wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

32-4 Industrial Communication Systems
32.2.2 Manchester Bus Powered
Manchester Bus Powered (MBP) stands for transmission technology with the following attributes:
• Manchester Coding (M)
• Bus Powering (BP)
MBP is synchronous transmission with a defined transmission rate of 31.25.kb/s and Manchester coding.
This transmission technology is frequently used in process automation as it satisfies the key demands of
the chemical and petrochemical industries for intrinsic safety and bus powering using two-wire technology.
This means that Profibus can also be used in potentially explosive areas with the attribute intrinsically
safe.
The intrinsically safe transmission technology of MBP is usually limited to a specific segment (field
devices in hazardous areas) of a plant, which are then linked to the RS485 segment (control system and
engineering devices in the control room) over segment coupler or links. Segment couplers are signal
converters that modulate the RS485 signals to the MBP signal level. These are transparent from the
bus protocol standpoint. In contrast, links have their own intrinsic intelligence. They map all the field
devices connected to the MBP segment as a single slave in the RS485 segment. There is no limit to the
transmission rate in the RS485 segment, so that fast networks can also be implemented using field
devices with MBP connection, e.g., for control tasks.
Tree or line structures (and any combination of the two) are network topologies supported by
Profibus with MBP transmission. In a line structure, stations are connected to the trunk cable using
tree adapters. The tree topology is comparable to the classic field installation method. The multi-core
master cable is replaced by the two-wire bus master cable; the field distributor retains its function of
connecting the field devices and detecting the bus terminator impedance. When using a tree topology,
all field devices connected to the fieldbus segment are wired in parallel in the field distributor.
In all cases, the maximum permissible spur line lengths must be taken into account when calculating
the over-all line length. In intrinsically safe applications, a spur line has a maximum permissible
length of 30.m.
A shielded two-wire cable is used as the transmission medium. The bus trunk cable has a passive line
terminator at each end, which comprises an RC element connected in series with R = 100 Ω and C = 2.μF.
The bus terminator is already integrated in the segment coupler or link. When using MBP technology,
incorrect connection of a field device (i.e., polarity reversal) has no effect on the functionality of the bus
as these devices are usually fitted with an automatic polarity detection function.
The number of stations that can be connected to a segment is limited to 32. However, this number
may be further determined by the protection type selected and bus powering (if any).
In intrinsically safe networks, both the maximum feed voltage and the maximum feed current are
defined within strict limits. But the output of the supply unit is limited even for nonintrinsically safe networks.
The FISCO model considerably simplifies the planning, installation, and expansion of Profibus
networks in potentially explosive areas.
32.2.3 Fiber Optics
Some fieldbus application conditions place restrictions on wire-bound transmission technology,
such as those in environments with very high electromagnetic interference or when particularly
large distances need to be covered. Fiber-optic transmission over fiber-optic conductors is suitable
in such cases. The transmission characteristics support not only star and ring topology structures
but also line structures. In the simplest case, a fiber-optic network is implemented using electrical/
optical transformers that are connected to the device and the fiber optics over a RS485 interface.
This allows you to switch between RS485 and fiber-optic transmission within a plant, depending on
the circumstances.
© 2011 by Taylor and Francis Group, LLC