Profibus PA
What is PROFIBUS PA?
Profibus PA is system solution for connecting automation systems with field instruments. Profibus PA is based on Profibus DP acc. to EN 50170. The Profibus PA profile defines the behaviour of the field devices and ensures full interoperability and interchangeability of the field devices of different manufacturers. Profibus PA operates either with intrinsically safe transmission technology (acc. to IEC 1158-2) or standard transmission technology (acc. to RS485). Profibus PA allows power supply and data transmission over the same two-wire cable and can be implemented in explosion endangered areas with intrinsically safe transmission technology according to IEC 1158-2. Intrinsically safe and non-intrinsically safe bus segments are separated by segment couplers.
What is a GSD file?
A GSD file is used to identify a PROFIBUS DP/PA device (Master or Slave). It contains data that make it possible to use a manufacturer-independent configuration tool. Typical information in a GSD file is supplier information, baud rate, timing, supported options/features and available I/O signals. A GSD file must be available for every DP/PA slave.
What are Ident Numbers?
Every DP/PA device type has to have an individual Ident Number. This number is required by the DP master to be able to identify the connected device types. The master compares the Ident Number of every connected device with the Ident Number in the configuration database. User data transfer in the adjustment phase is only possible when the right Profibus DP slave is connected under the correct address. This ensures very good protection against parameterization errors.
Cable specification
The PROFIBUS standard defines two variations of the bus cable, type A and type B. Type A is recommended for all new installations. Type A is especially suitable for high transmission speeds (>500 kB) and permits a network distance twice as large as Type B.
Technical specification Type A:
Impedance: 35 to 165 Ohm at frequencies from 3 to 20 MHz.
Cable capacity: < 30 pF per meter.
Core diameter: > 0.34 mm², corresponds to AWG 22.
Cable type: twisted pair cable. 1x2 or 2x2 or 1x4 connection.
Resistance: < 110 Ohm per km
Signal attenuation: max. 9 dB over total length of line section.
Shielding: CU shielding braid or shielding braid with shielding foil
Max. bus length: 200 m at 1500 kbit/s, up to 1.2 km at 93.75 kbit/s. Extendable via repeaters
How is the cable screen connected?
It is recommended to connect the screen on both ends to the protective ground (low inductivity). In case of widely branched installations with separate potentials, the screen should be connected to protective ground only at one end of the bus cable. The connection between screen and protective ground is made preferably via the metal case and screw coupling of the D-Sub connector. If this is not possible the connection can also be made via PIN 1 of the D-Sub connector. In such a cases, leave the cable screen bare at an appropriate spot and connect it to the ground terminal of the housing. The cable should be as short as possible
Why is a bus termination necessary?
Termination of a bus cable is carried out to prevent signal reflections on the PROFIBUS cable. Wrong or missing cable termination can cause transmission errors.
Further information: www.profibus.de