Abstract: A microcomputer is adapted to act as a communications processor for a personal computer of the type having its address and data busses supplied to a female connector forming part of a system expansion slot. The communications processor is formed on a printed circuit card and includes at least two I/O ports, one connected to its edge connector for insertion into the slot connector of the personal computer, and the other formed on its rear edge so as to be accessible to external connectors. The communications processor has a power supply energized through a connector on its rear edge and a back-up battery power supply. The communication processor can receive communications from and transmit to remote sources while the personal computer is being used for other purposes. Receiver messages are stored in the RAM of the communications processor and are unloaded to the personal computer under control of its keyboard at the operator's convenience.