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15.1 OVERVIEW OF HART COMMUNICATION
HART (highway addressable remote transducer) is a digital communication system in which two frequencies are
superimposed on the 4 to 20 mA output signal from the transmitter. A 1200 Hz sine wave represents the digit 1,
and a 2400 Hz sine wave represents the digit 0. Because the average value of a sine wave is zero, the digital sig-
nal adds no dc component to the analog signal. HART permits digital communication while retaining the analog
signal for process control.
The HART protocol, originally developed by Fisher-Rosemount, is now overseen by the independent HART
Communication Foundation. The Foundation ensures that all HART devices can communicate with one another.
For more information about HART communications, call the HART Communication Foundation at (512) 794-0369.
The internet address is http://www.hartcomm.org.
15.2 HART INTERFACE DEVICES
HART communicators allow the user to view measurement data (pH, ORP and temperature), program the transmit-
ter, and download information from the transmitter for transfer to a computer for analysis. Downloaded information
can also be sent to another HART transmitter. Either a hand-held communicator, such as the Rosemount Model 275,
or a computer can be used. HART interface devices operate from any wiring termination point in the 4 - 20 mA loop.
A minimum load of 250 ohms must be present between the transmitter and the power supply. See Figure 15-1.
MODEL 3081 pH/ORP SECTION 15.0
THEORY - REMOTE COMMUNICATIONS
SECTION 15.0
THEORY - REMOTE COMMUNICATIONS
15.1 Overview of HART Communications
15.2 HART Interface Devices
15.3 AMS Communication
4-20 mA + Digital
250
ohm
Control System
Computer
Model 3081 pH
Smart
Transmitter
Bridge
Hand Held
Communicator
(“Configurator”)
FIGURE 15-1. HART Communicators.
Both the Rosemount Model 275 and a computer can be used to communicate with a
HART transmitter. The 250 ohm load (minimum) must be present between the transmit-
ter and the power supply.