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Plug & Play


Plug and Play Sensors Soon To Have A Standard

The IEEE P1451.4 is a long overdue and pragmatic solution to provide plug and play capability to "dumb" sensors with the addition of a 1-wire, 1-dollar memory chip. The memory chip contains a TEDS (Technical Electronic Data Sheet) that provides the connecting system with full sensor details including sensor type, serial number and calibration information.

The IEEE Standard provides two methods by which the TEDS chip is accessed. The first (Class 1) is appropriate for 2-wire sensors with in-built conditioning and requiring excitation. Reversing the excitation to -5V makes the TEDS chip available. This has the significant merit of retaining a simple co-axial cable connection to the sensor. The arrangement has already become popular with accelerometer manufactures where sensors are typically excited with a high impedance 4mA current source and the output signal modulates the voltage of the excitation .

The second method (Class 2) of connection is via one or two dedicated wires in parallel with the sensor's traditional wire set. This makes the TEDS available anytime and can be applied to any sensor type, including multi-parameter sensors.

A third method has been proposed in the form of a Virtual TEDS - an Internet accessible data base of sensor TEDS. Obviously Virtual TEDS will not have the same convenience as P1451.4 TEDS because the association between the physical sensor and the Virtual TEDS will need to be made by the user, rather than occur automatically.

IEEE P1451.4 is just one part of a series of IEEE Std 1451Standards. The TEDS concept originally appeared in the IEEE Std 1451-2 as part of the move towards smart sensors, with the processing of the sensors data being done in the Network-Capable Application Processor (NCAP) packaged with the smart sensor. What IEEE P1451.4 does is move the NCAP to the data acquisition device (that is a plug-in DAQ board & computer, data logger or stand alone DAQ unit).

With the intension of keeping the size of the TEDS as small as possible, the new standard defines a number of templates , allowing a more compact representation of the data. The host must have some understand of the templates to enable it to decode the TEDS information.

The IEEE Std 1451.1 and IEEE Std 1451.2 standards have struggled to gain wide acceptance, probably largely due to the structure of the industry. An aim of the standards was to allow sensor manufactures to focus on their core competencies while others provided the network interfaces. Unfortunately the "others" failed to materialize in sufficient numbers to give the standards critical mass. With the benefit of hindsight, perhaps the adoption of the IEEE 1451 series of standards would have been quicker had IEEE P1451.4 been the first to be released.

The new IEEE1451-4 standard is a commendable effort. Largely championed by National Instruments and a group of sensor manufacturers, we can look forward to the wider adoption of the standard due to its support of legacy sensors and its support by major players. For the users of sensors it will simplify the data acquisition task and lead to better sensor asset management.

Keywords: IEEE P1451.4, TEDS, plug and play, sensors.