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HART interface to communicate with measurement devices.

HART modem

FundamentalWhat is a HART modem ?Signal modulationLocal linkRemote linkBasic connections and cablingBasic electrical drawing

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Automation > Resources > Technologies > Networks > HART > HART modem

HART MODEMSignal modulation, Local link usage, Remote link usage, HART modem basic wiring and cabling, HART modembasic electrical drawing.

The HART Modem To Exchange Data With Instrumentation Devices

A HART modem is a modulation / demodulation interface used in instrumentation to exchange databetween a terminal and a measurement device in compliance with the HART protocol.

This modem can be embedded, which is the case for a whole of measurement devices, mobile terminal anddata acquisition systems.

The HART modem can also be installed as an external peripheral. It could be useful because numerousinstrumentation software used to setting up and to calibrate a measurement sensor or to establish a diagnostic(such as the Pactware software), often need a serial port to communicate with the transducer.

Which is the case for many programmable logic controllers linked with these devices too.

Then a HART modem can be a smart tool for an automation engineer to configure a sensor or another device,or to maintain it by linking either locally (directly) or remotely.

This interface is usually called a "HART modem", because it operates the modulation and the demodulation(which is the definition of a "modem", indeed).

It is sometimes also called "HART communicator" or "HART transmitter".

HART Signal Modulation

The basics of the HART compliant data transmission is to add the HART data digital signal to theanalogical signal of the measure in the 4­20 mA current loop.

The data to be exchanged can be data used for the configuration of the instrument, parameters, calibration data,diagnostics data or services data.

Then only one medium is used to transfer the both signals.

The digital data exchanged with a HART transducer are frequency modulated :

A bit which value is 1 is coded by the emitter with a 1200 Hz frequency signal.A bit which value is 0 is coded by the emitter with a 2400 Hz frequency signal.

The range of the modulated digital signal is about 1 milliampere (± 0.5 mA around the analog signal of themeasure).

The data transfer parameters are imposed by the technology and are specified in the protocol as a standard :

Data transfer rate: 1200 Bps.Quantity of start bits: 1.Quantity of data bits: 8.Parity: odd.

Quantity of stop bits: 1.

When a peripheral (a programmable logic controller, a computer, a mobile terminal) wishes to exchange datawith a measurement device in compliance with the HART protocol, it will need the services of an embedded orexternal HART MODEM to modulate and demodulate the data.

From the terminal to the sensor, the HART modem modulates the data received from the communication port.Then it adds this signal to the analogical signal of the measure, to build the 4­20 mA composite signal.

From the transducer to the terminal, the HART modem extracts the modulated signal from the 4­20 mA

composite signal and demodulates it. Then it sends the data to the communication port.

HART MODEM Local link ­ Direct Connection To A Peripheral

To connect locally a computer and a device by using a serial link and an external HART modem for instance,it is enough to connect directly the communication port of the computer to the communication port of themodem.

The HART modems are available with various interfaces : RS232 serial port, USB, Wireless HART,BlueTooth and more.

Nevertheless as far as we are concerned, there are still many programmable logic controllers and measurementdevices exchanging data through a RS232 serial link.

Hart Modem Remote Link With A Virtual Communication Serial Port

An automation engineer, an instrumentation engineer or a user are not always close to the measurement sensorthey want access to.

The technician who maintains the device can be hundreds or thousands kilometers far away the transducer.

One solution is to use the virtual communication port technology.

This consist in installing a virtual serial communication port client manager (the software eVcom forinstance) on the machine where the instrumentation software runs.

The virtual communication serial port server can be a programmable logic controller (a PLC, a router or aninternet switch providing this function.

The manager emulates all the signals ­hardware and software­ of the serial port of the machine usedby the instrumentation software.Using the TELNET protocol, the pilot of the virtual serial port redirects these signals over theEthernet networks towards the serial port server addressed with an IP.The virtual communication port server can be a programmable logic controller, a router or a switch, forinstance, providing this functionality.The server will use the data received over Ethernet to build the hardware and software signals tosend to the serial port of the HART modem. Then, the modem will modulate the data of the request,

and will add this modulated signal to the analog signal of the measure. Finally, the result is a 4­20 mAcurrent loop composite signal.Next, the sensor will compute the data of the request and will send the answer backwards.

This technology is compatible with all the media using the Ethernet protocols : the cabled networks (copper,optical), the wireless networks (wifi, bluetooth, radio modems), the mobile networks (GPRS, 3G) and so on.

HART MODEM Cabling, Wiring And Connecting Basics

Internally, the HART modem uses a digital analogical converter (DAC) working with the voltage to read andwrite the data transferred over the 4­20 mA current loop.

Consequently, when the HART modem needs to read data from the transducer, the current of the 4­20 mAloop has to be converted into voltage by the system.

And when the HART modem needs to write data into the transducer, the voltage has to be converted intocurrent by the system.

To do that, the HART protocol standard prescribes to insert a 230 Ω to 1100 Ω resistor in the 4­20 mAcurrent loop of the measurement sensor.

To be able to adjust correctly the value of the resistor, it is necessary to know the global resistivity of the4­20 mA loop (which depends on the cable features, on its length which can exceed 1000 meters, and on thenetwork topology according to the quantity of devices).

It is to note that in some particular circumstances, to keep a good quality in the communication between thetransducer and the HART modem, the measurement resistor value can be lower than the minimum valueprescribed by the HART protocol specifications (ie. less than 230 Ω).

Basic Electrical Drawing

This example of drawing is composed of two 4­20 mA current loops.

Each of the current loop is supplied with a 24 VDC power line, which is a common low voltage power supplyin the electrical cabinets installed in the European factories.

(It could be any other low voltage direct current, such as a 48 VDC power supply for instance, according to thecountry and the common usages.)

Each of these current loops contains :

One 4­20 mA PLC analogical input (called "%IWx").One HART sensor and its transmitter including a 4­20 mA converter.

("Q" means a flow sensor while "L" means a level sensor).

One load resistor dedicated to the HART modem connection.

In this example, the HART modem is connected to the current loop on the left, parallel to its 250 ohms loadresistor.

The outgoing HART modem RS 232 serial line can be connected to a personal computer, to a controller or toany other HART compliant device.

The load resistor dedicated to a HART modem in the loop on the right is wired, but none modem is connected.

Whether a permanent connection is not necessary, or whether there is only a single HART modem inorder to operate the configuration, to read and write the parameters or to diagnose an analogical sensor,only connect the HART modem according to the drawing, operate the job, then disconnect the HARTmodem from the first current loop and connect it to the second one.

Once all operations have been done, it is possible to disconnect the modem from the both loops, lettingthe load resistors wired in situation (this avoids to open the line before using the HART modem again).

Whether the HART modem must be permanently connected (to be use remotely for instance), the HARTmodem stay continuously connected to the loops it is needed.

It is to notice that, according to all which concerns analogical sensors and analogical measuring, it is necessaryto take care of the power supply quality.

It is better to use a very stable isolated power supply (the better choice being a switched mode power supply ­SMPS), in addition to the ones used commonly for the digital inputs and outputs, or for some other devices.

This additional power supply should be exclusively dedicated to the analogical sensors and to all the othermeasuring devices.

It is also necessary to check whether the 4­20 mA PLC analogical inputs are galvanic isolated or not.

Actually, for economical reasons, and although it is not logical, some low cost programmable logic controllersprovide 4­20 mA analogical inputs which are not isolated, with common connections at the incoming or at theoutgoing of the embedded digital analogical converters (DAC).

First, that implies that is not possible to insert two non isolated analogical inputs serially in the same currentloop.

Second, that implies (in relation with the first implication above) that, in order to use each of the non isolated4­20 mA analogical inputs inside different loops, it is not possible to insert them anywhere.

Whether the common connection is the outgoing point of the DAC, such as in the illustration above, thecommon connection of both analogical inputs has to be connected to the "0 VDC" (or to the "­") of thepower supply.

The other components and devices have to be connected serially upstream in their respective currentloop (that is toward the "+" or the "+ 24 VDC"of the power supply).

Whether the common connection is the incoming point of the DAC, the common connection of bothanalogical inputs has to be connected to the "+ 24 VDC" (or the "+") of the power supply.

The other components and devices have to be connected serially downstream in their respective currentloop (that is toward the "­" or the "0 VDC" of the power supply).

HART modem Resources

HART Electronic Interface ­ Technical Notes.

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