Portable computers, more narrowly defined, are distinct from desktop replacement computers in that they usually were constructed from full-specification desktop components, and often do not incorporate features associated with laptops or mobile devices. A portable computer in this usage, versus a laptop or other mobile computing device, have a standard motherboard or backplane providing plug-in slots for add-in cards. This allows mission specific cards such as test, A/D, or communication protocol (IEEE-488, 1553) to be installed. Portable computers also provide for more disk storage by using standard disk drives and provide for multiple drives.