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PROFIBUS Optional Board Operating instructions
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Criteria such as high transmission speed and simple, low-cost installation are of
critical importance in the selection of the transmission system. The RS-485
requires a screened copper cable with twisted-pair wires.
The transmission speed can be selected within the 9.6 kbaud to 12 Mbaud range.
It is set globally for all devices on the bus during system start-up.
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All devices are connected in a bus structure (line). Up to 32 nodes (master or
slaves) can be interconnected within one segment. The bus is terminated by
an active bus terminator at the beginning and end of each segment. To ensure
fault-free operation, both bus terminators must have a voltage supply at all times.
The bus terminators can normally be activated either in the devices themselves or
on the bus termination connectors.
To accommodate more than 32 nodes or increase the scope of the network,
repeaters (cable amplifiers) can be installed to link the individual bus segments.
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PROFIBUS can be implemented by means of fiber optics for installation in
environments susceptible to high levels of radiated emission, where electrical
isolation is essential or to increase the bus range with high transmission speeds.
Various fiber types are available to suit different range, price and application
requirements.
The following table shows a list of suitable fiber types:
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Multimode glass fiber Medium range, 2-3 km
Monomode glass fiber Long range, > 15 km
Plastic fiber Short range, < 80 m
PCS/HCS fiber Short range, approx. 500 m
PROFIBUS segments implemented by fiber optics are configured either as a star
or ring structure. Couplers between RS-485 and fiber optic transmission systems
are also available, allowing the two types of system to be deployed alternately as
required within the same installation.
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PROFIBUS operates according to the token passing principle, i.e. the active
stations (masters) receive transmit authorization in a logical ring for a defined time
window. Within this window, the master can communicate with other masters or
exchange data with the relevant slaves in a subordinate master-slave process.
PROFIBUS-DP primarily utilizes the master-slave process for this purpose; in most
cases, it exchanges data with drives such as MICROMASTER4 cyclically.
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