Page 248 - Instrumentation Reference Book 3E

232 Microprocessor-based and intelligent transmitters
Report distribution: When an event has System). Powerful control strategies can be built
occurred, such as a diagnostic alarm, and the by linking blocks from several devices together.
device receives the token it transmits its message, Field control is also the key to the scalability of
e.g., to the host. This eliminates the need for the the FCS. A traditional control system has finite
host to frequently poll a device to catch infre- resources thus adding field instruments and loops
quent events, thereby reducing the communica- make, it slower and forces costly expansion
tion load. of controllers and I/O-subsystems. However, in
System management is responsible for automatic a FCS, each added device prbvides more comput-
device detection and address assignment, making ing power and memory, making the system larger
with minor negative impact on performance.
the addition and removal of devices a plug-and- Because UO and controller hardware is minimized,
play operation eliminating human errors. System expansion is comparatively economical. Interoper-
management also takes care of time synchroniza- ability and level of system integration also bene-
tion between all devices as well as block and fits because the semantics of the data is defined,
publishing schedule to ensure an exact execution. e.g., for any manufacturer, controller mode is set
To ensure interoperability all device types have
their communications conformance tested. In by the MODE-BLK parameter, and the code
for Manual is always the same.
addition to this all device-types have their func- There is some doubt whether in safety-critical
tion block programming checked and registered.
applications communication between transmitter,
controller, and final control element via a shared
13.12.1.2 Profibus-PA fieldbus is acceptable. At the time of writing no
fieldbus technology has been approved for certi-
Profibus allows multiple masters to be connected fied critical control systems, nor for field control,
to the network but only one can write values. A nor for device configuration, measurement
token is passed from master to master, and the variables, or actuation. Once more, reliability
master that holds the token has the right to access for multidrop networks is available: this may be
the bus. The token rotation time is not fixed, but considered. Certainly, the increased diagnostics
varies at an average of 50 percent. A master-slave that can be communicated using fieldbus can
communication relationship is always used. increase both safety and availability of systems,
The master requests a value such as the process and control in the field is more fault-tolerant than
variable requested by the central controller from the centralized controls of the legacy DCS.
a transmitter on the network, called slave, which
responds with the value. One master can also
write, e.g., a manipulated variable into a valve 13.12.3 Future of analog instruments
positioner. Although the number of microprocessor-based
To ensure interoperability all device types have
their communications conformance tested. and intelligent transmitters being installed is
rising steadily, it will be a long time before
the transition is complete because of the large
installed base of traditional instruments. Smar
13.12.2 Field-mounted control
provides a range of current-to-fieldbus (IF302),
The FOUNDATION Fieldbus specification is fieldbus-to-current (FI302) and fieldbus-to-
unique among fieldbus technologies in that it is pneumatic signal converters (FP302) for interfacing
the only one to specify a communications proto- between the old analog world and the FOUNDA-
col, but also a programming language for build- TION or Profibus fieldbus. These are ideal not
ing control strategies. This is a graphical only for migration of an existing plant, but also
programming language using thc familiar func- to interface devices not yet available in a fieldbus
tion block concept: an extensive library of function version in a new installation. There was initially
blocks for inputs, outputs, control, arithmetic, some concern regarding the use of software-based
and selection, etc. Have been specified, along with instruments in safety-critical applications because
guidelines to how non-standard function blocks of the possibility that they may fail in some unex-
shall be designed in order to be interoperable. pected manner. However, several years of reliable
The function blocks primarily execute in field operation and refined design techniques have
devices such as transmitters and positioners resulted, in that the first few safety-certified
thereby reducing the need for. and off- HART devices have now emerged.
loading, centralized controllers. FOUNDATION
Fieldbus thus creates a new architecture taking 13.12.4 Sensor validation
the old concept of DCS a step further, more
distributed, more digital. This networked archi- Many of the diagnostic routines applied in early
tecture is often referred to as FCS (Field Control microprocessor-based and intelligent transmitters