10 - HART overview

Abstract

This chapter focuses on the fundamental operation of highway addressable remote transducer (HART). The HART protocol, originally developed by Rosemount, is a typical smart instrumentation fieldbus. It is regarded as an open standard, available to all manufacturers. Its main advantage is that it enables an instrumentation engineer to keep the existing 4-20 mA instrumentation cabling and to use, simultaneously, the same wires to carry digital information superimposed on the analog signal. This enables most companies to capitalize on their existing investment in 4-20 mA instrumentation cabling and associated systems and to add further capability of HART without incurring major costs. HART is a hybrid analog and digital protocol, as opposed to most fieldbus systems, which are purely digital. The HART protocol uses the frequency shift keying (FSK) technique based on the Bell 202 communications standard. Two individual frequencies of 1200 and 2200 Hz, representing digits 1 and 0 respectively, are used. The average value of the sine, which is superimposed on the 4-20 mA signal, is zero. Hence, the 4-20 mA analog information is not affected. The HART protocol offers two formats for digital transmission of data, namely, poll/response mode and burst or broadcast mode.