In addition to its diagnostics (error counters, corrupted slaves lists, peripheral fault lists, etc) the new stainless steel enclosed AS-interface master (from Bihl+Wiedemann) now has a wide range of new, useful features - they include:
Recognition of duplicate addresses
When two AS-interface (ASi) slaves have the same address the master cannot send a request to each slave separately. Until recently, different masters reacted differently to this situation, without actually recognising the problem. As both slaves' responses overlap on the line, it is impossible for the master to recognise a correct response. This results in an unstable network, without any indication to the user. The new master solves this irritating problem. A duplicate address is recognised and a corresponding error message is displayed. The configuration error and duplicate address flags are set and a list of duplicate slaves is updated.
Earth fault detector
Communication on the ASi bus is still possible if one of the ASi lines short-circuits to plant earth. However, the data signals in such a case can be highly influenced by electro-magnetic disturbances. Circuits with a line-to-earth fault are likely to have a high number of communication errors, the cause of which used to be a mystery to the user. The new master's earth fault detector will now immediately raise an alarm in the event of an earth fault, a message is displayed on the master's display and an earth fault flag is set.
Over-voltage detector
The new master detects severe common mode oscillations. ASi is designed as an earth-free system. This design can sometimes cause the potential of both ASi lines to fluctuate with reference to ground. This may not cause communication errors on the bus, but it can influence externally connected sensors. A sensor with inductive characteristics may generate an erroneous signal during such common mode oscillations, causing the master to temporarily receive an incorrect input signal. The new master warns the user of this danger in the form of an over-voltage flag and corresponding error message.
Noise detector
ASi is designed to operate at a high threshold noise level, even if this means that telegrams are repeated occasionally. However, if this threshold is exceeded, an unacceptable number of telegram disturbances is created. The master detects alternating signals produced by neither the master nor the slaves and subsequently sets a noise flag. This helps the user identify the source of these disturbances. The noise flag could for example be raised cyclically upon the cyclic activation of a frequency inverter, thus identifying the source.
Adding short-circuit protection
It is well known that a short-circuit between the communication lines of a fieldbus network will pull down the entire network. This will cause downtime on that section of the plant until the short-circuit is located and corrected, but with the whole network down, where does one begin looking?
Stonel offers a range of drop connectors for the above networks with a protection circuit on each drop, ensuring that a short circuit on a drop will not affect the rest of the bus. A trip current of 240 mA for AS-interface and 40 mA for Profibus PA and FF isolates the drop from the rest of the network, and visibly indicates the short by means of an LED.
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