Abstract: A device (12) for interfacing an external peripheral unit (18) to a store controller (16) and store terminal (14) of a point of sale (POS) system has been provided. The interface device includes an MPU (50) for passively monitoring the data coming from the store terminal without causing any interruption to the POS system. When a specific key sequence indicative of a request for use of the peripheral unit has been detected, the device interrupts the flow of data between the store controller and store terminal and enables the MPU to now send data to the store controller and store terminal. Additionally, the device provides a plurality of data paths for allowing the MPU to read data from the store terminal, for storing and reading data from the store controller, and for transferring data between the MPU and the peripheral unit.