collects the data from the various decoders and transmits the data to the host alternatively the data processed by each decoder can be sent for example through a hub to a data processing system for example a personal computer for storage and possible further processing the data acquired by each linear camera 1 and sent to each decoder are sent to the host when the object 3 has travelled a preset distance from the source of the u201c trigger event u201d signal reaching a so called data transmission position indicated merely by way of example by line t in fig1 the fact that the data transmission acquired from the linear camera 1 occurs only when an object 3 reaches the data transmission position enables the detected codes to be assigned to a determined object 3 in order to enable the object 3 to be traced without the risk that the codes detected on an object 3 being assigned to another object 3 alternatively the data acquired by each linear camera 1 are sent to the host as soon as they are available the data being associated with the value of the signal generated by the advance sensor 8 the mode for image acquisition can be preset on the linear camera 1 or on the control device on the basis of the known resolution of the advance sensor 8 associated with the conveyor belt 2 alternatively the linear camera 1 or the control device can be configured in such a manner as to recognise automatically the resolution of the advance sensor 8 that is associated with the conveyor belt.