| 摘要 |
The invention relates to a method for operating a field device and to a field device (1) for process instrumentation, wherein the field device (1) comprises an analog output having an electrical connection, to which a two wire line (2) for the transmission of an analog current signal (1) can be connected. During operation, on a two wire line (2) in an error state, in which, by means of the field device (1), a low value for the current strength of the analog current signal (I) is set for the display of the error state, the field device (1) determines the product of a predetermined load (R) and the increase in the current strength of the analog current signal (I), which is required during normal operation for a change in state, determines the difference between a value, which is currently determined in the error state, of the voltage (Vk) at the clamps (6, 7) of the electrical connection, and the determined product, and verifies, prior to a state transition into normal operation, whether the difference determined exceeds a predetermined minimum clamp voltage. As a result, in an advantageous manner, an automatic setting of a hysteresis is achieved in case of the detection of an undervoltage, which setting largely makes do without manual input of parameters, and nevertheless prevents a back and forth of the field device (1) between the error state and normal operation, and thus an oscillation of the analog output signal. In addition, a load resistance (R) and a minimum supply voltage (Vq) can be determined automatically. |
