http://www.analog.com/library/analogDialogue/archives/46-10/hart_modems.html
HART Communication Networks Are Improved by Small, Flexible, Low-Power Modem ICs
The need to measure, control, and communicate with machinery and equipment has existed since the earliest days of the Industrial Revolution, with instrumentation systems employing sensors and actuators becoming the backbone of the modern manufacturing plant. Communication employing 4-mA-to-20-mA analog current signals to carry data and settings over wires has long been in widespread use. But instrumentation has matured from those purely analog systems to the “smart” systems used today, augmenting communications capability by the likes of the HART® (highway addressable remote transducer) protocol. Simply put, dc and low-frequency current signals are modulated by independent, higher-frequency signals that switch between a pair of frequencies (Figure 1)—a technique known as frequency-shift keying (FSK).
Figure 1. HART communication.
This article describes the technology’s implementation, provides some application examples, discusses some of the devices employed by modern silicon integrated-circuit technology to aid the system designer, and—to illustrate the technology—introduces today’s most compact, lowest power, and widest supply-voltage, fully compliant HART modem (modulate-demodulate) IC—the Analog Devices AD5700.
——————————————————————————————————————-