iButton
A highly durable chip with a unique address from Maxim Integrated Products that is used for many purposes, such as building access, personal information, asset management, guard tours and tracking. Housed in a 16mm aluminum or stainless steel can that looks like a watch battery, iButtons are small enough to be attached to a key fob or finger ring.
Data are transferred in and out of the iButton when the can makes momentary contact with a reader/writer connected to a computer via a simple Maxim interface (see 1-Wire).
iButton and Reader
Looking like a watch battery, this iButton is resting on top of the reader. iButtons are rugged identification devices, and their top and side are the two contacts.
A Unique Serial Number
Every iButton has its own serial number laser etched into the case.