An iButton is a chip housed in a stainless steel enclosure. The electrical interface is reduced to the absolute minimum, i.e., a single data line plus a ground reference.
The energy needed for operation is "stolen" from the data line ("parasitic power"). The logical function of the DS1990A is a simple serial number. This globally unique registration number is common to all iButtons.
For read operations all devices are satisfied with a 5kOhm pull-up resistor to supply energy and to terminate the 1-Wire bus.
The I Button Reader application was designed to be a small interface that can read data from iButton device.