Rockwell Automation 1770-HT8 SMART TRANSMITTER - Manual (Page 19)

Introducing the Smart Transmitter
Chapter 1
Interface
1-8
The HART field communications protocol carries digital information with
the analog signal over industry-standard 4-20 mA process control loops.
Both the digital and analog signals occur simultaneously on the same loop
wiring without disrupting the process signal.
The HART protocol uses the Frequency Shift Keying technique, based on
the Bell 202 communication standard. Digital communication is
accomplished by superimposing a frequency signal over the 4-20 mA
current, as shown in Figure 1.9. Two individual frequencies, 1200 and 2200
Hz, represent the digits 1 and 0. The sine wave formed by the two frequency
levels has an average value of zero, so digital communication takes place
without disruption to the analog signal.
Figure 1.9
Analog and Digital Signals on 4Ć20 mA Current
90047
+0.5 mA
0
-0.5 mA
Analog
Signal
1200 Hz
1"
2200 Hz
0"
Average Current Change During Communication = 0
Field devices (transducers, actuators) can use the HART protocol to
transmit or receive a process variable as a 4-20 mA analog signal at the
same time as they are transmitting or receiving device or process data (e.g.
smart pressure, temperature, density, etc.) as a modulated digital signal.
The analog signal, with its faster update rate, can be used for control, while
the digital signal can be used for diagnostic, maintenance and additional
process data. Communication can be in either poll/response or burst
transmission mode.
The HART protocol supports digital communication from both a control
system and a hand-held communications device. It also allows multidrop
networking by which several smart HART field devices can be connected
to a single twisted-pair wire, and can operate over leased telephone lines.
The HART Protocol