iButton is a concept based upon Maxim's 1-wire interface. The concept encompasses for e.g. memory, serial number ICs and temperature sensors, mounted on a coin-shaped package which only requires two connections for reading, earth and data in/out. The system is used in everything from access control and payment identification systems and temperature logging of transport products. There are also systems of accessories such as holders, probes, USB interface and starter kit. 

• All coin cell memories contain a unique laser printed serial number