Hart Loop Converter New Hart Loop Converter Increases the Performance of 4…20 mA Field Devices

| Editor: Marion Henig

A new Hart Loop Converter converts up to three digital Hart variables that could include the 4…20mA transmitter current signal and makes them available to the host control system. These three variables enable not only the actual measured result but also process-related parameters such as reaction-time and temperature to be determined and used selectively to control the process — using the same cable and incurring no extra cost.

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More information with the new Hart Loop Converter... (Pictures: Pepperl+Fuchs)
More information with the new Hart Loop Converter... (Pictures: Pepperl+Fuchs)

The bi-directional communication between field devices and host systems via Hart protocol is standard practice in the process industry today. Due to the existence of the standard, digital Hart protocol, the potential of more intelligent field devices is increased and the need for a universal fieldbus for process data is essentially not required. Supporting this position is the fact that the majority of actuators and sensors are now Hart-compatible. These types of instruments are used for measuring process variables like flow, level, temperature, pressure or pH value, and have proven their worth over many years in the field. The advantages of Hart protocol range from safe plant operation to diagnosis options and higher flexibility. Furthermore, the conventional, analog 4…20 mA Interface in a 2-wire system can continue to be used, so there are no extra costs for the customer.

Hart is a protocol for Master/Slave communication and permits up to two master devices — primary and secondary. The primary devices are host devices, such as a DCS, while a secondary may, for instance, be a handheld communication device. A secondary device can be installed without any adverse effect on communications between the field device and the host. There are two modes for bi-directional communication. The modes used regularly are Command and Answer, in which the field device is polled to provide dynamic measured values or instrument data. In order to increase the sampling rate, the burst mode can be used. In this case, the field device provides an updated measured value packet three to four times per second without the need to be polled. However, the burst mode is only appropriate for point-to-point connections. The measuring circuit includes a Hart-compatible field device that is linked to a control system via an analog 4…20 mA interface in a conventional 2-wire system. Hart communication between the master and slave also takes place over this connection.

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