Communication Protocol - ABB 691HT Operating Instructions Manual

COMMUNICATION PROTOCOL

The communication between the Hand Held Communi-
cator and the field devices, like Smart Transmitters, is
based on HART protocol, that permits simultaneous
transmission of the industry-standard 4 to 20 mA 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. The other layers are not necessary for this type
of communication.
The Layer 1, the Physical Layer, physically connects
the devices. It is based on the Bell 202 FSK ( Frequency
Shift Keying ) standard, a ±0.4 mA signal modulation
superimposed on the 4 to 20 mA analog output signal.
The Data Transfer Rate is 1200 Baud. Two frequencies
, 1200 and 2200 Hz, in sinusoidal form, are used to code
respectively the bit "1" and "0. The figure below gives
the modulation envelope respectively for a "1" and a "0"
bit.
f = 1200 Hz
+ 0.4 mA
Level of the
analog signal
- 0.4 mA
Bit = 1
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
process dynamic, no disturbance or interference occurs
on the analog process signal.
The Layer 2, the Data Link Layer, provides to form and
check the frame of the messages, accordingly to the
Hart protocol specification. The frame includes a double
parity check, horizontal, at level of each byte transmitted,
and vertical, in form of a parity byte added at the end of
the frame, in order to insure the maximum data integrity.
The figure below gives the structure of a typical frame.
Preamble SD AD CD BC RC
Start Delimiter
Preamble ( 5 to 20 "FF" )
- 4 -
f = 2200 Hz
Bit = 0
Data Parity
Contents
Response code
Byte count
Command code
Addresses
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
Master and of the issuing of appropriate error messages
in case of communication malfunction.
The Layer 7, the Application Layer is based on the use
of HART Commands, a set of commands sent to a field
device in order to obtain data or information and to
remotely change configuration's parameters.
The Hart Command Set is structured in three classes of
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
the HART Protocol. This class includes the commands
of reading of the process variable and those of reading
of Universal Information like the Tag, ranges and limits,
date and message,Serial Number,etc.
Common Practice Commands is a class of commands
commonly used by a large number of smart devices :
unlike the Universal Commands their implementation is
not mandatory but the use of this set of commands
increases the compatibility between devices using 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
common to other type of equipment. This class includes
the commands related to the specific design of the
device, such as the command for the sensor trimming or
the command to read and interpret the product code,
the sensor materials, etc.
Another small set of commands is reserved for the
manufacturer for use during the manufacturing process.
Any command sent by a Master Device, either the
Primary Master or the Secondary Master, requires a
response message that necessarily includes 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
Application Layer, for the part concerning the application
and in case of errors an error message will be issued: a
specific section of this manual lists the error messages.