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HART interface to communicate with measurement devices.

HART modem

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Virtual serial port technology basics  


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

The HART Modem To Exchange Data With Instrumentation Devices

A HART modem is a modulation / demodulation interface used in instrumentation to exchange data between 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 and data acquisition systems.

The HART modem can also be installed as an external peripheral. It could be useful because numerous instrumentation 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 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, diagnostics data or services data.

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

HART - Digital signal modulation by frequency modulation (bit = 1 : f = 1200 Hz ; bit = 0 : f = 2400 Hz) with a level of 10 mA, that is ± 0,5 mA around the analog signal of the measure over the 4-20 mA current loop.

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

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

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

HART - 4-20 mA analogical composite signal - Sum of the modulated digital signal added to the analogical signal of the measure.

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 an embedded or external 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 4-20 mA composite signal.

From the transducer to the terminal, the HART modem extracts the modulated signal from the 4-20 mA composite signal and demodulates it. Then it sends the data to the communication port.

HART MODEM Local link - Direct Connection To A Peripheral
HART modem local link - Terminal + RS232 link + HART modem + HART transducer (measurement sensor).

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 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 measurement devices 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 sensor they want access to.

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

HART modem remote link - Local terminal with a virtual serial port client + Ethernet + remote virtual serial port server  + RS232 serial link + HART modem + 4-20 mA current loop + HART transducer (measurement device).

One solution is to use the virtual communication port technology.

This consist in installing a virtual serial communication port client manager (the software eVcom for instance) 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 an internet switch providing this function.

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
HART modem - Cabling and wiring fundamental drawing - RS232 interface + resistor + 4-20 mA current loop + measure sensor + power supply.

Internally, the HART 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 HART modem needs to write data into the transducer, the voltage has to be converted into current by the system.

To do that, the HART protocol standard prescribes to insert a 230 Ω to 1100 Ω resistor in the 4-20 mA current 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 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 HART modem, the measurement resistor value can be lower than the minimum value prescribed by the HART protocol specifications (ie. less than 230 Ω).

Basic Electrical Drawing
HART modem basic electrical drawing including two 4-20 mA current loops, each of them containing one 4-20 mA PLC analogical input, one HART sensor, and one HART modem with its load resistor.

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.

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

Each of these current loops contains :

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

The outgoing HART 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 a HART modem in the loop on the right is wired, but none modem is connected.

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.

4-20 mA non isolated analogical input.

It is also necessary to 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, with common connections 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.

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