when transmitting digital information according to the nmra standard from a control unit 10 to a decoder or consumer 20 ( locomotive decoder or stationary decoder according to fig3 ), the schemata in fig1 is used to code the bit values 0 and 1 in which possible time points t [ send ] for return signals according to the invention are indicated . a data packet transmitted in that manner is shown in fig2 . the preamble is a header for a data packet and consists of a sequence of at least ten “ 1 ” bits . the packet start bit is the first “ 0 ” bit which follows the preamble . it concludes the preamble and signals that the next bits represent an address byte . after transmission of the address byte , there is another “ 0 ” bit as indication for a following data byte in the form of a data byte start bit . the error detection byte serves to recognize transmission errors . the packet end bit at the end of a data byte denotes the end of the data packet and generally belongs to the preamble of the following packet . in the example , the evaluation unit 30 receives the track signal shown in fig1 and 2 from the control unit 10 . the consumer 20 constantly detects and evaluates the track signal in a principally known manner and evaluates and carries out the control information contained in the data packets addressed to it . in this way , both the evaluation means 30 and the consumer 20 can use the square wave in the track to trigger and time the generation and detection of the return signal . the evaluation unit 30 supplies the return of feed - back information in the track signal detected by it to the central control unit 10 for further processing . [ 0032 ] fig2 shows a possible return or feed - back transmission of a byte according to the track format in fig1 . if a consumer 20 has fully received a data packet addressed to it , this consumer can feed back information via the evaluation unit 30 to the control unit 10 in the following data packet . for this purpose , the consumer 20 modulates the above mentioned higher frequency on the data packet , which the evaluation means 30 then demodulates again and in thereby detects a bit information of the return or feed - back signal . in the present example , a frequency of 1 mhz is used for the return signal which is far higher than the frequency of the track signal of 5 to 10 khz . moreover , the return signal according to fig1 is sent during the second signal half of a zero information (“ 0 ” bit ), since during this period , the digital track signal modulated by the control unit for a longer period of time does not exhibit a change in signal level , whose signal edges could lead to incorrect evaluation . by means of this triggering when producing the return signal and particularly when detecting the return signal , the signal distortions and interference present in the track signal itself are eliminated and inhibited from reaching the detection oscillating circuits , detection filters and detection counters in the evaluation means , which are sensitively tuned to the 1 mhz return signal . the available signal edge free periods between alternating polarities in the form of the second half of the zero information is long enough , compared to the short periods of the return signal , so that the return of feedback signal can reliably be detected in oscillating circuits used in the evaluation means 30 . the oscillating circuits have enough time to oscillate to the main frequency and to detect the bit value 1 which represents a return signal according to the present embodiment . the bit value 0 represents that the transmission frequency of the sender frequency in the consumer &# 39 ; s return signal is not present . besides , another allocation of the detection and non - detection of the return signal to the bit values 0 or 1 can be freely set . in order to achieve the highest quality , the nmra track format has the possibility to introduce stretched “ 0 ” bits as indicated in fig1 . thereby , the period of the second zero bit half can even be lengthened . in order to implement the described method , it is necessary that all consumers which momentarily do not send a return signal have a high impedance for the selected transmission frequency ( here , for 1 mhz ). depending upon the detection means in the evaluation means , it is also possible to select considerably lower frequencies , for example , down to 300 khz or even lower , for the return signal . in this case hardware expenditure may be higher and it might be necessary to lengthen the signal edge free periods between alternating polarities being used for transmission and detection of the return signal . alternatively , frequencies higher than 1 mhz are also possible for the return signal . the track signal format in fig1 and 2 shows at least eleven zero bits due to the use of the error correction byte in a valid data packet . thus it is possible to transmit from the consumer more than only 1 - bit information as a return signal which , in the simplest case , represents confirmation of receipt of the control signal . allowing for the synchronisation bit , at least ten data bits can be transmitted in the return signal to the control unit . of these , only eight bits corresponding to one byte are suitably used . therefore , it is possible , provided corresponding sensors are installed in the locomotives and other consumers , to transmit information about the current speed , acceleration , temperature and energy consumption of the driving motor or the energy consumption of stationary consumers and the like to the control unit . according to fig4 a 1 mhz oscillator 40 is provided in consumer 20 . the oscillator receives an oscillation enable signal from a scanning device 50 which scans the track signal and produces a synchronising signal to the predefined track signal period used . in this example , this period is the second half of the zero bits in the data packet following the data packet addressed to the consumer 20 . upon receipt of the oscillator enable signal , the oscillator 40 drives an otherwise open transistor switch 60 with 1 mhz . the switch 60 is connected to the track in series via a working impedance z . in the diagram , the impedance is provided behind a rectifier 70 which serves to supply energy to the consumer 20 as in known in the art . the series circuit of impedance z and switch 60 can also be directly connected to the track . in the example shown in fig4 the track voltage is superimposed by means of a current modulation with the return or feedback signal . this solution is technically simple and space - saving . the aforementioned necessary high impedance of a non - sending consumer for the return signal frequency is guaranteed by the inherent hardware of the consumer which is realized when switch 60 is open , i . e . disabled . according to fig5 the evaluation means 30 includes a detector 31 in the power circuit , which is supplied with the track signal . the square wave signal according to fig1 generated by the control unit is represented in fig6 a and indicated in fig5 . the track signal exhibits a number of signal distortions and interference as explained above , as well as a possibly existing return or feedback signal . in fig6 a , the return signal is located in the signal edge free periods between alternating polarities marked r of the modulated control voltage from the control unit 10 . the detector 31 obtains a detection signal from the track signal according to fig6 b . a subsequent signal limiting and pre - amplifying circuit 32 provides the normalized detection signal according to fig6 c , in which the return signal already occurs more clearly . a gate switch 34 connected to the signal limiting and pre - amplifying circuit 32 is provided in the form of an analogue switch , and filters out the time periods r used for the return signal from the normalized detection signal according to fig6 c . for this , the control contact of the gate switch 34 receives a synchronisation signal from a synchronisation device 33 . the synchronisation device 33 has the same principle construction as a sensor device 50 and receives the control signal sent to the track by the control unit 10 according to fig1 . alternatively , the track signal according to fig6 a can also be sensed by the synchronisation device 33 . as a result of this synchronisation , it is ensured that a filter amplifier 35 , here in the form of a high quality active band pass , which is tuned to 1 mhz , receives the reduced signal according to fig6 d . the output signal of the filter amplifier 35 according to fig6 e is demodulated in a demodulator 36 . the demodulated signal according to fig6 f is compared in a comparator 37 with a threshold value and the output signal according to fig6 g is supplied to the control unit 10 . [ 0041 ] fig7 shows a preferred embodiment for the detector 31 , according to which a measuring resistance is provided in a connection line from the control unit 10 to the track . the measuring resistance converts the existing track signal with or without superimposed return signal to a proportional voltage . the voltage measured over the measuring resistance is pre - filtered in a band pass and supplied to the signal limiting and pre - amplifying circuit 32 , which has been provided as a differential amplifier . otherwise , fig7 corresponds to fig5 . [ 0042 ] fig8 shows an alternative in which a detector 31 ′ is a measuring sensor which , for example , contains a differentiator which converts the square wave return signal contained in the detected signal into a pulse series . a counter 38 synchronized to the signal periods r by the switch 34 counts the pulses in each time period r . furthermore , the counter 38 is controlled by the synchronisation device 33 such that it is set to zero outside the time periods r , and counts the pulses during the time period r . for this purpose a gate switch is used . apart from the pulse series resulting from the return signal , the counted pulses can also be various interference pulses . as a consequence of the predefined high frequency of the return signal &# 39 ; s pulse series , these interference pulses , however , can be neglected in case of a sufficiently high count value . in this way , if the counter has counted , for example , up to 64 pulses , it can reliably be concluded that the counted pulses mainly result from the return signal and are not caused by interference and other sporadic signal distortions . besides , considerably lower frequencies for the return signal are also sufficient to “ lift ” the count value resulting from the return signal above contributions of the interference signals in the overall count value . a subsequent digital comparator 39 compares the count value of detector 30 with a set point value at the end of the time period r . if the count value exceeds the set point value , the comparator 39 generates a signal which represents a detected return signal during the time period r . the comparison control in the example is performed such that a possible detected return signal is transmitted to the control unit as long as the comparison control no longer transmits a release signal to the comparator 39 . as another alternative , it is also conceivable not to use a high frequency square wave modulation as return signal according to fig4 . instead , a correspondingly high frequency pulse series generated by the consumer as return or feed back signal can be directly coupled to the track and then detected using principle of fig8 . in the implemented embodiments the following hardware components and parameters have been used : control unit 10 : lz100 with an amplifier lv101 , both of lenz elektronik gmbh ; while there has been shown and described what are at present considered the preferred embodiment of the invention , it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention defined by the appended claims .
a method and device which digitally controls moveable and / or stationary electrical consumers in a model railway . the power for the consumers is supplied over the track in the form of a square wave voltage signal which is frequency and / or pulse width modulated according to digital control information for the consumers generated by a central control unit of the model railway . a consumer , after having received a control information addressed to said consumer , applies a return signal to the track , which return signal has a higher frequency than the frequency of the modulated square wave voltage . this return signal is detected by synchronizing the detection to the square wave voltage such that the return signal is detected in periods of the square wave voltage signal which are free of signal edges .