Wikipedia
ESMexpress is a very compact computer-on-module (COM) standard. It is a complete processor module that currently supports several low-power Intel and PowerPC platforms. Apart from a CPU component, every module also includes memory and a range of serial communication interfaces such as PCI Express, Gigabit Ethernet, USB, SATA, SDVO, LVDS and HD audio. These interfaces are defined in the form factor's specification, and signals are assigned to two 120-pin connectors. This fixed pin mapping ensures that different ESMexpress modules can be exchanged more easily. Consequently, ESMexpress typically does not have an onboard FPGA. The idea behind this is to implement very specialized functions in an FPGA on the COM's carrier board to ease upgrades of the system CPU through exchange of the ESMexpress module.
The ESMexpress standard includes only one form factor: 95 x 125 mm. The most important specialty of this computer-on-module is its surrounding mechanical concept, which was designed for especially severe environmental conditions, for example in railway or avionics, or in the automation industry. The large temperature ranges prevalent in such fields of use are met by the modules through a cooling concept that does not need a fan for the CPU: the computer board is embedded into an aluminum frame and cover that can lead the heat off the processor by conduction cooling. At the same time this mechanical set-up provides stability to withstand vibration, but also electromagnetic compatibility (EMC).