A complete computer on a single printed circuit board, including clock, microprocessor, RAM, flash memory, Ethernet and I/O controllers. Also called a "system-on-module" (SOM), a computer-on-module (COM) typically does not have I/O sockets for direct connection to peripherals as do single board computers. They must be plugged into the main board of another system, which completes the I/O pathways to the outside world.ETX, XTX and COM ExpressEmbedded Technology Extended (ETX) provides a standard form factor for x86-based computer-on-modules and includes support for the older ISA bus. Express Technology for ETX (XTX) excludes ISA and adds four lanes of PCI Express. COM Express/ETXexpress supports 16 lanes of PCI Express for graphics and bumps Ethernet from 10/100 to 10/100/1000. See single board computer.