- 4 -
Since the energy balance added to the current loop
virtually equal to zero and the frequency used for the
communication very high compared to that of the
processdynamic,nodisturbanceorinterferenceoccurs
on the analog process signal.
TheLayer2,theDataLinkLayer,providestoformand
check the frame of the messages, accordingly to the
Hart protocolspecification.Theframeincludesadouble
paritycheck,horizontal,at levelofeachbytetransmitted,
and vertical, in form of a parity byte added at the end of
theframe,inordertoinsurethemaximumdataintegrity.
The figure below gives the structure of a typical frame.
Thecommunication betweentheHandHeldCommuni-
cator and the field devices, like Smart Transmitters, is
based on HART protocol, that permits simultaneous
transmissionoftheindustry-standard4to20mA analog
signal and of the digital signals carrying the
communication.
The HART protocol follows the OSI ( Open Systems
Interconnection) reference model proposed by the
ISO ( International Standard Organization ) but uses a
collapsed OSI model implementing only the 1, 2 and 7
layers.Theotherlayers are notnecessary forthistype
of communication.
The Layer 1, the Physical Layer, physically connects
thedevices.Itis basedontheBell202FSK(Frequency
Shift Keying ) standard, a ±0.4 mA signal modulation
superimposed on the 4 to 20 mA analog output signal.
TheDataTransferRateis1200Baud.Twofrequencies
,1200and2200Hz,insinusoidalform,areusedtocode
respectively the bit "1" and "0. The figure below gives
themodulationenveloperespectivelyfora"1"anda"0"
bit.
Preamble SD AD CD BC RC Data Parity
Contents
Response code
Byte count
Command code
Addresses
Start Delimiter
Preamble ( 5 to 20 "FF" )
The Data Link Layer is also responsible for the
communication between the field devices and the
configurators, either the Hand Held Communicator, a
Secondary Master, or the P.C. Configurator, a Primary
Masterandoftheissuingofappropriateerror messages
in case of communication malfunction.
TheLayer7,theApplicationLayer isbasedontheuse
of HARTCommands,asetofcommands senttoafield
device in order to obtain data or information and to
remotely change configuration's parameters.
TheHart CommandSetisstructuredinthreeclassesof
commands:
Universal commands that are implemented and then
recognized by all field devices irrespective of the
manufacturer: the implementation of this class of
commands is mandatory for each manufacturer using
theHART Protocol.Thisclass includesthecommands
of reading of the process variable and those of reading
ofUniversalInformationliketheTag, rangesandlimits,
date and message,Serial Number,etc.
CommonPracticeCommandsisaclassofcommands
commonly used by a large number of smart devices :
unliketheUniversalCommandstheirimplementationis
not mandatory but the use of this set of commands
increasesthecompatibilitybetweendevicesusing HART
protocol. This class includes commands to change
common parameters like range values, engineering
units, to perform loop test and so on.
Device-Specific Commands is a class of commands
implemented for a specific device and therefore not
commontoothertypeofequipment.Thisclass includes
the commands related to the specific design of the
device,suchasthecommandforthesensortrimmingor
the command to read and interpret the product code,
the sensor materials, etc.
Another small set of commands is reserved for the
manufacturerforuseduringthemanufacturingprocess.
Any command sent by a Master Device, either the
Primary Master or the Secondary Master, requires a
responsemessagethatnecessarilyincludes a Specific
Response Code: the response code gives information
about the correct interpretation and execution of the
received command.
The response code pertains to the Data Link Layer, for
the part concerning the communication, or to the
ApplicationLayer,forthepartconcerningtheapplication
andincaseoferrorsanerror message will be issued: a
specificsectionof thismanuallists theerrormessages.
+ 0.4 mA
- 0.4 mA Bit = 1 Bit = 0
f = 1200 Hz f = 2200 Hz
Level of the
analog signal
COMMUNICATION PROTOCOL
