Page 246 - Instrumentation Reference Book 3E

230 Microprocessor- based and intelligent transmitters

for the physical layer in the HART document ing of faults and also that the status of the trans-
(HART Communication Foundation). mitter can be checked without having to go to its
installed position, take off its cover, and carry out
tests, or worse, disconnect it and take it to a
13.11 User experience with workshop for maintenance. Even having discon-
microprocessor-based and nected it and found no fault, the transmitter will
intelligent transmitters have been identified as one that requires atten-
tion. More seriously, it is quite likely that a fault
Reports of users’ experience are dominated by is introduced by the unnecessary handling.
references to differential pressure measurements In general, users have recognized a reduced
used for flow measurements. This arises not only need for repair and maintenance. This may be
because of the large number in use but also due in part to the communication facility of the
because these were the first microprocessor-based intelligent transmitters which facilitates the keep-
and intelligent transmitters to become available. ing of better maintenance records.
The ability to set the range of an instrument
before or at the time of installation was one of
the earliest, and still is one of the most valuable 13.12 Fieldbus function and
benefits of the new technology. From the user’s benefits
viewpoint, orders for instrumentation can be
released earlier because there is no penalty for Of the completely digital fieldbus technologies
ordering the wrong range and the holding of mentioned above only FOUNDATION Fieldbus
spares is greatly reduced. Perhaps the most dra- and Profibus-PA are used for process instrumen-
matic change has been in temperature transmit- tation such as valve positioners and the transmit-
ters, some of which can accommodate virtually ters discussed in this chapter. Both of these are
any type and range of thermocouple or resistance based on the IEC 61 158-2 physical layer standard
temperature detector likely to be used in a plant. giving them the unique capability to provide both
There are fewer reports of the benefits derived power and communication on the same pair of
from the ability to re-range the transmitter during wires, be intrinsically safe, and run as far as
plant operation except for some batch processes 1.9 km using ordinary twisted-pair cable. Other
where a large turn-down is involved and one fieldbus technologies are used in discrete factory
transmitter can be used instead of two. In such automation and building automation. The main
instances, the use of a single instrument not only difference between FOUNDATION Fieldbus and
saves the cost of an instrument but also reduces Profibus-PA is that FOUNDATION Fieldbus
the cost and complication of the installation. is not only a communications protocol, but also
The most significant benefit arising from the a programming language form building control
use of “intelligent” transmitters is in the commis- strategies.
sioning time. In some instances this more than
compensates for the additional cost of the trans- 13.12.1 FOUNDATIONTM Fieldbus and
mitters compared with their analog counter- Profibus-PA
parts. The improvement arises from the ability
to address the instrument from the control The IEC 61158-2 deals only with the physical
room or the marshalling cabinet, to confirm layer that is based on the Manchester coding
that the correct type and range has been principle of multiplexing the data stream with a
installed and that the system responds correctly clock signal in order to achieve synchronous data
when the transmitter output is forced to 4 or transmission. Data bits of 1 and 0 are represented
20 mA. With analog instrumentation, this would by falling and rising edges respectively trans-
involve obtaining access to the transmitter, per- mitted at 31.25 kbitls superimposed on the 9-32
haps with a ladder, and making adjustments VDC power. Because the Manchester waveform
while using a radio link with the control room. has a 50 percent duty cycle the d.c. power remains
The difference can be even more marked for sufficiently stable.
transmitters involved in a complex re-ranging Several devices may be multidropped on a net-
sequence, which can now be controlled without work. If the devices are not drawing power from
anyone being present at the point of measure- the bus as many as 32 are possible. If the devices
ment. are bus powered the number should be limited to
Although one might expect these more com- 12-16; that is also a good number for best perform-
plex instruments to be less reliable than the older, ance. When intrinsic safety is required the power is
simpler ones, user perceptions indicate the oppos- limited and therefore only four devices can be
ite. This may be due to the fact that the built-in connected on the hazardous area side segment
diagnostic routines provide an immediate warn- of a linear barrier such as the Smar SB302