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Cost-Efficient and Advanced HART and FF Communicators

I’m new to HART, what do I need to know?

HART Signal over 4-20mA

Basic Information 

HART is a waveform added to the 4-20mA current loop that transmits digital information between the instrument and a host.  A transmitter with HART capability sends its measurement output on the 4-20mA loop and can also send information to a host using the added waveform.  A host initiates the communication exchange and also includes a user interface for viewing the instrument data.  Before the telephone system went digital, the Caller ID feature used the same technology as HART to transmit digital information.

HART Bluetooth System Diagram

HART System Diagram

System Components

A HART Network consists of:

HART transmitter - the device that performs a measurement, a pressure transmitter for example.

Host - Hardware and Software that contains the User Interface.  A windows PC with DevCom2000 software for instance.

Loop Resistor - HART requires 200 to 1K of loop resistance.  A typical loop resister value is 250 ohms.

HART Modem - Device used to decode the HART signal into serial data for the Host to use.  One side of the modem is the HART connection, and the other side is an interface to the host device.  It can be USB or Bluetooth for example.

Device Descriptor (DD) - A file written by the device manufacturer that tells the Host what data is in the HART transmitters, how to get that data out, and how to display that data.


Communication issues sometimes occur but are usually simple to solve.  The error message provided by the host can narrow down the cause.

COM Port Error

The connection between the host device and the HART modem is not configured correctly.  When a HART modem is connected to a Windows device, a COM port is assigned to the modem by the Windows operating system.  This COM port must be entered into the host program.  This error means the COM port set in the host is not set to the COM port where the HART modem is located.  Use Windows Device Manager to determine the COM port assigned to the Modem then enter that value in the host software.

Bluetooth Device Not Found

This is also a error meaning the connection between the host device and the HART modem is not configured correctly.  For mobile devices that use Bluetooth, the host must know what Bluetooth device to use for the HART connection.  This error means the connection cannot be made from the host device to the HART modem.  Is the modem turned on?  Was the correct Bluetooth device selected in the host?

HART Device Not Found

The connection between the host device and modem is good.  The issue is between the HART modem and the HART transmitter.  Typically this cause is not enough loop resistance in the 4-20mA loop.  HART requires 200 to 1000 ohms of loop resistance.  A 250 ohm resistor is standard.  This error can also be caused when there are two hosts on the HART network.  In this case, one host needs to be designated the "Primary HART Master" and the other host is designated the "Secondary HART Master".  If both hosts are the same, communications will not occur.  Check the HART Master setting in each host to make sure they are not the same.

Device Descriptor Not Found

All physical connections are working properly and the host is communicating with the HART transmitter.  However, the DD file that the host relies on is missing.  A generic Device Descriptor can be used, but not all parameters and features of the HART transmitter are not available.  Contact the HART transmitter manufacturer to obtain the DD file that is needed.  Or contact the FieldComm Group to obtain the missing DD file.  Most hosts allow the user to add DD files to the host library. 

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