## Update on the IEEE 1451 Smart Transducer Interface Standard

Steven Chan

AEPTEC Microsystems / 3E Technologies

The proliferation of sensor and instrument busses has introduced new
ways to interface and communicate with transducers. The widespread
availability of microelectronics, computers, and networks provide a good
opportunity to network a large arrays of transducers to measure,
characterize, model, and monitor many large structures, machinery, and
mechanical systems. Nevertheless, these new ways have been useful only
to segments of the transducer community. In addition, the increased use
of large number of transducers has also created a need for keeping track
of the transducers and their associated manufacturer data. The
availability of economical off-the-shelf memory chips has help to
implement built-in electronic data sheets in small transducers. This has
significant contribution in building smart transducers with
self-identification capability through the use of electronic data
sheets. The transducer community also recognized the need for a common
way of connecting these smart transducers and thus began the work on the
IEEE 1451 Smart Transducer Interface Standard.

The IEEE 1451 provides a set of common interfaces for connecting
transducers (sensors and actuators) to existing instrumentation and
control networks and lays a path for the sensor community to design
systems for future growth. It is intended to provide an easy upgrade
path for connectivity of products from any manufacturer of transducers
or networks. The IEEE 1451 Standard can be basically viewed as a
software and hardware oriented interfaces. The software portion is an
information model defining the behaviors of a smart transducer using
object model approach and the path for network connectivity. This work
has been completed and become the IEEE 1451.1-1999 Standard \[1\]. The
sensor usage crosses various industries, therefore the hardware portion
of the IEEE 1451 Standard is divided into 1451.2, 1451.3, and 1451.4 to
meet their specific needs. The first one, focused on an interface for
transducers with lower signal bandwidth requirements, has been completed
and designated as the IEEE 1451.2-1977 Standard \[2\]. These two
Standards can be acquired from IEEE \[3\].

The proposed IEEE 1451.3, A Smart Transducer Interface for Sensors and
Actuators - Digital Communication and Transducer Electronic Data Sheet
(TEDS) Formats for Distributed Multidrop Systems is proposed to utilize
spread-spectrum modulation techniques to allow the following functions
to be performed over a single cable:

1.  synchronizing data acquisition for an array of sensors;

2.  communicating simultaneously with an array of transducer bus
    interface modules (TBIM);

3.  providing power for operation of all transducers on the bus and
    their associated electronics.

The IEEE 1451.3 working group is currently developing the draft document
focusing on defining the TEDS, T-Block, physical layer, and
synchronization issues.

Today, we are primarily focusing on the progress of the proposed IEEE
1451.4 Standard.

The proposed IEEE 1451.4, A Smart Transducer Interface for Sensors and
Actuators - Mixed-Mode Communication Protocols and Transducer Electronic
Data Sheet (TEDS) Formats.

The drive for transducers with built-in identification, manufacture data
such as calibration, and extended functionality has increased sharply
over the last few years. The transducer community, in a concerted effort
with NIST, started the work on the proposed IEEE 1451.4 standard to meet
the demands and needs of the changing industry. The IEEE 1451.4 Working
Group is charged to define a universally accepted, mixed-mode transducer
interface standard (i.e. able to work both in analog signal transmission
mode and in digital communication mode, but not simultaneously). The
initial focus was on 2-wires piezoelectric accelerometers, however,
generic transducers such as strain gauge, thermal couples, RTD, are
going to be included.

The main objectives of the proposed standard are to:

-   Enable plug and play at the transducer level by providing a common
    IEEE 1451.4 Transducer communication interface compatible with
    legacy transducers.

-   Enable and simplify the creation of smart transducers.

-   Facilitate the support of multiple networks.

-   Make a bridge between the legacy transducers and the networked
    transducers

-   Enable implementation of smart transducers with minimal use of
    memory

The proposed standard describes the following:

-   An IEEE 1451.4 Transducer contains a Mixed-Mode Interface (MMI) and
    a Transducer Electronic Data Sheet (TEDS).

-   MMI is a two-wire, master-slave, multi-drop, serial connection. MMI
    requires a single master device to initiate each transaction, with
    each slave or node according to a defined transaction timing
    sequence. The MMI uses two wires for power supply, time-shared
    analog signal and digital data connection.

-   The TEDS is fixed and dynamic data contained in one or more memory
    nodes on the MMI. (NOTE -- The digital interface is not part of TEDS
    templates). The TEDS residing in the IEEE 1451.4 Transducer,
    contains fields that describe the type, operation, and attributes of
    one or more transducer elements (sensors or actuators).

-   A TEDS Template is a software object, which describes the data
    structure of the TEDS. It is implemented in Description Language and
    resides in a Transducer block (T-block).

-   The Description Language is a scripted and tagged language, which
    provides a standard method to describe the functionality of an IEEE
    1451.4 Transducer.

-   A Transducer Block is a software object describing the IEEE 1451.4
    Transducer, it resides in a network capable application processor
    (NCAP) which is the master device (e.g. an instrument or data
    acquisition system). A T-block is used to access, decode and encode
    the TEDS using the Description Language.

**IEEE 1451.4 Transducer**

An IEEE 1451.4 Transducer contains a Transducer Electronic Data Sheet
(TEDS) and a Mixed-Mode Interface (MMI). The context for the mixed-mode
transducer and interface is shown below.

![](media/image1.wmf)

Figure 1. Context for the Mixed-Mode Transducer and Interface

The IEEE 1451.4 Transducer is a sensor or actuator with typically one
addressable device, which here will be referred to as a node, containing
TEDS. The digital communication can be used to read the TEDS information
and to configure an IEEE 1451.4 Transducer. Multiple IEEE 1451.4
Transducers with switch nodes can be connected in a multi-drop
configuration with maximum one of these Transducers in "active" mode and
the rest in "passive" mode. The switch nodes can be used to change the
functional mode of each IEEE 1451.4 Transducer.

The TEDS residing in the IEEE 1451.4 Transducer, provides
self-describing capability. The TEDS contains fields that describe the
type, operation, and attributes of one or more transducer elements
(sensors or actuators). By requiring that the TEDS be physically
associated with the IEEE 1451.4 Transducer, the resulting hardware
partition encapsulates the measurement aspects in an IEEE 1451.4
Transducer, while the application-related aspects can reside in the NCAP
or alternatively be stored in the TEDS

An IEEE 1451.4 protocol is used to separate the time critical part of
the communication of the IEEE 1451.4 interface from the T-block. The
T-block object located in the NCAP handles the interpretation of the
TEDS data for the end user. Further processing of the data may take
place both in the NCAP and in other processors in larger systems. The
NCAP includes an IEEE 1451.1 object model with an IEEE 1451.4 T-block.

To support various types of transducers, the working group has expanded
the interface definition to include two Classes of interface -- 2-wire
Class 1 and multiple-wire Class 2 interfaces. Examples of various
configurations are shown below.

![](media/image2.wmf)

Figure 2. Example of IEEE 1451.4, Class 1, Two-wire Interface, with
Constant-current Powered Sensor

![](media/image3.wmf)

Figure 3. Example of IEEE 1451.4, Class 1, Two-wire Interface, with
Voltage Powered Sensor

![](media/image4.wmf)

Figure 4. Example of IEEE 1451.4, Class 2, Multi-wire Interface, with
4-20mA Sensor

![](media/image5.wmf)

Figure 5. Example of IEEE 1451.4, Class 2, Multi-wire Interface, with
Bridge Sensor

![](media/image6.wmf)

Figure 6. Example of IEEE 1451.4, Class 2, Multi-wire Interface, with
shared wire.

**Digital Communication Protocol**

The Dallas Semiconductor MicroLAN was the basis for much of the hardware
development work leading to this proposed standard. The digital
communication protocol adopted in this proposed standard is licensed by
Dallas Semiconductor Corporation. Supplying power to and transferring
data to and from, one or more slave devices, or nodes, arrayed on a
single wire network, from a master device, requires strict adherence to
a protocol based upon the following rules:

-   This is a master-slave, multi-drop, 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.

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-   Each node contains a unique address (serial number plus family
    code), identifiable by the master, allowing individual access among
    multiple nodes, on a common bus.

-   A hierarchy of commands, issued by the master, controls every
    transaction between the master and any node.

-   A defined set of interface signaling pulse sequences, based upon
    timing and asserted by the master, including power/recovery,
    initialization, read and write, controls the data transfer between
    devices.

The IEEE 1451.4 Mixed-Mode Interface can be used for control networks
and data acquisition in a variety of applications such as portable
instruments and data acquisition plug-in cards for PCs.

**Current Efforts**

The working group targets at balloting in March. The group is currently
finalizing the following areas to support the majority of popular
transducers and make the standard easy to use.

***T-Block -*** A universal modeling language (UML) is required for
development, demonstration, validation and debugging of the T-Block. The
UML will be used to validate the high-level structure of the T-Block.

***Reference implementation -*** Reference implementations of a standard
can be a valuable development tool for companies interested in
developing their own software for the IEEE 1451.x and other sensor
networks. Following the open source model, a Consortium proposal is
under reviewed to provide a UML and C++ (or C where applicable)
reference implementation of the 1451.4 standard based on a Windows 95/98
platform with a transducer interface board. When managed and used
correctly, open source code can be a valuable tool for assuring
compatibility across manufacturers, increasing code reliability and an
extensive user base.

***XML -*** The working group is considering the use of XML to describe
TEDS.

***A Generic TEDS Template -*** A generic template 1451.4 that would
support the majority of transducers in popular use should be feasible.
Since not every transducer type in the world need be addressed,
limitations of minority transducers are acceptable. The working group
intends to detail the information necessary in the TEDS of a transducer
using such a generic template.

**Further information**

If you are interested in the proposed IEEE 1451.4 standard, please
contact Steven Chen at <schen@3eti.com>[. Likewise, for]{.underline} the
proposed IEEE 1451.3 standard, contact Larry Malchodi at
<Larry.Malchodi@PSS.Boeing.com>.

**Reference**

1\. "IEEE Std 1451.1-1999, Standard for a Smart Transducer Interface for
Sensors and Actuators - Network Capable Application Processor (NCAP)
Information Model," Institute of Electrical and Electronics Engineers,
Inc., Piscataway, New Jersey 08855, June 25, 1999.

2\. "IEEE Std 1451.2-1997, Standard for a Smart Transducer Interface for
Sensors and Actuators - Transducer to Microprocessor Communication
Protocols and Transducer Electronic Data Sheet (TEDS) Formats,"
Institute of Electrical and Electronics Engineers, Inc., Piscataway, New
Jersey 08855, September 26, 1997.

3\. IEEE website <http://standards.ieee.org/sds/index.html>, or through
the IEEE customer service department by calling 1-(800)-678-4333 (IEEE)
in the US and Canada, 1-(732)-981-0600 from outside of the US and
Canada, or by Faxing 1-(732)-981-9667.