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General Application Profiles

General application profiles describe functions and characteristics that relate to more than just one application. They can also be used in conjunction with specific application profiles.

 

4.1 PROFIsafe

For considerable time, the distributed fieldbus technology for factory and process automation was subject to the restriction that safety tasks could only be solved using conventional technology in a second layer or distributed over special buses. With PROFIsafe, PROFIBUS has created a comprehensive, open solution for safety-relevant applications that satisfies most known safety criteria.

PROFIsafe defines how fail-safe devices (emergency stop pushbuttons, light arrays, overfill cutouts, etc.) can communicate over PROFIBUS with fail-safe controllers so safely that they can be used for safety-related automation tasks up to KAT4 compliant with EN954, AK6 or SIL3 (Safety Integrity Level). It implements safe communications over a profile, i.e. over a special format of user data and a special protocol.

The specification has been jointly drawn up by manufacturers, users, standardization committees and inspectorates (TÜV, BIA). It is based on the relevant standards, primarily the IEC 61508, which address the concerns of software development in particular.

PROFIsafe takes into account a number of error possibilities that can occur in serial bus communications, such as the delay, loss or repetition of data, incorrect sequences, addressing or corrupt data.

There are a range of remedial measures, the following of which have been selected for PROFIsafe:

  • Consecutive numbering of safety telegrams.
  • Timeout for incoming message frames and their acknowledgment.
  • Identifier between sender and receiver ("password").
  • Additional data security (Cyclic Redundancy Check, CRC).

By skillfully combining these remedial measures in connection with a patented "SIL monitor" (monitoring of the frequency of failed messages) PROFIsafe achieves safety classes up to SIL 3 and beyond.

PROFIsafe is a single-channel software solution, which is implemented in the devices as an additional layer "above" layer 7 (see Fig. 17); the standard PROFIBUS components, such as lines, ASICs or protocols, remain unchanged. This ensures redundancy mode and retrofit capability.

Fail-safe mode with PROFIsafe

Fig. 17: Fail-safe mode with PROFIsafe

Devices with the PROFIsafe profile can be operated in coexistence with standard devices without restriction on the same cable.

PROFIsafe uses acyclic communication and can be used with RS485, fiber optic or MBP transmission technology. This ensures both fast response times (important for the manufacturing industry) and intrinsically safe operation (important for process automation).

In process technology, it is only necessary to provide and prepare one standard device type for failsafe or normal operation, as the fail-safe functionality can be configured during application (SIL2 for operational reliability).

As a generic software driver, PROFIsafe is available for a wide range of development and runtime environments. The specification can be found in the document "PROFIsafe, Profile for Safety Technology", Order No. 3.092.

 

4.2 HART on PROFIBUS DP

In view of the large number of HART devices installed in the field, the integration of these devices in existing or new PROFIBUS systems is of key importance to most users.

The PROFIBUS "HART" specification offers an open solution for this problem. It includes the benefits of the PROFIBUS communication mechanisms without any changes required to the PROFIBUS protocol and services, the PROFIBUS PDUs (Protocol Data Units) or the state machines and functional characteristics.

This specification defines a profile of PROFIBUS that is implemented in the master and slave above layer 7, thus enabling mapping of the HART client-master-server model on PROFIBUS. The cooperation of the HART Foundation on the specification work ensures complete conformity with the HART specifications.

The HART-client application is integrated in a PROFIBUS master and the HART master in a PROFIBUS slave (see Fig. 19), whereby the latter serves as a multiplexer and handles communication to the HART devices.

For the transmission of HART messages, a communication channel has been defined that operates independently of the MS1 and MS2 connections. An HMD (HART Master Device) can support several clients. The number of clients depends on the implementation.

HART devices can be connected with the HMD to PROFIBUS over different components (PROFIBUS Guideline "PROFIBUS Profile for HART“ Order No. 3.102).

 

4.3 Time Stamp

When recording timing functions in networks, particularly those such as diagnosis or fault location, it is useful to be able to provide certain events and actions with a time stamp, which enables precise time assignment.

Time stamp and alarm messages

Fig. 18: Time stamp and alarm messages

For this purpose, PROFIBUS offers the time stamp profile. Precondition is clock control in the slaves through a clock master over MS3 services. An event can be given a precise system time stamp and read out accordingly. A concept of graded messages is used. The message types are summarized under the term "Alerts" and are divided into high-priority "alarms" (these transmit a diagnosis message) and low-priority "events". In both cases, the master acyclically reads (using the MS1 services) the time-stamped process values and alarm messages from the alarm and event buffer of the field device (see Fig. 18). Please refer to the corresponding document, the PROFIBUS Guideline "Time Stamp", Order No. 2.192.


Integration of HART devices into PROFIBUS DP

Fig. 19:Integration of HART devices into PROFIBUS DP


4.4 Slave Redundancy

The installation of field devices with redundant communication behavior is desired in many applications. For this reason, PROFIBUS has drawn up the specification for a slave-redundancy mechanism that describes the following device characteristics (see Fig. 20):

  • Slave devices contain two different PROFIBUS interfaces that are called primary and backup (slave interface).

These may be either in a single device or distributed over two devices.

  • The devices are equipped with two independent protocol stacks with a special redundancy expansion.
  • A redundancy communication (RedCom) runs between the protocol stacks, i.e. within a device or between two devices, that is independent of PROFIBUS and whose performance capability is largely determined by the redundancy reversing times.

In normal mode, communications are sent exclusively over the primary slave; only this slave is configured, it also sends the diagnosis data of the backup slave. In the event that the primary slave fails the backup slave takes over its functions, either because it has detected the failure itself or because it was requested to do so by the master. In addition, the master monitors all slaves and sends a diagnosis message as soon as the backup slave fails and there is no further redundancy.

A redundant slave device can be operated on one PROFIBUS line or, in the event of an additional line redundancy, on two PROFIBUS lines. The advantages of this redundancy solution for the user are as follows:

  • Only one device version required to implement different redundancy structures.
  • Master, line and slave redundancy are available independently of one another.
  • No additional configuration of the backup slave required, thus no need for complex tools.
  • Complete monitoring of both slave parts possible.
  • The slave device has no influence on the bus load and therefore on the dynamic response of PROFIBUS.

The redundancy of PROFIBUS slave devices provides high availability, short reversing times, no data loss and ensures fault tolerance. Please refer to the corresponding document, the PROFIBUS Guideline "Specification Slave Redundancy", Order No. 2.212.

Slave redundancy in PROFIBUS

Fig. 20: Slave redundancy in PROFIBUS

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