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. 
 
 
 

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. 
 
 
 

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

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

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

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

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

Digital Communication Protocol 
 

The Dallas Semiconductor MicroLANā 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.
• 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 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.