Page 242 - Instrumentation Reference Book 3E

226 Microprocessor-based and intelligent transmitters
tools and technical staff to operate them may meat, and equipment data are transferred digi-
represent a significant investment. This will deter tally. The analog signal remains unaffected and
manufacturers from becoming involved until an can be used for control in the normal way. HART
obvious standard emerges. data give access to maintenance, diagnostic, and
The following outline description and informa- other operational data.
tion is reproduced, with permission. from the
bulletin prepared by the HART Communication Multi-drop This mode requires only a single
Foundation.
pair of wires and, if applicable, safety barriers
and an auxiliary power supply for up to 15 field
13.10.4 The HART protocol devices (see Figure 13.35). Multi-drop connection
is particularly useful for supervising installations
13.10.4.1 Method of operution
that are widely spaced, such as pipelines, feeding
The HART protocol operates using the frequency stations. and tank farms.
shift keying (FSK) principle, which is based on HART instruments can be used in either mode.
the Bell 202 (Bell 1976) Cominunication Stand- In point-to-point operations, the field device has
ard. The digital signal is made up from two address 0, setting the current output to 4-20 mA.
frequencies (1200 Hz and 2200 Hz) representing In multi-drop mode, all device addresses are
bits 1 and 0, respectively. Sine waves of these greater than zero and each device sets its output
frequencies are superimposed on the d.c. analog current to 4mA. For this mode of operation,
signal cables to give simultaneous analog and controllers and indicators must be equipped with
digital communications. Because the average a HART modem.
value of the FSK signal is always zero, the HART devices can communicate using coni-
4-20mA signal is not affected (Figure 13.33). pany-leased telephone lines (Bell 1973). In this
This produces genuine, simultaneous commu- situation only a local power supply is required
nication with a response time of approximately by the field device and the master can be many
500 ins for each field device, without interrupting kilometers away. However, most European coun-
any analog signal transmission that might be tak- tries do not permit Bell 202 signals to be used with
ing place. national carrier equipment, so HART products
Up to two master devices may be connected to should not be used in this way.
each HART loop. The primary one is generally a Any number of field devices can be operated on
management system or a PC while the secondary leased lines, as long as they are individually sup-
one can be a hand-held terminal or laptop com- plied with auxiliary power, independently of the
puter. A standard hand-held terminal (called the communication. If only one power supply is used
HART Communicator) is available to make field for all the field devices, the number is limited
operations as uniform as possible. Further net- to 15.
working options are provided by gateways.
13.10.4.17 HART protocol structure
Point-to-point Figure 13.34 shows some HART follows the basic open systems interconnec-
examples of point-to-point mode. The conven- tion (OSI) reference model, developed by the Inter-
tional 4-20 mA signal continues to be used for national Organization for Standardization (ISO)
analog transmission while measurement, adjust-
(DIN IS0 7498). The OS1 model provides the
structure and elements of a communication sys-
tem. The HART protocol uses a reduced OS1
Simultaneous analog and digital model, implementing only layers 1, 2, and 7 (see
Table 13.12).
Layer 1, the physical layer. operates on the
FSK principle, based on the Bell 202 Communi-
cation Standard:
(1) Data transfer rate: 1200 bitls
(2) Logic “0” frequency: 2200 Hz
(3) Logic “1” frequency: 1200Hz
1200 Hz 2200 Hz
“1” ‘0” The vast majority of existing wiring is used for
this type of digital communication. For short
distances, unshielded, 0.2 mm2 two-wire lines are
Figure 13.33 Becausethe mean harmonic signal value is
zero, digital communication made no difference to any suitable. For longer distances (up to 1500m),
existing analog signal. single, shielded bundles of 0.2 mm2 twisted pairs