An implantable medical device has a medical unit, such as a pacemaker lead, and a casing at least partially enclosing the medical unit. Each slave includes a processor for performing an I2C address allocating program, which includes a signal setting module, a delay controlling module and an I2C address calculating module. The switch assembly includes a miniature reed switch and a miniature latching magnet affixed directly to one of the reeds or the associated lead wire. If the recording of the file system has failed, the address information recording means records the address information in the lead-in region, but does not thus record the address information if it is already recorded in the lead-in region, or the like. In some embodiments, a color change indicates that the timing device has expired.