Slave Redundancy
PROFIBUS specification "slave redundancy" (IEC standardized)
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Redundancy for high system availability
In fieldbus technology, data transmission is done over a single trunk cable. For applications which require particular high system availability, e.g. in chemical or petrochemical plants or power stations, redundancy concepts are implemented. Redundancy can extend to all system components such as power supply, data transmission lines, masters, gateways, or field devices (slaves). In early times, only vendor-specific redundancy concepts have been on the market. In 2004, PROFIBUS International published the “Slave Redundancy” specification 1.2, which describes the basic mechanisms for a redundant slave to be used in several control systems with several master implementations.
Fig. 1 shows the main PROFIBUS redundancy solutions: Master, line and slave redundancy whereby each part is responsible for its own redundancy aspects. The concepts can be combined with one another as desired. Thus, a redundant slave can be connected to a single line and/or to non-redundant master or even with full line redundancy.
Fig. 1: PROFIBUS redundancy concepts
PROFIBUS slave redundancy
The PROFIBUS slave redundancy specification Version 1.2, November 2004 (Order No. 2.212) is standardized in IEC 61158-5, 61158-6 and 61784-1. It describes the basic mechanisms for a redundant slave to be used in control systems with several master implementations. A redundant PROFIBUS slave has two independent communication interfaces with a PROFIBUS-independent communication in between. One of the slave interfaces acts as the primary and one as the backup slave.
The specification defines the “slave interface module” redundancy from the PROFIBUS point of view. The redundancy structure from the process I/O point of view is not part of this specification. In other words, whether the slave offers redundant I/O components to the process environment or not depends on a particular slave implementation.
The concept is supported by the PROFIBUS DPV2 communication protocol. It controls the communication between the master and the redundant slave.
The configuration types supported
The specification incorporates two different redundancy configurations (fig. 3).
System redundancy
is composed of two complete systems each containing master, line and slave.
Flying redundancy
uses integrated redundand masters and slaves.
Fig. 2: Flying and system redundancy according to PROFIBUS Spec. 2.212
Functions and benefits
- All three redundancy levels (slave, line, master) can be configured independendly accrding to the user reqirements.
- Very fast redundancy switch-over. For that, the master controls the slave switch-over and the slave the master switch-over.
- Redundancy switch-over can be initiated from both the slave and the master
- Already existing redundancy structures remain completely unaffected and preserved. PROFIBUS slave redundancy can therefore easily implemented into existing concepts.
- Redundancy switch-over is supported by the DPV2 functionality. Separate adaption to the master is not required
- Slave devices of different suppliers supporting PROFIBUS slave redundancy can be connected to a redundant bus system without any engineering effort
- Only one common slave interface implementation for all redundancy levels
- The system operator can freely select the products based on their functionality without limiting the availability of the system.