How do you use yours?

ONE OF THE STRONGEST ARGUMENTS FOR INSTALLING A FIELDBUS for safety-related data is that it can reduce downtime by helping to pinpoint which item of safety equipment has tripped the safety circuit. Meanwhile, it is being suggested that the many users of conventional fieldbuses are not fully exploiting diagnostics on those systems.

Murrelektronik has therefore launched the MVK family of fieldbus modules, with models available for use with Profibus, Interbus, DeviceNet and CANopen. These allow information about short-circuits and cable breaks to be fed back from individual standard sensors and actuators to the master controller via the bus. Of course, once the data has reached the master, it can also be disseminated much more widely thanks to modern communications technology. Note that this level of single-point diagnostics can be achieved with standard sensors, though an adaptor has to be fitted between the sensor and the cable to gain the full functionality. However, if the sensor meets the DESINA II standard, it can also transmit other fault data, such as dirt build-up on optical sensors or reduced signal strength on proximity detectors. And regardless of whether the sensor is a standard item or a DESINA II item, the PLC can be programmed with a look-up table so that the maintenance engineer can be told that if sensor XYZ has failed, the part number for the replacement is ABC123.

These modules sound as if they could make life much easier when it comes to running a machine or plant - as well as commissioning it - and they could help to reduce the cost of ownership through a reduction in downtime. But does this imply that existing fieldbuses are lacking something?

Well it seems that much of this diagnostic functionality is, in fact, already available in other fieldbus networks. Take DeviceNet as an example: problems such as open-wire faults and short circuits can be reported via the fieldbus and, if you like, the overall system can be set to forward a text message to a maintenance engineer's mobile phone, giving him the fault location and the part number of the faulty component, for instance. However, it's all a question of how much trouble you are prepared to go to when programming the system, because all of these nice-to-have features need someone to set them up - and this has not often been done. It also relies on field devices that meet all of the requirements of the fieldbus standard.

And here lies another issue: whereas some vendors have opted to concentrate on supplying field devices that are compatible with just one or two fieldbuses - and therefore provide full functionality - other vendors have tried to supply devices that are compatible with a large number of fieldbuses. For this category of supplier, it is clearly going to be much more challenging to develop devices that can take full advantage of the functionality that is available.

Another thing to watch out for is how the fieldbus vendors are defining their system's diagnostic capability. Some may claim 'single channel diagnostics', but you should check whether that means, for example, open wire and short circuit detection on every input and output or, perhaps, open wire protection on the inputs and overload detection for a group of outputs. Just for the record, Murrelektronik's 'single point diagnostics' means open wire and short circuit detection on every input and every output.

So what's the situation now? If you want to make full use of diagnostics, make sure you ask all the right questions of the fieldbus and device vendors to check that you can do what you want to. And beware that you will need to invest in a little more programming time to make it all work as you want it to. The Murrelektronik MVK modules seem to have more capability for providing diagnostics for standard sensors and might therefore be more attractive for retrofits and refurbishments, but don't forget that you will need adapters on some of the field devices to get the most out of the MVK boxes.