A PC-based diagnostic system is modular-based, and includes a motherboard, a backplane board, a PCMCIA board, and a keypad adapter board. The motherboard communicates with I/O modules connected to the backplane board, PCMCIA modules connected to the PCMCIA board, and with a keypad or a mouse through the keypad adapter board. Based on the type of device to be tested, various diagnostic programs can be accessed internally to provide the signals used to perform the test. A system controller and a processor are provided on the motherboard to control the transfer of data and control from the motherboard, through the backplane, and to the I/O modules. Additional select lines are provided to select one or more I/O modules during a test. An ISA bus is also provided in a standard arrangement.