The present invention relates to a device as described in the preamble of claim 1. Such a device has been used for a long time now and generally comprises several cam discs which are substantially circular, the discs being provided with stop cams at their circumferential edge. The stop cams can cooperate with a pawl which locks the rotation of a motor shaft as soon as the pawl falls behind a stop cam. For instance several discs may be mounted next to each other on the motor shaft, with each disc providing a different stop pattern. As soon as a different stop pattern is required, the pawl is suitably pulled out or slid inwards in the direction of the shaft. For a long time now, such devices have been used on conveyors which are to feed and discharge packaging units intermittently, more particularly cyclically, for them to be loaded at particular well-defined positions or stations. In particular, such conveyors are used for cyclically feeding trays or egg cases which are to be filled with eggs, such packaging units being provided with patterns of rows of nests. Such conveyors cross sorting conveyors, typically by passing under them, and their movements are adjusted to each other in such a way that the eggs are discharged from the sorting conveyor only when a packaging unit is positioned in the correct position. After the packaging unit has been filled, for instance in steps, row by row, it is discharged. Such a movement, also referred to as packaging cycle, can be composed of combinations of acceleration, continuous travel, deceleration, and standstill. During and/or after discharge of the filled packaging unit, a next, empty packaging unit can be positioned. The packaging cycle referred to can be repeated for each next P ackaging unit . Although the above-described device has been found to be reliable and durable, only a limited number of movement patterns are possible because of the limitations of the cam disc mechanism. Furthermore, with such cams, only simple movement patterns are possible. In particular for worldwide application, specifically involving a great many different kinds of packaging units, as well as in view of requisite higher handling rates, such a device is less suitable. In order to provide for the need indicated, and furthermore to obviate the established shortcomings, the device according to the present invention is characterized in that the device is provided with a combination of a transmitter and a receiver, for instance a detector, for scanning and detecting the code fixed on the disc, in particular to set the speed of the conveyor. During use, for instance scanning signals and detection signals are generated which are processed in an electrical driver circuit for the conveyor. More particularly, the disc is circular. Further, the code provided on the disc can represent at least one packaging cycle, or correspond to at least one packaging cycle. With great advantage, such discs can for instance be simply exchanged, allowing the conveyor to be rendered suitable for a great many different packaging units and associated packaging cycles. With great advantage, thus a conveyor which is easy to operate can be obtained. In a further embodiment, the device is characterized in that the disc is provided with holes, for instance near the circumferential edge, such a hole for instance corresponding to a said stop position. A further particular advantage of this is that when such devices are delivered and installed, prepunched discs can be delivered along with the device, so that the user can set new packaging cycles if necessary. In still further embodiments, the device is characterized in that with the holes electromagnetic induction signals are obtained, or that with the holes an optical ray path is created and interrupted, or also that with the holes a mechanical switch is energized, with which corresponding electrical signals are obtained. Discs with holes mounted on a drive shaft of a motor are known in the art of sorting conveyor control. Inductively coupled proximity sensors detect the passage of a hole, whereby signals are generated in an electric circuit. Such signaling is used to link, with the position of a hole, a position of the sorting conveyor chain to a position in the shift register of the control of a sorting machine. This enables advanced sorting of large numbers of products which have been labeled and classified in detection processes and take up a 5 well-defined position on the sorting conveyor. This signaling does not in any¬ way relate to the speed control of the sorting conveyor. In a further preferred embodiment of the invention, the device is characterized in that furthermore a signal processor is comprised for processing the scanning signals and the detection signals, with a single rotation of the disc 0 being for instance converted and fixed in a control signal sequence which is supplied to the drive circuit for each next packaging cycle. What can be accomplished in this way in a very suitable manner is that at start-up of packaging, the conveyor is made ready for use through a single reading procedure. Furthermore, this means that in combination with the 5 further embodiments and features mentioned, a packaging machine of simple operation is obtained. More particularly, with such a control, for instance a so-called PLC (= Programmable Logic Controller), the use of a computer and the associated operation, specifically in environments such as those of remote regions or also those of developing countries, can be avoided. o Further details of the invention will be elucidated with reference to a drawing, in which: Fig. 1 shows an elevation of a part of a conveyor provided with the device according to an exemplary embodiment of the invention, Fig. 2 shows another disc according to an exemplary embodiment of the 5 invention, and Fig. 3 shows an example of a movement pattern for a packaging cycle associated with the disc according to Fig. 1. In Fig. 1, in a housing 1, an endless conveyor 10 is arranged. This conveyor 10 typically comprises an assembly of chains, or toothed belts, and end o wheels, which may or may not be combined with conveyor belts connected to these chains or toothed belts. To move packaging units in conveying direction T, in this exemplary embodiment, carriers 11 are fastened onto the chains or belts, which provides for accurate positioning. By means of a chain or toothed belt 21, end wheel 12 is connected to and driven by a motor 20. However, the end wheel 5 can also be mounted directly on the motor shaft. Fig. 1 furthermore shows an adjustment plate 13. This adjustment plate 13 is for instance connected at one end to the axle of end wheel 12 and at the other end to the housing 1 through a combination of a set screw and a slot, and thus the positions of the carriers 11 can be adjusted to enable the packaging units, for instance trays, to be filled 0 from above with eggs as described hereinabove, without damages or breakage. On the axle of end wheel 12, a disc 30 is mounted by means of a combination 33 of positioning holes and clamps for positioning and fixation. The disc 30 will rotate in direction R in accordance with the conveying direction T. Provided in the disc 30 are several circular holes 31, 32. These holes function as 5 a code for the movement which the conveyor 10 with the packaging units .thereon will have to follow. How this works will be elucidated hereinafter. In the exemplary embodiment, the chain or toothed belt 21 and the axle of the end wheel 12 for instance constitute a transmission between the disc 30 and the motor, such that the motor 20 causes the disc to rotate during operation of the o motor. In Fig. 2 a similar disc 30 is represented, but now with a different pattern of holes 31 adjacent the circumferential edge. The combination 33 is represented in detail, with a central mounting block hole 34, as well as three fitted holes 35. When placing and mounting such a disc on the axle, hole 34 fits around a block 5 (not shown) as well onto three corresponding dowel pins passing through the fitted holes 35, whereupon the block is rotated and, likewise by way of corresponding recesses, is resiliently clamped on the dowel pins and secured. The working principle and use of this code are as follows. When, after such a disc 30 has been mounted on the axle of end wheel 12, the motor driving 0 the axle has been switched on, the disc 30 will start to rotate. The edge of the disc thereby passes detectors (not shown in Fig. 1) which detect the presence of a hole. As mentioned above, such detection can be done in various well-known ways, for instance electromagnetically or optically. Also, in a generally known manner, mechanical scanners can be used. More particularly, typically, inductively or capacitively coupled proximity sensors are used, or also photocell units. The signals obtained upon passage of a hole 31 stand for the stop positions which the conveyor must follow in the then desired packaging cycle. The passage of the holes 31 and 32 situated on the same radius constitutes the beginning of the cycle. In the present case, one revolution corresponds to one packaging cycle, that is, the movement pattern between acceleration and stopping for one and the same packaging unit, such as, for instance, an egg case or a tray. Such acceleration is initiated as soon as a signal is produced that indicates that filling in the occupied position has been completed. The whole packaging cycle, and hence a single rotation of the disc 30, is always traversed in a same period of time. Also, the time interval between two consecutive holes, that is, for each acceleration-continuous travel-deceleration part, is the same. Hence, with a greater angle, or distance, between two holes on the disc 30, the speed will have to be higher. In the exemplary embodiment according to Fig. 1, the meaning of two holes corresponds to two stop positions, that is, stopping twice. The small angle, or the small distance, between two holes then means that in a fixed time interval the packaging unit is displaced a small distance over the conveyor, and the large angle, or also the large distance, between two holes then means that the same packaging unit, in the same fixed time interval, is displaced a large distance over the conveyor. Accordingly, the speeds on these two distances will differ. These two time intervals together constitute the total time for this packaging cycle. In this way, the distance between neighboring holes measured in a rotation direction of the disc can for instance be substantially proportional to a predetermined speed with which the conveyor is to displace a packaging unit during a respective part of a packaging cycle during operation. Further, the distance between neighboring holes measured in a rotation direction of the disc can be substantially proportional to a predetermined distance over which the conveyor is to displace a packaging unit during a respective part of a packaging cycle during operation. The signals, i.e. the scanning signals and the detection signals, obtained upon such a first rotation, are converted by a signal processor, as for instance a PLC as mentioned above, to a control signal sequence. This sequence is subsequently fixed by the signal processor and followed for each next packaging cycle for this type of packaging unit. This control signal sequence drives the motor and consequently produces a movement pattern. More particularly, for the above-described movement pattern, a packaging cycle is represented in Fig. 3, showing a velocity -time diagram (υ-t diagram). The lines indicated with Sl and S2 are the distances traveled by the packaging unit during the two above-mentioned acceleration-continuous travel-deceleration parts. It is clear to see that the required time intervals for such parts are equal but that the distances are different. This cycle is for instance used for an egg case having two filling rows, whereby after the first stop the first row is filled, the case is then displaced over a distance Sl, then the second row is filled, and finally the case is further displaced from under the sorting conveyor over a distance S2 which is greater than Sl, while simultaneously a new case is positioned at the first stop of the next cycle. Such a υ-t diagram, that is, the corresponding signals that are needed for energizing the motor, is supplied by the signal processor which has been set in this manner, for instance by the manufacturer. It is further to be seen in such a diagram that there are portions where v = 0. During those portions, at least the packaging unit is filled. In addition, waiting time may be necessary, for instance because products are yet to arrive in position. Another possibility is that after filling, at standstill, a cover is to be folded over a packaging unit and to be closed. Other combinations for waiting and filling are equally conceivable. In any case, an acceleration -continuous travel-deceleration phase will not be initiated until a signal to that end has been produced by for instance the filling unit or the cover control just mentioned. After the above explanation, for the disc 30 in Fig. 2, a tray may be contemplated having in the conveying direction T five rows to be filled. It will be clear to anyone skilled in the art that a disc according to the invention can be designed in many ways other than with holes, for instance with recesses in the circumferential edge, with square openings instead of circular holes, with thickened portions or also protrusions in one direction or the other. Furthermore, it will be clear that the function of the disc is that of a memory, which implies that further details of a movement can be encoded on it, but also other control data of the conveyor. Further small modifications or variants are understood to fall within the scope of protection of the- appended claims.