital output to a microprocessor-based network adapter. IEEE 1451.2 was also the first standard to introduce the concept of the TEDS. The concept of the embedded, self-identifying TEDS is arguably the most popular component of the standards, and is a key element throughout the entire family of IEEE 1451 standards.

Conclusion

IEEE 1451.4 defines a relatively simple, straightforward mechanism for adding smart, plug and play capabilities to traditional analog sensors. These plug and play sensors can bring real, immediate benefits in ease of use and productivity to any measurement and automation system that uses sensors. Additionally, IEEE 1451.4 defines a standard framework for sensor description, embodied specifically in the TEDS, that can scale from today's traditional analog sensors to tomorrow's smart networked sensors.
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