Data transmission Data are exchanged over PROFIBUS DP by means of standard telegrams which are transmitted via the RS-485 interface. The permissible telegram length of reference data is specified in the PROFIBUS DP protocol at 244 bytes. it should be noted, however, that some controllers only support an maximum telegram length of 122 bytes.
The PROFIBUS DP devices manufactured by Endress+Hauser may transmit both input and output values, see also Table 5.17. In general:
• Measured value and status inputs are transmitted in 5 bytes.
• Display values and status outputs are transmitted in 5 bytes
• Control output values generally require 1 byte per action, the actual number available being dependent upon device.
An instrument with several measured values can transmit correspondingly more bytes. In the case of the flowmeter Promass 63, for example, a cyclic telegram up to 37 bytes (25 bytes input and 12 byte output data) is transmitted at maximum configuration, see Section 3.4.2. The values to be transmitted are determined during the commissioning of the network. The total number of inputs and outputs enabled then determine the telegram length. The same telegram is used for
transmission to and from the PLC.
GSD
(device database file)
In order to integrate the field devices into the bus system, the PROFIBUS DP system requires a description of the device parameters such as output data, input data, data format, data length and the transmission rates supported. These data are contained in the GSD device database file, which is required by the PROFIBUS DP master during the commissioning of the communication system.
In addition, device bit maps are required, which appear as icons in the network tree. Further information on device database files is to be found in Chapter 7.2.
Bus address A prerequisite for communication on the bus is the correct addressing of the participants. Every participant in the PROFIBUS DP system is assigned a unique address between 0 and 125 (cyclic data transmission). Normally the low addresses are assigned to the masters. The addresses may be assigned by DIP switch, on-site operating elements or by an operating program, e.g. FieldCare.
The exact way in which this is done should be taken from the appropriate device manual. For Endress+Hauser devices, the factory default address is always 126. This must be changed during commissioning and no device in the system should use it for cyclic communication.
Transmission rate All participants in a PROFIBUS DP system must support the governing transmission rate. This means that the speed of data exchange is determined by the slowest participant. In the case of
Endress+Hauser devices that are designed for PROFIBUS DP, all transmission rates from 9.6 kbits/
s to 12 Mbit/s are supported. For other manufacturers, please consult the relevant operating manual.
Bus parameters In addition to the transmission rate, all active participants on the bus must operate with the same bus parameters. If FieldCare is in use, the bus parameters can be set by using the corresponding Communication DTM for PROFIBUS. Individual device rates are set within their DTMs. More information on bus parameters are to be found in the Chapter 4.
3.5 Applications in hazardous areas
All devices and terminators that are installed in hazardous areas as well as all associated electrical apparatus (e.g. PA links or segment couplers) must be approved for the corresponding atmospheres.
RS-485 IS RS-485 IS is a recent innovation in response to an increasing market demand for the use of RS-485 in explosion-hazardous areas. A corresponding PROFIBUS guideline is now available that specifies the configuration of intrinsically safe RS-485 solutions with simple device interchangeability. In contrast to the FISCO model (see Chapters 2.2.1 and 6.1), for which there is only one active supply device per segment, all stations represent active sources. The devices are supplied with external energy and can supply this to the bus. RS-485 IS segments are coupled to RS-485 safe segments by means of so-called "Fieldbus Isolating Repeaters". Up to 32 stations can be connected to the intrinsically safe bus circuit, provided the conditions in Table 3-7 arre upheld. More details can be found in the specification.
Endress+Hauser devices do not support RS-485 IS at the present moment, but offers equivalent solutions with standard devices.
The following table shows all safety-relevant values for the entire bus system.
Table 3-4: PROFIBUS RS_485 IS: list of safety-relevant parameters
Parameter Description Value Remarks
Bus system
Maximum input voltage Ui [V] ± 4.2
Maximum input current Ii [A] 4.8
Maximum inductance to resistance ratio
L ’/R’ [µH/Ω] 15 For the whole operation Temperaturee range of the bus system
Number of devices NTN ≤ 32
Communication device
Maximum output voltage Uo [V] ± 4.2
Maximum output current Iο [mA] 149 Total current from wires A, B and supply for bus termination
Maximum input voltage Ui [V] ± 4.2
Maximum internal inductance Li [H] 0
Maximum internal capacitance Ci [nF] N/A Insignificant for safety
External active bus termination
Maximum output voltage Uo [V] ± 4.2
Maximum output current Iο [mA] 16
Maximum input voltage Ui [V] ± 4.2
Maximum internal inductance Li [H] 0
Maximum internal capacitance Ci [nF] N/A Insignificant for safety
If a PROFIBUS DP segment runs through an explosiion hazardous area, it must be executed with a degree of protection "increased safety" "e".
• For copper cabling, the number of devices per segment is limited to four.
• Proof of instrinsic safety is required for every segment, since every intrinsically safe component has different electrical characteristics.
• The cable and spurs must be included in the calculation.
• Exchange of a component from one manufacturer by a component from another manufacture is always requires renewed proof of intrinsic safety.
Mixed network PROFIBUS DP/PA
Since PROFIBUS PA systems are designed for use in hazardous areas, it is much easier install a segment there. For this reason, a PROFIBUS PA segment is used to extend the PROFIBUS DP segment into a hazardous area.
PROFIBUS PA networks are connected to PROFIBUS DP networks by a segment coupler or link, see Chapter 3.2.
Fig. 3-6: The PROFIBUS DP system can be extended into a hazardous area by using a segment coupler/link.
PLC
Class 1 master Class 2 master
e.g. FieldCare
segment coupler / link PROFIBUS DP
PROFIBUS DP-Slaves
PROFIBUS PA-Slaves
PROFIBUS PA
4 PROFIBUS PA Basics
This chapter presents the basic principles behind PROFIBUS PA. It is structured as follows:
• Synopsis
• Segment couplers and links
• Topology
• Bus access method
• Network configuration
• Applications in hazardous areas
• Operating mode
• FISCO
• Fieldbus barriers
4.1 Synopsis
Application PROFIBUS-PA has been designed to satisfy the requirements of process engineering. There are three major differences to a PROFIBUS DP system:
• PROFIBUS PA supports the use of devices in explosion hazardous areas without the need for specific proof of intrinsic safety.
• The devices can be powered over the bus cable (two-wire devices).
• The data are transferred via the IEC 61158-2 physical layer (MBP), which allows great freedom in the selection of the bus topology.
The most important technical data are listed in Table 4-1.
Participants Depending upon the application, the participants on a PROFIBUS PA segment might be actuators, sensors and a segment coupler or link, see Chapter 4.2. Endress+Hauser offers PROFIBUS PA instrumentation for the most important process variables, i.e. analysis, flow, level, pressure and temperature. A complete list is to be found in Chapter 10.
Fig. 4-1: PROFIBUS PA-system PLC
Class 1 master Class 2 master
e.g. FieldCare
segment coupler
PROFIBUS DP
PROFIBUS PA-Slaves PROFIBUS PA DP/PA link or
Table 4-1: Technical data PROFIBUS PA