The DS1990A and DS1990R iButtons provide globally unique 64-bit serial numbers. These are utilized in a variety of applications such as guard tour, asset tracking, driver ids, and many more. These iButton models are typically compatible with each other, with their primary technical difference being the DS1990R's guaranteed presence pulse on contact. In applications where time is of the essence, the DS1990R's guaranteed presence pulse on contact may quicken the detection of the iButton. However, the DS1990A is the most popular model due to pricing and stock availability.