Multidrop Communication; Appendix A Principle Of Operation; Hart Communication - Fisher FIELDVUE DLC3010 Instruction Manual

Instruction Manual
D102748X012
Appendix A Principle of

HART Communication

The HART (Highway Addressable Remote Transducer) protocol gives field devices the capability of communicating
instrument and process data digitally. This digital communication occurs over the same two‐wire loop that provides
the 4-20 mA process control signal, without disrupting the process signal. In this way, the analog process signal, with
its faster update rate, can be used for control. At the same time, the HART protocol allows access to digital diagnostic,
maintenance, and additional process data. The protocol provides total system integration via a host device.
The HART protocol uses the frequency shift keying (FSK) technique based on the Bell 202 communication standard. By
superimposing a frequency signal over the 4-20 mA current, digital communication is attained. Two individual
frequencies of 1200 and 2200 Hz are superimposed as a sinewave over the 4-20 mA current loop. These frequencies
represent the digits 1 and 0 (see figure A‐1). The average value of this sinewave is zero, therefore no DC value is added
to the 4-20 mA signal. Thus, true simultaneous communication is achieved without interrupting the process signal.
Figure A‐1. HART Frequency Shift Keying Technique
+0.5 mA
0
-0.5 mA
AVERAGE CURRENT CHANGE DURING COMMUNICATION = 0
A6174
The HART protocol allows the capability of multidropping, networking several devices to a single communications line.
This process is well suited for monitoring remote applications such as pipelines, custody transfer sites, and tank farms.

Multidrop Communication

"Multidropping" refers to the connection of several digital level controllers or transmitters to a single communications
transmission line. Communication between the host and the field instruments takes place digitally with the analog
output of the instruments deactivated. With the HART communications protocol, up to 15 field instruments can be
connected on a single twisted pair of wires or over leased phone lines. Multidrop installations are not recommended
where intrinsic safety is a requirement.
The application of a multidrop installation requires consideration of the update rate necessary from each instrument,
the combination of instrument models, and the length of the transmission line. Communication with the field
instruments can be accomplished with commercially available Bell 202 modems and a host implementing the HART
protocol. Each instrument is identified by a unique address (1-15) and responds to the commands defined in the HART
protocol.
Figure A‐2 shows a typical multidrop network. Do not use this figure as an installation diagram. Contact your Emerson
Process Management sales office with specific requirements for multidrop applications.
OperationA‐
1200 Hz 2200 Hz
"1" "0"
ANALOG
SIGNAL
Principle of Operation
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