Operating method for a peripheral module of a programmable logic controller, - wherein the peripheral module outputs, via a terminal of the peripheral module, a supply potential and, in this respect, an associated supply current to a field unit (4, 6) connected to the peripheral module, - wherein, in a normal mode, the supply potential has a base voltage value (U+) and, in this respect, the supply current has a base current value, - wherein the peripheral module changes over from time to time from normal mode into a test mode, in which the peripheral module -- changes the supply potential from the base voltage value (U+) to a modified voltage value (U-) by driving a switching device which is upstream of the terminal within the peripheral module for a defined valid time interval, -- checks whether, once the defined time interval has elapsed, an expected change has occurred due to the modification of the supply potential, -- changes back over into normal mode if the expected change occurs and -- changes over to alarm mode if the expected change does not occur, - wherein, in alarm mode, the peripheral module outputs an alarm message (M') to a central unit of the programmable logic controller and/or, via outputs of the peripheral module, transfers actuators connected to the peripheral module into a safe state, - wherein, in the course of commissioning of the peripheral module, the peripheral module initially changes over prior to normal mode into a calibration mode, characterised in that, in calibration mode, the peripheral module - initially supplies the base voltage value (U+) to the terminal via the switching device, - then, at a start time, changes the supply potential to the modified value (U-) and acquires a response time at which the expected change occurs, and - finally, using the start time and the response time, ascertains the valid time interval. - wherein the peripheral module outputs, via a terminal of the peripheral module, a supply potential and, in this respect, an associated supply current to a field unit (4, 6) connected to the peripheral module, - wherein, in a normal mode, the supply potential has a base voltage value (U+) and, in this respect, the supply current has a base current value, - wherein the peripheral module changes over from time to time from normal mode into a test mode, in which the peripheral module -- changes the supply potential from the base voltage value (U+) to a modified voltage value (U-) by driving a switching device which is upstream of the terminal within the peripheral module for a defined valid time interval, -- checks whether, once the defined time interval has elapsed, an expected change has occurred due to the modification of the supply potential, -- changes back over into normal mode if the expected change occurs and -- changes over to alarm mode if the expected change does not occur, -- changes the supply potential from the base voltage value (U+) to a modified voltage value (U-) by driving a switching device which is upstream of the terminal within the peripheral module for a defined valid time interval, -- checks whether, once the defined time interval has elapsed, an expected change has occurred due to the modification of the supply potential, -- changes back over into normal mode if the expected change occurs and -- changes over to alarm mode if the expected change does not occur, - wherein, in alarm mode, the peripheral module outputs an alarm message (M') to a central unit of the programmable logic controller and/or, via outputs of the peripheral module, transfers actuators connected to the peripheral module into a safe state, - wherein, in the course of commissioning of the peripheral module, the peripheral module initially changes over prior to normal mode into a calibration mode, - initially supplies the base voltage value (U+) to the terminal via the switching device, - then, at a start time, changes the supply potential to the modified value (U-) and acquires a response time at which the expected change occurs, and - finally, using the start time and the response time, ascertains the valid time interval.