24/11/03 - Information: The new master generation of B+W: Double addresses? - The problem has been solved!
To cut a long story short: AS-Interface is simple and will remain simple. But the higher its success on the market, the more users ask for subtleties and special functions. The company Bihl + Wiedemann1 has accomplished another degree of perfection with the development of a new master, without overstraining users or endangering the simplicity of the system. Bihl + Wiedemann complement the common AS-Interface standard by a variety of diagnostic information. Consequently, the new generation of masters – mainly in the form of gateways or "links" to higher buses – is capable of managing nearly any situation in the field. The most important news: The formerly annoying problem of duplicate addresses has been solved! The new master generation recognises that fault.
Even the design of the new master is an eye-catcher: A stainless steel housing with IP20 degree of protection (fig. 1) to fit in smoothly in a switching cabinet. Due to its size, its overall form, and the positioning of its sockets, the master can be neatly fitted in a 100 mm-high cabinet, even in complicated spatial circumstances. A newly introduced second socket may surprise: It is not meant for the power supply – this function is normally still fulfilled for Bihl + Wiedemann masters by the AS-Interface network – but can be used to extract data of the master, the network, and their functions. That is an option which is targeted to get all data directly as part of an on-site diagnosis .
On-site diagnosis:
Internally, the master has been supplemented with several new application functions (fig. 2). They all draw on the extended possibility of diagnosis of the master, without an (areal or programmed) detour through the host. Bihl + Wiedemann hence enlarges its concept of "on-site diagnosis": The user is given the capacity to query as many information as possible on the connected network only by connecting to the master, but without having to connect to a higher PC, the host. Following this strategy, the master, even though technically it "only" serves as a gateway, is considered an intelligent product.
In most cases, this diagnostic information is not important for the user (therefore: "AS-Interface – the simplest automation networking solution"). This is also the reason why the adoption of AS-Interface will not be more complicated for the user when using this new master. But any time the user wants to gradually construct a network, to check his network more thoroughly, or if he wants to identify an error, she or he will gladly resort to one of these options. The user thus receives an optimised support "on the spot".
Three layer model with three ports:
Technically, the master has been extended beyond the three typical layers as defined by the user organisation for each AS-Interface – master (fig. 2). It goes without saying that these extensions neither violate the specification nor nullify parts of it.
On the lowest level, the Bus Interface, all bus telegrams are generated as analog signals, incoming signals are detected and checked for possible errors. That's standard. A new feature on this layer is the additional surveillance of telegrams on the bus, which recognises duplicate addresses, earth faults, EMC-disturbances, and strong common mode disturbances and relays them upwards.
The Master Layer, which works on top of the bus interface, primarily executes all the master functions as set down by the general specification (CS 2.11), as well as the already introduced application functions of Bihl + Wiedemann: the "Analyser functions" (configuration, diagnosis, error statistics, error treatment, display of the list of periphery errors, treatment of slave malfunctions), the reading or conscious setting of actual periphery data without the host, the logging of warning and readiness messages, as well as the listing of safe slaves and the analysis of causes for a stop in Safety at Work applications. Here, the error statistics is based on the telegram control functions within the bus interface. A new application function is the treatment of the mentioned information for the surveillance of the bus physics. It edits all these data in such a way that they can be provided to the user according to his or her wish.
The new options in detail:
The highest level, the Host Level, optionally now provides a diagnosis interface with an RS232 socket, in addition to the interface to different hosts (gateways to higher bus systems) and to the graphical display. As all three possibilities access the master layer, identical information can not only (as was already possible up to now) be provided via the higher fieldbus and via the display on the unit, but it can now also be read via the RS232 socket directly to a PC. Thus, comfortable options for displaying and documentation are available on-site via a notebook (fig. 3). Customers having to work with different buses can – in spite of differences in the buses – always use the same master functions and the same method of diagnosis.
In recognising duplicate addresses, the new master solves an old, irritating problem: Due to the system specification, it has usually been difficult in practice to reliably recognise the assignment of one address to two slaves. The master now identifies this case and issues a corresponding error message (fig. 4). Meanwhile, it sets the flag "configuration error", so that the higher PC will be informed and can react accordingly. The user does not have to fear a system deficiency anymore.
For the first time, the master also contains an earth fault detector, which so far has only been sold either as a stand-alone component of the net or as the supplement to a power supply. An accidental earth fault in the network causes the system to be more vulnerable against electro-magnetic disturbances; a double earth fault might trigger a stepping error in special circumstances. Therefore, an earth fault detector has been recommended for critical applications for a long time. It detects already the first earth fault and thus hedges the system. If the earth fault detector, as in the case of the new Bihl + Wiedemann masters, is implemented in the master, it has the "direct line" to the controller, which can process its signal immediately. The default setting releases a diagnostic signal is in the higher fieldbus. According to the application-specific settings, an alarm, an immediate stop of the application, or a systematic shutdown of the application can be triggered. Alternatively, an instant transition to the offline phase can be forced directly on the master layer .
The third new function is the noise detector. It continually checks the analogue signal on the bus line and detects disturbances (noise) in the pause time of the signal if they exceed a certain level. In that manner emerging problems can be covered in a state in which telegram repetitions do not yet occur. Critical situations can thus be analysed more easily, such as disturbances that are cyclically linked to the course of a process, e.g. the activation of a motor.
The over voltage detector has a similar function; it recognises severe common mode oscillations on the AS-Interface line, which sometimes disturb externally connected sensors. The design of AS-Interface as an earth free system may in fact sometimes lead to a highly fluctuating potential, which affects both conductors of a cable in the same manner. Due to the high common mode rejection, this will not trigger an error in the bus communication; however, a sensor (e.g. an inductive sensing device with low current consumption) may generate a short erroneous signal if it is not specifically protected. Although this happens very seldom and in extreme cases only (e.g. when fabricating PET-bottles) where other system will fail completely, this case cannot easy to be identified by other means. The transmission via AS-Interface remains technically correct, but the sensor signal is temporarily wrong. The overvoltage detector registers this danger which can then be similarly treated as EMC disturbances. In both cases the disturbances can be issued acutely or as a sum signal over the run time of the master. Again the application limits for AS-Interface will thus be extended again.
Finally, in comparison to the previous masters, some changes in the displaying of information have been implemented which will simplify the operation (to some extent plaintext messages, and a multi-lingual, simplified operator guidance). Optionally, the master can be supplied by an external power supply (instead of through AS-Interface). In that case, the master will remain operational after a loss of power on the AS-Interface network, and the host can receive the message "AS-Interface power supply failure" instead of the more general "AS-Interface network breakdown". This possibility is explicitly demanded by some users.
What remains the same?
The general construction of the master, such as illustrated in fig. 2, persists. Bus-interface and master layer are the basis for the entire master family, which again comprises about a dozen different gateways. This core, which is always the same, is accessed from outside. Now, however, three instead of previously two options of communication are available. This is the (distant) access via any fieldbus (always in the same manner!), the fast and easy on-site communication via 4 buttons and the graphical display, as well as the more detailed possibilities of local documentation via the diagnosis port. The main functions of AS-Interface still have the – also temporal – absolute priority over all other functions. The master therefore remains interoperable with all other components that fulfil the general specification. This also means that all central features, such as the self-configuration of the master or the "auto addressing" of a substituted slave, remain operational. For the user who only wants to connect an executable system, AS-Interface remains as easy as before. All additional functions have a lower priority from the purely technical point of view. They influence neither the cycle time nor the communication to the host.
This may completely change from the point of view of a user as soon as he wishes to have more information on an application or if it does not run immaculately from the start. Then all the application functions described in this article will have a high priority to the user, as they are suitable to issue a very complete diagnosis of the network and to identify errors quickly. In addition, the user receives this information on the spot.
 |
Article as PDF file |
Literature
(1) See:
www.bihl-wiedemann.de
(2) Madelung, O.W.: Safety at Work (Master mit
Applikationsfunktionen); SPS Magazin, Heft 4+5, April 2002, p. 76-79, or Safety
at Work? Of course! And Convenient! In: www.bihl-wiedemann.de
Author
Dr. Otto W. Madelung, Technical
Consultancy Dr. Madelung, www.madelung-online.de