Box computers built on SBCs are mostly characterized by their form factors: PC/104, PC/104-Plus and PCI/104, with other form factors (EPIC, 3.5” SBC, Embedded System Module (ESM), ISA, ETX, EBX, Pico-ITX, Mini-ITX, Micro-ATX, 4.5” SBC, QSeven, CoreExpress, COM Express, nano ETXExpress and XTX) compatible with these three form factors. Early box PCs had a form factor of PC/104 or EBX but later an in between form factors (mini-ITX, ETX and EPIC) were developed. The EPIC form factor is a 4.5 x 6.5” board with pin header or PC-style I/O connections and is an international and licensed standard. There are other non-standard SBCs too that resemble EPIC.
3.5” SBC is another improved specification that expand using PC/104 or PCI/104 and offers complete features of a PC including expandable memory sockets, USB 2.0 connectors, integrated audio and graphics capabilities, dual gigabit Ethernet connectors, multiple serial ports, etc. on a 146 x 102 mm board, the size of a 3.5” floppy disk. Further, ESM boards are compatible with PCI/104 standard and are 149 x 71 mm Computer on Module (COM) standard boards that can be stacked with other compatible boards.
Other form factors that are compatible with the above form factors are the ISA slot boards which are either full length (13.8 x 4.8”) or half-length (7.1 x 4.8”), 3.6 x 3.8” and PC/104-Plus, 3.7 x 4.4” highly integrated and compact COM board used as an Integrated Circuit, Embedded Board, eXpandable (EBX) which is the size of a 5 1/2 “ floppy that is an expanded design of PC/104 and PC/104 Plus, and which host many modern operating systems with open interfaces. Also, the 100 x 72 mm pico-ITX is another board in box PCs that is known as the world’s smallest x86 mainboard that has an ultra-low power processor but offers full features of a regular x86 mainboard. The Micro-ATX is a 243.84 mm x 243.84 mm board with the mini-ITX (170 mm x 170 mm) smaller than mirco-ATX while 4.5” SBCs are low power fanless box PCs while the XTX is an open standard form factor based on ETX with newer interfaces like SATA and PCI Express rather than the obsolete ISA bus. Also, there are a range of other form factors that have been in use like the 70 x 70 mm QSeven Computer on Module, proprietary 58 x 65 mm CoreExpress and the 55 x 84 mm nanoETXexpress.
Further, the type of interface in a box PC SBC has also been used to characterize the different box PCs and is found to use standard PC bus interfaces like the ISA, PCI and PCI Express along with the more industry standardized interfaces like VMEbus, Compact-PCI and STD Bus.
