A data logger records measurements of AC/DC current, temperature, relative humidity, time of use of devices, or any other parameter, at set intervals over a time period. The devices come with sensors, microprocessors and internal memory, and are powered by batteries. They are available in various types, as standalone devices or integrated as part of a larger external sensor. The top accessories for data loggers are connection cables, battery packs, terminal blocks, power supplies, wall holders, current clamps and more. PC measuring instruments come in various types. I/O cards, used for PC's, have options to connect input and output devices, supporting plug and play operations. I/O cards are made in analogue or digital versions. The common accessories include cable assemblies and terminal blocks. Measuring and controlling software may run on USB or via LAN, RS232, 485, GPIB or CAN bus. The top tools in this category are relay outputs, counter or frequency meters, distributed DAC systems and others. A frequency counter measures frequency of an event. The counter accumulates the number of events occurring within the specified time period. At the end of the preset period, the value of the counter is displayed and the counter reset. A DAC system, or digital-to-analogue converter, converts digital audio into analogue audio signals, for transmission to headphones.