User's Manual
Communication
PROFIBUS PA
Introduction
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Siemens ST PCS 7 · November 2007
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Overview
Direct interfacing of the devices in the field, especially in the
hazardous area, together with the information content of the
communication, are of significant importance in the process in-
dustry. PROFIBUS PA, which permits both digital data transmis-
sion and the power supply on a two-wire line with the intrinsi-
cally-safe MBP transmission technology (Manchester Coded;
Bus Powered) is tailored to these requirements. It is optimally
suitable for direct integration of the pneumatic actuators, sole-
noid valves and sensors positioned in operating environments
up to Ex zone 1 or 0 into the process control system.
The typical response time of a transmitter of approx. 10 ms indi-
cates that short cycle times can be achieved with the
PROFIBUS PA even in the case of a segment configuration with
up to 31 devices. Practically all typical applications of the pro-
cess industry can be implemented - both with small or large
plant distances. Bidirectional communication and high informa-
tion content allow enhanced diagnostics for fast and exact fault
detection and elimination. The standardized communications
services guarantee interoperability and replaceability between
multi-vendor field devices and remote parameterization of the
field devices during operation.
The PROFIsafe profile allows seamless integration of safety
communication into the PROFIBUS PA. You need not configure
a separate safety bus for your safety-related applications. The
PROFIBUS PA with the PROFIsafe profile is incorporated in
"Safety Integrated for Process Automation". This comprehensive
range of products and services from Siemens for failsafe, fault-
tolerant applications in the process industry offers you attractive
and cost-effective alternatives to separate safety systems.
You can define the degree of redundancy separately for the con-
troller, fieldbus and I/O levels of your plant depending on the au-
tomation task and the derived safety requirements, and match
them to the field instrumentation (Flexible Modular Redundancy,
FMR). You can find an overview of the redundant architectures
of PROFIBUS PA under "Design".
The PROFIBUS PA is based on electrical transmission compo-
nents. A shielded two-wire cable is used for digital data trans-
mission and for the power supply. With line, tree and ring topol-
ogies, it is possible to configure networks with spur lines up to
120 m and bus segments up to approx. 1.9 km for operating en-
vironments up to Ex zone 2. With bus segments for hazardous
areas in line with zone 1, the max. possible length per spur line
is reduced to 30 m and per bus segment to 1 km. The line
lengths actually achievable in practice depend on the number of
PA devices and their current consumption. Bus segments are
terminated either automatically (with active field distributors AFD
in the case of ring topologies) or with the passive terminating el-
ement for PROFIBUS PA (SpliTConnect terminator).
The DP/PA Link is preferred for the gateway from PROFIBUS PA
to PROFIBUS DP. When using the DP/PA Link, the transmission
rate on the PROFIBUS DP is independent of the subordinate
PROFIBUS PA segments. The configuration of the DP/PA Link
depends on the operating environment (Ex zone) or the selected
redundancy architecture. The types of coupler described in the
Section "Gateways - DP/PA link and DP/PA coupler" are used for
the configuration. With a small amount of data (small quantity
framework) and low timing requirements, the DP/PA coupler can
also be operated in stand-alone mode as a gateway.
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Benefits
Advantages provided by distributed field automation with appli-
cation of the PROFIBUS PA profile included low hardware over-
head, cost-effective engineering, increased operational safety
and problem-free maintenance. These advantages are under-
lined by the following features:
• Modularity and uniformity from the sensor up to the control
level permit new plant concepts
• Implementation of intrinsically-safe applications through use
of the fieldbus in hazardous areas
• Flexible Modular Redundancy (FMR) from the automation sys-
tem (controller) down to the PROFIBUS PA field device with re-
dundant architectures for environments up to Ex zone 2 (ring
and line topologies with coupler redundancy)
• Safety-related and fault-tolerant applications with low device
and cabling overheads
• Reduced configuration costs through simple, central engi-
neering of the field devices (PROFIBUS PA and HART with
SIMATIC PDM, also cross-vendor)
• Simple installation using two wire cable for common power
supply and data transmission
• Reduced commissioning costs through simplified loop check
• Low servicing costs thanks to simple wiring and comprehen-
sive diagnostics facilities
© Siemens AG 2007