HART modem basic knowledge

Signal modulation | Local link usage | Remote link usage | HART modem basic wiring and cabling

A HART modem is a modulation / demodulation interface used in instrumentation to exchange datas 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 softwares used to setup and to calibrate a measurement sensor or to establish a diagnostic (such as the Pactware software), often need a serial port to dialog with the transducer. Which is the case for many programmable logic controllers linked with theese 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.

HART - Signal modulation basics

HART - Digital signal modulation by frequency modulation (bit is 1 : frequency = 1200 Hz ; bit is 0 : frequency = 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 basics of the HART compliant data transmission is to add the HART datas digital signal (configuration, parameters, datas calibration, diagnostics, services) to the analog signal of the measure in the 4-20 mA current loop. Then only one medium is used to transfer the both signals.

The digital datas 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 the measure).

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

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

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

When a peripheral (a programmable logic controller, a computer, a mobile terminal) wishes to exchange datas 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 datas.

From the terminal to the sensor, the HART modem modulates the datas received from the communication port. Then it adds this signal to the analog 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 datas to the communication port.

HART MODEM - Local link

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 datas through a RS232 serial link.

HART MODEM - Remote link

An automation engineer, an instrumention 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.

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 manager emulates all the signals - hardware and softwares - of the serial port of the machine used by the instrumentation software.

Using the TELNET protocol, the pilot of the virtual serial port redirects theese signals over the Ethernet 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, for instance, providing this functionality.

The server will use the datas received over ethernet to build the hardware and software signals to send to the serial port of the HART modem. Then, the modem will modulate the datas of the request, and will add this modulated signal to the analog signal of the measure. Finally, the result is a 4-20 mA current loop composite signal.

Next, the sensor will compute the datas of the request and will send the answer backwards.

HART MODEM - Cabling and wiring 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 analog convertor (DAC) working with the voltage to read and write the datas transferred over the 4-20 mA current loop.

Consenquently, when the HART modem needs to read datas 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 datas 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).

Last updated : July 04, 2010.