The original PC/104 bus derives from the ISA bus. It includes all the signals found on the ISA bus, with additional ground pins added to ensure bus integrity. Signal timing and voltage levels are identical to the ISA bus, with lower current requirements. The PC/104 specification defines two versions of the bus, 8-bit or 16-bit. The 8-bit version corresponds to the IBM XT and consists of 64 pins. The 16-bit version corresponds to the IBM AT and adds 40 additional pins, bringing the total to 104 (hence the name "PC/104").