HART MODEM

A « HART » modem is a signal modulation / demodulation interface used in instrumentation to exchange data efficiently between a terminal and a measurement device over a serial link and in compliance with the HART protocol ( for highway addressable remote transducer protocol ). It is used to encode and to decode digital data from a transmission medium to another ( from its serial communication port towards the 4-20 mA loop where the instrument is connected to and vice versa ). Transmitted data can be digitalized measurement values, service data, diagnostic data, calibration data or configuration data. They can also be orders or/and parameter command values sent to the instruments.

This equipment can be embedded, which is the case for a whole of measurement devices, mobile terminal and data acquisition systems. It can also be installed as an external peripheral, because many instrumentation software used for sensor installation or sensor maintenance ( such as the Pactware software for instance ), are still operating a serial port to communicate with the instrument transducer ( yes indeed, even into the Ethernet age ! ). This is of the the case for many programmable logic controllers linked with these devices too.

Such a device - which you can find references with the « HART communicator » name or with the « HART transmitter » name about - can be a smart tool for an automation engineer to set up or to maintain a measurement device by linking it either locally or remotely.

The basics of the HART compliant data transmission consists in a signal modulation, operated by adding the digital data signal to the analogical signal of the measure in the 4-20 mA current loop. The data to be exchanged can be data used for the configuration of the instrument, parameters, calibration data, diagnostic data or service data. Then only one single medium is needed, and this is sufficient to transfer the two 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 analogical signal of the measure ).

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 data with a measurement device in compliance with the HART protocol, it will need the services of either an embedded or en external dedicated modem to modulate and demodulate the data.

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

From the transducer to the terminal, the interface separates the modulated signal from the 4-20 mA composite signal and demodulates it. Then it sends the data to its communication port.

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 the modem.

The 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 measurement devices exchanging data through a RS232 serial link.

Measurement devices are not always immediately accessible to an automation engineer, to an instrumentation engineer or to an end-user. The technician who maintains the device can be hundreds or thousands kilometres far away the transducer and the ability to connect remotely can allow him to save a lot of time.

One possible solution is to use the virtual communication port technology.

• This consist in installing a virtual serial communication port client manager ( the eVcom software for instance ) on the machine where the instrumentation software runs, the virtual port acting as an interface between Ethernet and the serial port driver of the software.
• As the virtual communication serial port server can be any programmable controller ( a PLC, a router or an Ethernet switch for example ) providing this function.

• The manager emulates all the hardware and software signals of the serial port in the machine used by the instrumentation software.
• Using the TELNET protocol, the pilot of the virtual serial port redirects these signals over the Ethernet networks towards the virtual serial port server IP address.
• The server will use the data received over Ethernet to build the hardware and software signals to send to the serial port of the HART modem, which will modulate in frequency the data of the request over the 4-20 mA current loop of the sensor.
• 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.

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

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

To do that in an efficient way, the HART protocol standard prescribes to insert a 230 Ω to 1100 Ω load resistor in the 4-20 mA current loop of the measurement sensor. This terminator helps to adapt the impedance of the serial line in order to preserve a reasonable signal amplitude.

To be able to adjust correctly the value of the resistor, it is necessary to know the global resistivity of the 4-20 mA loop ( which depends on the cable features, on its length which can exceed 1000 meters, and on the network 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 the transducer and the modem, the measurement resistor value can be lower than the 230 Ω minimum value prescribed by the HART protocol specifications.

This example of drawing is composed of two 4-20 mA current loops. Each of the current loop is supplied with a 24 VDC power line, which is a common low voltage power supply in the electrical cabinets installed in the European factories.

Each of these current loops contains :

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

  («Q» means a flow sensor while «L» means a level sensor).

• One load resistor dedicated to the HART modem connection.

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In the example shown in this figure :

• The HART modem is connected to the current loop on the left, parallel to its 250 ohms load resistor.
• The outgoing modem RS 232 serial line can be connected to a personal computer, to a controller or to any other HART compliant device.
• The load resistor dedicated to the modem, located into 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 modem in order to operate the configuration, to read and write the parameters or to diagnose an analogical sensor, just connect the modem according to the drawing, operate the job, then disconnect the modem 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, letting the 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 device 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 necessary to 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 other measuring devices.

Check whether the 4-20 mA PLC analogical inputs are galvanic isolated or not.

Actually, for economical reasons and although it is not logical, some low cost programmable logic controllers provide 4-20 mA analogical inputs which are not isolated because they contain some internal common connection at the incoming or at the outgoing 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 current loop.
• Second, that implies ( in relation with the first implication above ) that, in order to use each of the non isolated 4-20 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, the common connection of both analogical inputs has to be connected to the « 0 VDC» ( or to the « - » ) of the power supply. The other components and the other devices have to be connected serially upstream in their respective current loop ( 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 both analogical inputs has to be connected to the « + 24 VDC » ( or the « + » ) of the power supply. The other components and the other devices have to be connected serially downstream in their respective current loop ( that is toward the « - » or the « 0 VDC » of the power supply ).

• HART Electronic Interface - Technical Notes.

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• Moreover you can also access the folder in relation with the hardware associated to the HART protocol.