A modular plug-in instrumentation system for enabling a personal computer to perform instrumentation functions, including receiving analog and digital signals from an external device and transmitting analog and digital signals to an external device, includes a carrier module pluggable into a bus of the personal computer, and also includes an internal instrumentation bus into which a plurality of different interchangeable instrument modules can be plugged. The instrumentation bus includes a digital portion, and also includes a segmented analog portion that is extendable merely by plugging instrumentation modules into bus connectors which span gaps between the analog bus segments. Analog and/or digital signals are communicated between various instrument modules and external devices by means of cables connected to the instrument modules. The carrier module includes interface circuitry that buffers, reconfigures, and synchronizes digital data from the personal computer bus to be compatible with the digital portion of the instrumentation bus. One embodiment of the carrier module includes a microprocessor system and a dual port memory which allows simultaneous execution of different programs by the carrier module and the personal computer.