Abstract: A inexpensive and high-density modular I/O architecture is provided which accommodates both electrical and optical signals in high-availability configurations. By increasing the number of slots that can fit in a given chassis, a greater amount of functionality may be packed into a given piece of equipment, thereby saving facility space. The higher density also provides for a more cost effective overall solution as more circuit boards may be accommodated by the infrastructure of a single chassis. Furthermore, all active circuit modules are replaceable from the front of a chassis and all external I/O cable connections are made on its rear. These external cable connections need not be removed or touched when faulty circuitry is to be replaced. This makes faulty board replacement quick and convenient and eliminates confusion and possible incorrect reconnection of disconnected cables. It also eliminates signal interruption which would decrease equipment availability.