a system for detecting the presence in a compartment of a rack of a portable transmission unit of a number of portable transmission units , each being capable for transmitting or for receiving a signal containing an identification number assigned to the unit . all compartments of the rack or only compartments receiving or having received the units therein are scanned . the presence of a unit in a compartment is acknowledged by the unit after it has received a scanning signal , being identical for each scanned compartment , by providing a response signal comprising the identification number assigned to the unit . for each unit which is present , or until the actual scanning thereof was present , in a compartment a control circuit updates a datum , indicating the presence of the unit in a compartment , in a memory . updating of the datum for absence can be carried out if a response signal is not received within a predetermined time or if a detection means , assigned individually to each compartment , detects a removement from a compartment for which a presence datum was set before .