A system for collecting data from machines is particularly but not exclusively, suited for use as a vending machine audit system, and, in the preferred embodiment, comprises an audit controller which records transaction data relating to the operation of the machine and periodically is caused to transfer the data into a data storage module in the form of a non-volatile memory. The module can be removed and inserted into a down-loading machine at a central location which extracts the data and uses it to provide a transaction record.The audit controller stores in the module a predetermined indication code only after first checking that the transaction data has been correctly stored in the module. The indication code signifies to the down-loading machine that the data was correctly transferred to the module. There is thus no need to lock the module into the controller during data transfer, because removal of the module during data transfer will not result in the production of an incorrect transaction record.Transfer of data to the module takes place only if the module stores an appropriate security code. If the customer forgets his own security code, access to the data is permitted by storing in the module a "skeleton" code known to the manufacturer.