Field device electronics unit for a field device used in automation engineering, said electronics unit comprising at least: - a first and a second connection terminal (K1, K2) designed to connect the field device electronics unit to a cable so that an input current (lin) can be supplied to the field device electronics unit via the cable and a terminal voltage can be applied to the first and the second connection terminal; - a series regulator, which is designed to regulate the input current in such a way that data can be transmitted over the cable using the input current; - a shunt regulator downstream from the series regulator; - a first capacitor switched in parallel to the shunt regulator and designed to store energy; - a supply circuit switched in parallel to the shunt regulator and the first capacitor, wherein said circuit is designed to provide an operating voltage; - a control and/or evaluation unit connected downstream from the supply circuit and powered by the operating voltage, wherein said unit is designed to measure a buffer voltage (UPuffer) present via the first capacitor or a variable which depends on said voltage, and to make a decision about the field device electronics unit, or for at least a part of the field device electronics unit, on the basis of the measured buffer voltage or the dependent variable; - further having a comparison circuit, which is designed to measure a cross current (IQuer) which is in the first cross branch in which the shunt regulator is arranged, and to compare the cross current (IQuer) to a threshold value (ISchwelle) and to provide a signal representing the result of the comparison to the control and/or evaluation unit, wherein the control and/or evaluation unit is further designed to measure, if required, the buffer voltage (UPuffer) or a variable that depends on the voltage on the basis of the signal. - a first and a second connection terminal (K1, K2) designed to connect the field device electronics unit to a cable so that an input current (lin) can be supplied to the field device electronics unit via the cable and a terminal voltage can be applied to the first and the second connection terminal; - a series regulator, which is designed to regulate the input current in such a way that data can be transmitted over the cable using the input current; - a shunt regulator downstream from the series regulator; - a first capacitor switched in parallel to the shunt regulator and designed to store energy; - a supply circuit switched in parallel to the shunt regulator and the first capacitor, wherein said circuit is designed to provide an operating voltage; - a control and/or evaluation unit connected downstream from the supply circuit and powered by the operating voltage, wherein said unit is designed to measure a buffer voltage (UPuffer) present via the first capacitor or a variable which depends on said voltage, and to make a decision about the field device electronics unit, or for at least a part of the field device electronics unit, on the basis of the measured buffer voltage or the dependent variable; - further having a comparison circuit, which is designed to measure a cross current (IQuer) which is in the first cross branch in which the shunt regulator is arranged, and to compare the cross current (IQuer) to a threshold value (ISchwelle) and to provide a signal representing the result of the comparison to the control and/or evaluation unit, wherein the control and/or evaluation unit is further designed to measure, if required, the buffer voltage (UPuffer) or a variable that depends on the voltage on the basis of the signal.