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Clear, Quick, and Cost-Effective INTERBUS Diagnostics Ensures System Availability |
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The advantages offered by fieldbus systems have already been discussed in detail and have ensured that fieldbuses are always used in systems that require powerful automation solutions. The requirements that users place on fieldbuses have barely changed since their introduction. One of these requirements is the diagnostics offered by the bus and its connected devices, which is always emphasized in specialist publications. It is important that the savings, which can be made during the installation and startup of the bus system, are not counteracted by time-consuming troubleshooting and resulting downtimes. Therefore all leading fieldbus systems are equipped with extensive data protection mechanisms for faulty telegrams. One measurement for the immunity to interference of a code is the Hamming distance, which is between 2 and 6 for standard bus systems. These values apply to both linear systems and the INTERBUS ring system. An important criterion for maintaining data communication is the response of the master in the event of the following errors:
Clear Error Localization In all fieldbus systems, in the event of a line interrupt the devices after the interrupt are no longer reached. The error localization capability depends on the transmission system used. In linear systems, telegrams are still sent to all devices. However, these telegrams are lost because the devices are no longer able to respond. After a certain period, the master detects the data loss. However, it cannot precisely determine the error location because the physical position of the devices is not known. The system diagrams must be consulted so that the service or maintenance personnel can determine the probable error location. Unlike linear systems, the individual devices in the INTERBUS system are networked so that each one behaves as a separate bus segment. Their physical position is clear from the summation frame protocol. In the event of a line interrupt between the devices, the master activates each separate device in turn. To do this, the master opens an electronic switch, starting from the first device up until the error location, thus clearly identifying the inaccessible device. The controller board can then clearly assign the error location as well as the station or station name and display it in clear text. This is a huge advantage, particularly in large bus structures with numerous devices, where bus systems are typically used. Autonomous Startup If a device fails, the fieldbus behaves in the same way as for a line interrupt. However, the functional capability of the remaining stations differs in linear and ring systems. In a linear system, bus operation cannot be maintained because the condition of physical bus termination using a terminal resistor is no longer met. This can lead to reflections and coupling in and out within the bus configuration. The resulting interference level means that correct operation is not possible. In an INTERBUS ring system, the terminal resistor is opened and closed together with an electronic jumper, which ensures that the condition of the closed ring is always met. In the event of a line interrupt or device failure, the master can either place the devices in a safe state or start up the remaining bus configuration autonomously.
Operable Segment Parts Short circuits on the line are a major challenge in a bus system. In the event of a direct or indirect (e.g., via ground) short circuit on the line, the transmission path is blocked for the entire section. In linear systems, the transmission line is used equally for all devices, which means that the master cannot reach segment parts either. This rules out diagnostics or the approximate specification of the error location. In the INTERBUS system, the user is aided by the physical (data transmission) division of the system into different bus segments. As described for the line interrupt, the devices are activated by the master in turn and the ring is closed prior to the short circuit, which means that subsystems can be started up again. The error location is reported in clear text on the controller board. Linear systems also support a division into different segments. Repeaters, which are placed at specific points, can then perform diagnostic functions. However, a repeater cannot monitor the entire system; it can only cover a defined number of devices per segment. Furthermore, the use of repeaters incurs additional costs, which should not be underestimated, and increased configuration effort.
Preventive Troubleshooting Short-term transient effects, which occur for example in extreme electromagnetic environments or on temporary voltage dips, lead to temporary errors. These errors are controlled in bus systems by the Cyclic Redundancy Check (CRC) or by parity bits and checksums. This enables single, multiple or group errors to be detected with a high degree of certainty (Hamming distance). In linear systems, telegram errors are registered using the specified test method. However, since all devices are connected to the transmission medium in parallel, it is impossible to determine the position of errors in the system. In the INTERBUS system, the CRC covers not only the entire transmission path, but also each separate transmission section between the individual devices. A CRC testing character is generated at the incoming and outgoing interface. The test results are sent to the master within the cycle, which means that all the effects on one or more devices can be detected and the transmission sections can be clearly assigned. Errors that occur frequently can be read from the diagnostic register. This enables the user to implement specific measures for preventative troubleshooting (e.g., for flexible cable tracks). Optical Fiber Path Evaluation Included In applications with large drives, welding robots, etc. optical fibers are increasingly used for data transmission. Due to its peer-to-peer communication method, INTERBUS is particularly suitable for this transmission medium. Installation advantages compared with other fieldbuses include the support of combined systems (optical fiber/copper). In linear systems, the use of optical fibers - like bus segmentation - requires expensive repeaters, which simulate a ring structure. Optical fiber path evaluation in the INTERBUS system is another feature, which is not offered by other buses. In this system, a test pattern for the optical fiber cable is transmitted between the interfaces to determine the quality of the connection. If the cable deteriorates due to dirt, loose connections, bending, etc., the transmission power is increased automatically. If a critical value is reached, the system generates an error message so that the service personnel can intervene before the deterioration leads to expensive downtimes. Studies by the VDMA and ZVEI indicate that many bus errors are caused by direct or hidden installation faults. For this reason alone, bus diagnostics simplify startup and ensure the smooth operation of the system, even in the event of extensions, servicing, and maintenance work. Every bus system should automatically carry out comprehensive diagnostics of all connected bus devices without the need for the installation and configuration of additional tools. In linear systems, this demand can often only be met if the appropriate control software (e.g., Simatic, RS Networx, etc.) has been connected, which limits the user in the selection of automation components. Additional software tools for system diagnostics often cost several thousand Euro.
In the INTERBUS system, all diagnosed states can be displayed directly on the controller board. If the master has a diagnostic display, various display colors can be used so that serious errors are clearly visible even from a distance. In addition, each master has a diagnostic interface, which can be used to transfer all functions to visualization systems or other software tools. Tools like the newly developed Diag+ by Phonix Contact can be used to access data throughout the network, even in higher-level Ethernet structures, and to create system evaluations and transfer protocols. |
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