In process automation traditionally most of the field transmitters are connected over a current loop with 4-20 mA to the controlling device. This allows not only to transmit the measured value with the level of the current, but also provide the required electrical power to the field device with just one two wire cable of a length of more than thousend metres. These systems are also installed in hazardous areas. According to NAMUR a fieldbus in these applications has to fulfill these requirements. A special standard for instrumentation IEC/EN 60079-27 is describing requirements for the Fieldbus Intrinsically Safe Concept (FISCO) for installations in zone 0, 1 or 2.