Chemical Engineering October 2014 - 40

Cover Story
available. HART superimposes a
small alternating component on
the 4-20-mA output, with information
carried by frequency-shift keying:
1,200 Hz indicates a digital 1
and 2,200 Hz represents a 0. Since
this carrier signal sums to zero over
time, it has no effect on the 4-20mA
loop. The HART signal allows
two-way communication between
the control system (the " master, " in
networking terminology) and the
transmitter (the " slave " ) at a speed
of 1,200 bits/s. As stated by the
HART Foundation (Austin, Tex.;
www.hartcomm.org), which controls
the specification, " The digital
signal contains information from
the device including device status,
diagnostics, additional measured
or calculated values, [and so on].
Together, the two communication
channels provide a low-cost and
very robust complete field communication
solution that is easy to use
and configure. " [1] A handheld communicator
can also be connected to
a transmitter for local setup and
maintenance purposes.
HART has the advantage of using
the existing 4-20-mA field wiring,
which makes it simple and inexpensive
to set up. But this is also a
disadvantage. Because HART uses
a separate 4-20-mA loop for each
transmitter, it is expensive to add
additional transmitters to an existing
system.
There is also a multidropped version
of HART in which there are
no 4-20-mA loops; all transmitters
connect to the same cable, which
carries sufficient current to provide
4 mA to power all the connected
devices, and all process-variable
data are sent digitally in response
to interrogations from the master;
though this architecture is seldom
utilized. A wireless-HART protocol
is also available, as discussed later
in the article.
Digital fieldbuses. Since the mid1980s,
numerous networks that can
be classified in computer terms as
local area networks have been introduced;
currently in the process control
field the most popular are Foundation
Fieldbus and Profibus PA.
There are many others used in disEmerson
Process
Management
Figure 5. A remote seal and capillary
system consists of external sensing
diaphragms (which can be flush
or extended, as shown) mounted to
the process and connected to the DP
transmitter either directly or via oil-filled
capillaries
crete control applications and office
communications, including Ethernet,
MAP, CANbus and more, but these
are outside the scope of this article.
Digital fieldbuses make available
a great deal of information inside
the connected field devices; this can
greatly speed diagnostics. But one
of the biggest advantages of digital
field buses is that they greatly
reduce the expense of adding additional
field devices; because field devices
are tapped into a single cable,
the cost of a separate cable for each
field device is eliminated.
Foundation Fieldbus. Foundation
Fieldbus is controlled by the
Fieldbus Foundation (Austin, Tex.;
www.fieldbus.org). While there are
two versions of Foundation Fieldbus
(with more on the way), the
one used in process control applications,
and the most common
implementation for transmitters
is an H1 segment design. This uses
a two-conductor cable with devices
multidropped from it, and is entirely
digital. It uses a peer-to-peer
protocol: devices can communicate
with each other without a host,
and they can initiate communications
without a specific host command.
For example, if one device
experiences a problem, it can send
an alarm.
Because Foundation Fieldbus
uses a peer-to-peer protocol, individual
devices (valve actuators, for
example) can contain control software
(function blocks) and make
38 ChemiCal engineering www.Chemengonline.Com oCToBer 2014
decisions based on data from other
field devices. This makes it possible
to set up truly distributed control,
often called control in the field, although
such arrangements should
be attempted only by those with a
great deal of experience.
Profibus. Profibus is controlled
by Profibus and Profinet International
(PI; Karlsruhe, Germany;
www.profibus.com). Like Foundation
Fieldbus, Profibus' devices
are multidropped from a two-conductor
cable. There are several
versions of Profibus for different
purposes; the one most used in the
process industries for transmitters
is Profibus-PA (The name Profibus
originally meant Process Field Bus,
and the PA is for Process Automation).
Profibus-PA uses a master/
slave protocol: individual devices
respond only when interrogated by
the central master.
Both Foundation Fieldbus H1
and Profibus-PA provide a data
rate of 31.25 kbits/s, have a maximum
cable length of 1,900 m, and
can support up to 32 devices per
cable segment; though often device
counts are much lower on the segment
to support hazardous-area
installations as well as bandwidth
and expansion requirements. Power
is supplied to the field devices over
the cable.
Have you considered wireless?
One way to substantially reduce
wiring costs for transmitters is to
skip all wired connections, analog
or digital, and use a wireless system.
Alternatively, wireless can be
used to add capabilities to an existing
wired system: While HART is
an industry standard, of the 30 million
wired installed HART instruments,
less than 10% have remote
access to secondary data (checking
and re-ranging are done with handheld
units). There are now wireless
adaptors that can be plugged into a
HART-equipped 4-20-mA transmitter
and make it part of a wireless
network, unlocking configuration
data, status information, calibration
dates, and process data, both
the main PV output and internal
data like temperature - without
http://www.profibus.com http://www.hartcomm.org http://www.fieldbus.org http://www.Chemengonline.Com
Chemical Engineering October 2014

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