INSTRUMENTATION NETWORKS

Instrumentation networks are specialized industrial communication networks allowing automation devices to exchange data with instruments, devices and other systems dedicated to the measurement and to analogical commands. They can be measures transmitters, commands transmitters, or software applications used to adjust, to calibrate and to diagnose the instruments. All of these measurement networks are used to communicate with process sensors and actuators transducers which generally send and receive the current data. They can also be used to set the configuration, to adjust and to calibrate the instruments and their interfaces. Another important usage is the exchange of data for sensors and actuators maintenance and control.

This full duplex low cost network uses a master-slave architecture which has been expressively designed for industrial measurement communication usages. Its naming can be spelled DIN-MESSBUS or DIN-MEßBUS and it can be named « MEASUREMENT BUS » too.

One of its particularities is that once the polling has started, a slave node can be disconnected and connected again, an eventual interrupted data transfer will restart and continue without any data lose. Its main characteristics are its simplicity, its robustness, its free topology, and the simplicity of its medium based on the so popular RS 485 standard. Its message structure allows to transmit measurement values, configuration and diagnostic data.

Due to its flexibility and to its simplicity, « Din Messbus » is also used to carry quality data ( for quality management ), to process control and manufacturing control.

• Measurement Bus Characteristics.
• Short Description of the Application Services according to the DIN 66348-3 standard.
• The Measurement Bus : A simple way of Fieldbus Technology.

www.measurement-bus.de

• « DIN-Meßbus » : the DIN Measurement Bus.

[ Archives ]

The « IEEE-488 » bus was developed with the intention to communicate with smart instruments ( that is programmable ) and to provide a standardized interface to exchange data between devices from different manufacturers. The medium is a 24 twisted pairs ( parallel cabling ), 8 pairs of them are used to transmit the data and the command messages ( for a data length of 7 or 8 bits ), 5 cables are used to manage the bus, and 3 cables are for handshaking.

The Hewlett-Packard company initiated the development of this versatile interface, and called it « HP-IB » ( Hewlett Packard Instrumentation Bus ). Since « HPIB » became very popular, the « IEEE 488 » commission renamed it « GPIB » ( General Purpose Instrumentation Bus ). Later, the « IEEE 488 » has evolved leading the « IEEE 488.2 » version, with the integration of the SCPI interface ( standard commands for programmable instruments interface ), in order to avoid devices manufacturer extensions, and to provide a quite universal « GPIB » programming interface.

This well written and well illustrated page resumes all you need to quickly know about the HPIB bus : its history, its relevancy, its communication model, how it works, its commands, how the data are transmitted, plus the addressing, the message structures, the state model, the connector and a lot of things more.

www.hp9845.net

This paper introduces to this instrumentation networking system. It presents the communication principles ( commands, queries, data and events ), the programming rules, the instrument status by detailing all the information contained in the registers, the trigger mechanism and the complete list of the error codes.

It also shows how to build the commands and the requests and how to use the implemented synchronisation techniques before listing all the common commands currently used by all the instruments and before describing the standard subsystems.

Electronics group - Faculty of sciences -

Free university of Amsterdam - Netherlands.

This tutorial presents the « IEEE 488 » system, particularly the role of its different lines and the SCPI model included in the IEEE 488.2 standard.

Author : National Instruments

Source : www.psi.ch - Paul Sherrer institute.

HART ( « Highway Addressable Remote Transducer » ) is a popular digital communication protocol designed for industrial process measurement applications, based on the 4-20 mA current loop standard. This digital communication system has been especially designed to exchange configuration data, calibration data, adjustment data and diagnostic data. It is useful for installation and maintenance jobs.

The technology consists in both hardware and software protocol.

• Hardware side, the data are exchanged by modulating the 4-20 mA measurement signal.
  • The data exchanges need to use a HART interface   in order to modulate and to demodulate the signal, and it is exactly what the HART modem do.
  • The composite HART signal consists in a 1 mA range frequency modulation over the 4-20 mA analogical measurement signal in order to transmit simultaneously both the digital data and the analogical signal value by using a single medium.
  • The HART signal modulation consists in coding the data to be transmitted by assigning different frequencies to each bit value.
• Software side, the protocol defines how to process the communications and specifies the data structure.

The robustness of this networking solution benefits of the low noise characteristics of the 4-20 mA current loop. Operating over such a loop does not need any other medium installation than the measurement transmission line, that leads in a cheap communication system. Since HART uses rather low frequencies to modulate the signal, the transmission rate is quite low too, but it is not important because the purpose of HART is to exchange non time critical data.

The HART technology has evolved and is now available supporting wireless medium ( « Wireless HART » ) and Ethernet medium ( « HART-IP » ) too.

This hardware component is used as an interface to enable the exchange of data between a terminal ( computer, mobile terminal, programmable logic controller ) and instrumentation devices such as a measurement sensors over a 4-20 mA current loop with respect to the HART protocol.

This interface modulates and demodulates the signal in order to read and to write the data ( which is the definition of a modem, indeed ).

Terminal side, the data are carried over a serial communication port ( RS232, USB, Bluetooth ) which can be either a real port or a virtual serial port.

Such a modem allows an automation engineer to talk with a local sensor or to communicate with a remote sensor over ethernet.

Practically, the basic connection of a HART modem consists simply to insert a load resistor serially with the sensor into the current loop, after what it is enough to connect the modem in parallel of this resistor.

Consultant in industrial control and communication, Romilly BOWDEN provides a whole of informations in relation with this digital instrumentation communication system. You will learn what is HART in a short introduction, the revision history of the protocol, what were the main changes and a lot of other technical informations.

More, the author offers the program H-SIM, a useful utility software which help you by computing the available cable length according to the desired strength of the signal you need for a specific installation.

Romilly BOWDEN - www.romilly.co.uk

This huge folder presents the available techniques in use to implement HART communications into hardware, and concentrates on the electric and electronic point of view. In addition, the data exchanges process is explained clearly in a very detailed manner.

[ Archives ]

• HART Communications. ( )

  www.samsongroup.com

  The SAMSON company presents in a very clear way the fundamental knowledge needed to control the HART technology as well.

  The first section details the connections between devices, addressing, number of participants, point to point connections, multiplexed connections and higher level communications.

  It ends with two wires cabling explanations, with device interfacing description, and with considerations about load impedance.

  The second section describes the communication layers used by the protocol. At first the physical layer ( coding, connecting, plugging ), second the application layer and the data-link layer ( access control, communication services, datagram structure, noise immunity, timing and data transfer ).

  This section finishes with the presentation of the device description language ( DDL ) which allows any device to dialogue with any system.

The « HART Communication Foundation » provides short overviews about HART communication protocol, including HART over 4-20 mA current loop, Wireless HART communication protocol, HART-IP Ethernet protocol, and presents the history of the protocols, what they are, and how they work. In the technical articles section, you can download the « application guide », a 194 pages document quite well illustrated which presents quickly all the concepts in use and some industrial applications.

www.fieldcommgroup.org

« Lan extension for instruments » is an instrumentation network standard system particularly used in instrumentation testing, in test laboratories or in research and development departments. It results of the migration of the VXI / PXI systems ( which integrates natively the GPIB measurement capabilities ) towards an Ethernet system. The functions provided by GPIB are accessed over a dedicated gateway and the synchronization functions are operated through the integration of the IEEE 1588 real time communication standard.

The inheritance of the VXI / PXI capabilities, of the Ethernet and internet features, of the GPIB measurement performances and of the IEEE 1588 services leads LXI to be a very powerful, fast, flexible, reliable and compact test equipment network system.

LXI is the successor of the GPIB bus in many ways.

Since it uses Ethernet, LXI benefits of the internet capabilities too, by embedding a web server into the instruments, interfacing and controlling them with a web application running on a cheap personal computer for instance. Consequently, the devices do not need any front command panel any more, and the usage of popular and widespread technologies contribute to minimize the costs.

(*)  VXI ( VME eXtension for Instrumentation ) is a compact instrumentation network system which integrates the best features of both VME ( Versa Module Eurocard ) and GPIB buses.

(**) PXI ( PCI eXtension for Instrumentation ) is a more recent instrumentation bus system, which incorporates the leading characteristics of the PCI ( Peripheral Component Interface ) and of the GPIB buses altogether.

The LXI consortium promotes the LXI technology and is only too happy to share many informations.

The « About LXI » section introduces to the technology ( what is LXI ?, what are its benefits, what are its main features ).

The « resources » section provides good introductions to the standard and some other guides. You can also download there some related technical papers and watch some videos. A few tools are available too.

Last but not least, you can download all the specifications freely, including the current version and older versions.

www.lxistandard.org

This data exchange standard process consists in a specialized network dedicated to quite smart measurement devices, especially whose operating in the industry of energy and in the power distribution areas. This digital communication standard is particularly used to read consumption measurements and temperature measurement at low cost.

This system is a rather versatile network since it uses linear topology, star topology or tree topology and since it can be incorporated into a wireless network structure, doing all that by using a single master / slave structure linked to a client / server communication process.

One of its specificity is that the data telegram are not specified, letting the manufacturers free to define and to specify it.

In the wired versions, another meter bus » specificity is that the master and the slaves modulate the signals over the power line in order to communicate.

• The master queries are transmitted by modulating the supply voltage ( a 36 VDC signal level means a logical level of « 1 », a 24 VDC signal level means a logical level of « 0 » ).
• The slave answers are transmitted by modulating its current consumption ( a current consumption of 1.5 mA means a logical level of « 1 », a current consumption between 11 to 20 mA means a logical level of « 0 » ).

The oficial organization provides the complete M-bus V 4.8 technical documentation ( ), an old version in which you can learn the fundamental principles of the meter bus system.

By reading these 88 pages, you will get a lot of interesting informations about the technical bases which have been selected to design this technology with a lot of explanations about the main features the system offers : first by introducing to the network and to the available topologies it can use, then by offering many details on the way the different layers of the OSI model have been implemented in the system, that is the physical layer, the data link layer, the application layer and the network layer.

www.m-bus.com

• Profibus for Process Automation introduction.
• You will find numerous documents according to Profibus-PA in the downloading section technical descriptions, installation manuals, drivers, devices description files and software.

www.profibus.com

This tutorial introduces the fundamental knowledge about Profibus in general, and the basic technology of « Profibus-PA ». It explains the physical layer, the data addressing, the safety layer, the topologies, the connections, the user interface and the device description interface ( GSD files and EDD files ).

www.samsongroup.com