
PLX51-HART-4I / PLX51-HART-4O
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HART Input/Output
What is HART?
Multidrop Field Devices
User Manual
ProSoft Technology, Inc.
Page 122 of 127
7 What is HART?
7.1 Introduction to HART
HART is an acronym for Highway Addressable Remote Transducer. HART is able to
transfer digital information across a standard 4-20 mA loop, by superimposing the
digital data on the analog signal using Frequency Shift Keying (FSK). As the name
implies FSK changes the frequency of the carrier to represent the binary data 0 or 1.
A frequency of 1200 Hz represents a logical 1 and a frequency of 2200 Hz
represents a logic 0. Therefore, HART has a maximum transfer rate of 1200 bits per
second (bps).
The amplitude of the FSK modulation is typically 1mA. Due to the relatively high
frequency in comparison to changes of the analog signal, a low pass filter can be
employed to prevent the modulation from affecting the analog signal.
Figure 7.1. – HART FSK Modulation