Overview > io-link > secondary-device-functions > Secondary device functions Secondary device functions This section describes the secondary device functions of the product. Boot Cycle Counter Description ISDU Boot Cycle Counter Custom Boot Cycle Counter System Commands Operating Hours Counter Description Algorithm Process Data ISDU Current Operating Hours Total Operating Hours Custom Operating Hours Operating Hours Saving Mode System Commands Voltage and Current Monitoring Description Mathematics/Algorithm Process Data ISDU Voltage Monitoring Detection Time Duration System Commands Events