Abstract: A removable package for use primarily in an ambulatory monitoring device, such as a medical monitor includes a RAM and a power supply. The RAM package is connected to an external device via an edge connector which includes interlock means for sensing when the removable RAM package is properly connected to the external device in order to prevent inadvertent writes into the RAM. In a preferred embodiment of the invention disclosed, the removable RAM circuit also includes a microprocessor compatible real time timer which is connected to the same data and address buses as the RAM.