Information Technology Reference
In-Depth Information
19.4 Exercises
19.4.1
The evolution of instrumentation transmission has generally progressed:
(a)
3-15 psi
4-20 mA
serial link
Fieldbus
→
→
→
(b)
4-20 mA
3-15 psi
serial link
Fieldbus
→
→
→
(c)
3-15 psi
serial link
4-20 mA
Fieldbus
→
→
→
(d)
serial link
3-15 psi
4-20 mA
Fieldbus
→
→
→
19.4.2
A 500
resistor at the end of a 4-20mA transmission signal will give which volt-
age range:
Ω
(a)
0-5 V
(b)
0-10 V
(c)
1-5 V
(d)
2-10 V
19.4.3
Which field bus is used in the automotive industry:
(a)
CAN bus
(b)
Profibus
(a)
FOUNDATION
(b)
WorldFIP
19.4.4
Outline the advantages of Fieldbus compared with traditional methods. This
should include topology flexibility, interoperatability, wiring costs, parameter
resolution, and so on.
19.4.5
Discuss how Fieldbus will change the operation of a large instrumentation net-
work. Outline the functions that are likely to be controlled by the DCS and which
are likely to be controlled locally, in the field (refer to Figure 19.5).
19.5 Notes from the author
The instumentation industry has moved over the years from instrumentation networks made
from dumb instruments which reported back to a central controller, to smart instruments
with distributed control. Fieldbus is really the first true implementer of totally distributed
systems, but as the scope of the Fieldbus is limited to areas, there is still a need for a global
control system (such as a DCS). The Fieldbus is excellent at allowing local control and pa-
rameter conversion, but is not so good at providing a global control strategy. This disadvan-
tage is outweighed by reduced cabling costs, as Fieldbus connects onto a bus, and devices
easily connect to the bus.
Future instrumentation networks have no need to involve a complex main controller, as
the instruments themselves have the power to locally control. The function of the main con-
troller will change from getting involved with the low-level operations of instrumentation to
the high-level functions of advanced control, interarea control, centralised configuration,
alarm filtering and maintaining a global database.
Serial communication, such as RS-485, has allowed for multidrop serial communication
networks, and has proved to be an excellent method of providing a highly reliable, fast com-
munications channel. Unfortunately, it is still basically a communication channel and most
