Profibus (PROcess FIeld BUS) is a widely accepted international networking standard,
commonly found in process control and in large assembly and material handling
machines. It supports single-cable wiring of multi-input sensor blocks, pneumatic valves,
complex intelligent devices, smaller sub-networks (such as AS-i), and operator interfaces.
Profibus is nearly universal in Europe and also popular in North America, South
America, and parts of Africa and Asia. It is an open, vendor-independent standard. It
adheres to the OSI model and ensures that devices from a variety of different vendors can
communicate together easily and effectively. It has been standardized under the German
National standard as DIN 19 245 Parts 1 and 2 and, in addition, has also been ratified
under the European national standard EN 50170 Volume 2.
The development of Profibus was initiated by the BMFT (German Federal Ministry of
Research and Technology) in cooperation with several automation manufacturers in 1989.
The bus interfacing hardware is implemented on ASIC (application specific integrated
circuit) chips produced by multiple vendors, and is based on the RS-485 standard as well
as the European EN50170 Electrical specification. The standard is supported by the
Profibus Trade Organization, whose website can be found at www.profibus.com.
Profibus uses nine-pin D-type connectors (impedance terminated) or 12 mm quick-disconnect connectors. The number of nodes is limited to 127. The distance supported
is up to 24 km (with repeaters and fiber-optic transmission), with speeds varying from
9600 bps to 12 Mbps. The message size can be up to 244 bytes of data per node per
message (12 bytes of overhead for a maximum message length of 256 bytes), while the
medium access control mechanisms are polling and token passing.
Profibus supports two main types of devices, namely, masters and slaves:
1. Master devices control the bus and when they have the right to access the bus,
they may transfer messages without any remote request. These are referred to
as active stations.
2. Slave devices are typically peripheral devices i.e. transmitters/sensors and
actuators. They may only acknowledge received messages or, at the request of a
master, transmit messages to that master. These are also referred to as passive
stations.
There are several versions of the standard, namely, Profibus DP (master/slave),
Profibus FMS (multi-master/peer-to-peer), and Profibus PA (intrinsically safe).
• Profibus DP (distributed peripheral) allows the use of multiple master devices,
in which case each slave device is assigned to one master. This means that
multiple masters can read inputs from the device but only one master can write
outputs to that device. Profibus DP is designed for high-speed data transfer at
the sensor/actuator level (as opposed to Profibus FMS which tends to focus on
the higher automation levels) and is based around DIN 19 245 parts 1 and 2
since 1993. It is suitable as a replacement for the costly wiring of 24 V and
4–20 mA measurement signals. The data exchange for Profibus DP is generally
cyclic in nature. The central controller, which acts as the master, reads the input
data from the slave and sends the output data back to the slave. The bus cycle
time is much shorter than the program cycle time of the controller (less than
10 ms).
• Profibus FMS (fieldbus message specification) is a peer-to-peer messaging
format, which allows masters to communicate with one another. Just as in
Profibus DP, up to 126 nodes are available and all can be masters if desired.
FMS messages consume more overhead than DP messages.
• ‘COMBI mode’ is when FMS and DP are used simultaneously in the same
network, and some devices (such as Synergetic’s DP/FMS masters) support
this. This is most commonly used in situations where a PLC is being used in
conjunction with a PC, and the primary master communicates with the
secondary master via FMS. DP messages are sent via the same network to I/O
devices.
• The Profibus PA protocol is the same as the latest Profibus DP with V1
diagnostic extensions, except that voltage and current levels are reduced to
meet the requirements of intrinsic safety (class I division II) for the process
industry. Many DP/FMS master cards support Profibus PA, but barriers are
required to convert between DP and PA. PA devices are normally powered by
the network at intrinsically safe voltage and current levels, utilizing the
transmission technique specified in IEC 61158-2 (which Foundation Fieldbus
H1 uses as well).
Profibus PA/DP/FMS overview 305
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