An identification system for identifying plug-in or modular type peripheral devices adopted in a portable computer through one module bay is described. The identification system includes a voltage divider provided at the peripheral device. This voltage divider is connected to a signal line for supplying specific voltage level with the portable computer through a connector. Also, this signal line is coupled with a resistor which generates a reference voltage. An analog-to-digital converter is provided for converting voltage level produced at the signal line into the corresponding digital value. This digital value is supplied with the computer system to determine whether the digital value corresponds to a preset value designated to a specific peripheral device. To this end, a ROM BIOS stores a plurality of identification data corresponding to each digital value of the A/D converter. By detecting different analog voltage level as an identification signal through a signal line, using only one signal line is possible. This provides the most effective method for identifying more than three peripheral devices with the least signal lines and connector pins. Thus, the size of the connector can be reduced, and thereby saving space of the portable computer.