Method for producing a connection redundancy for a serial communication system having a master unit and a plurality of slave units, which are interconnected as a concatenation of point-to-point connections in line topology, and corresponding serial communication system

In a serial communication systems embodied as mono-master system in line topology, the invention achieves tolerance with respect to an arbitrary fault by using an additional return line to the master. This connection serves only for monitoring purposes during normal operation and it is only activated in the event of a line interruption, in order to undertake the communication to the “isolated” subscribers. In this case, two independent lines exist from the master to the slaves.

DETAILED DESCRIPTION OF THE INVENTION FIGS. 1 a, 1 b and 2 a, 2 b represent the prior art and have already been described above in detail. FIG. 3 shows a serial communication system according to the present invention with a return line during normal operation. The structure of the communication network essentially corresponds to that according to FIG. 1 b, i.e., to a serial communication system in line topology with communication subscribers connected as a concatenation of point-to-point connections. A mono-master network is shown as master unit M, generally arranged at one line end, and a plurality of slave units SL 1 to SLn. The master unit is connected to the first slave unit SL 1 by cable connection L 1 . The slave unit SL 1 is connected to the second slave unit SL 2 via cable L 2 and this continues up to the last slave unit SLn via cable L(n−1). Each cable can contain two lines for a bidirectional full-duplex connection with desired values being transmitted from the master unit M to the slave units SL 1 to SLn and the slave units SL 1 to SLn supplying respective actual values in the direction of the master unit M. In this case, the communication between the individual subscribers is effected in particular with the aid of telegrams T which are exchanged via the communication network. A return line L&plus; is led from the last slave unit SLn back to the master unit M. If the master unit M is not arranged at a line end, then the return line L&plus; is implemented between the two slave units constituting the line terminating subscribers. The resulting ring topology is identical in both cases; however, the traditional topology, with the master unit M at one line end, is assumed below. FIG. 4 shows a serial communication system according to the present invention with a return line, as described in FIG. 3 , after a line interruption U. In this case, the line interruption U occurs, for example, between the slave units SL 2 and SL 3 . Accordingly, the line L 2 is interrupted. In order to maintain communication between all the subscribers M, SL 1 . . . SLn, the return line L&plus; is activated. From the point of view of the master unit M, communication is now effected via two lines, in the first place via the previous line topology with line L 1 to the slave unit SL 1 and, further via the return line L&plus; as second communication line. The second communication line also makes it possible to reach all the slave units which are arranged downstream of the line interruption U via the cables or lines L(n−1)&plus; to L 3 &plus; which are assigned to the second line L&plus;. The latter relationship is illustrated by the fact that these lines are now likewise denoted with a “&plus;”. The main advantage of the present invention resides in the minimal hardware outlay required, since only a single cable L&plus; is additionally necessary in order to overcome all the difficulties mentioned above. The remaining functionality is provided in the region of the communication electronics of the slaves, or in the software of the master. This is described in more detail below. By adding a return line to the master unit M, mono-master systems in line topology according to FIG. 3 and FIG. 4 can be made absolutely tolerant with respect to line interruptions U. The novel method enables reliable and fast detection of an interruption U, even if the latter only occurs sporadically and fast activation of the return line L&plus;. The sequence when a disturbance occurs can be seen as follows: 1. localization of the possibly sporadic disturbance U; 2. production of a permanent interruption in order to ensure the independence of the two communication lines; (to this end, the master sends to the last subscriber upstream of the disturbed connection, in this case the slave unit SL 1 , the command for establishing the transmission of telegrams); and 3. activation of the second communication line L&plus;, L(n−1)&plus;, . . . L 3 &plus;; (to this end, the isolated subscribers are informed that the master unit M can now only be reached via the second line with the return line L&plus;. Accordingly, the data traffic can be taken up on the second line). After the elimination of the disturbance U, normal operation can be resumed as follows: 1. notification of the subscribers on the second communication line L&plus;, L(n−1)&plus;. . . L 3 &plus; that the communication is switched over again to the original first communication line L 1 . . . L(n−1); 2. Eliminate interruption U; (to this end, a command from the master unit M is issued to the last subscriber upstream of the formerly disturbed connection, in this case to the slave unit SL 1 , for resuming the transmission of telegrams T); and 3. reactivate a possibly implemented monitoring function via the return line L&plus;. This method will now be explained in a clock-synchronous communication. An essential requirement of the application in this case is that fewer than two bus cycles are permitted to elapse between the occurrence of a (possibly sporadic) line disturbance U and the activation of the second communication line, i.e. the undisturbed continuation of the communication to all the subscribers. A difficult task is reliable localization of an only sporadic disturbance U which, under certain circumstances, results merely in the loss of a single telegram T. The solution for this, according to the present invention, is to equip each of the slave units with two counters in order to determine, separately for both data directions of the full-duplex connection, the number of valid cyclic telegrams transmitted in the last transmission cycle. The telegrams that are in any case sent once per transmission cycle from the slave units SL 1 to SLn to the master unit M are extended by the counter readings in the case of the redundancy option described. The task of a control for the master unit M, e.g. in the form of a software, is to determine the disturbed connection from the counter readings of all the slave units SL 1 to SLn. The solution to this problem is essentially based on the insight that all the slave units SL 1 to SLn must transmit the same number n of telegrams in the “desired value direction”, while in the “actual value direction”, the number of telegrams to be transmitted in each case increases by one from slave unit to slave unit in the direction of the master unit M. This relationship is shown in FIG. 5 by each cable L 1 to L(n−1) and L&plus; comprising two lines. One serves for communication in the “desired value direction” (solid line), and the other line serves for communication in the “actual value direction” (broken line). In a manner corresponding to the number of slave units SL present, the number of n telegrams are sent from the master unit M in the “desired value direction” to the slave units SL 1 to SLn. Each slave unit SL 1 to SLn must transmit each of these n telegrams. The situation is different in the “actual value direction”, where each slave unit sends a telegram to the master unit M. Whereas the slave unit SLn which is furthest away from the master unit M does not have to transmit a telegram, the last slave unit SL 1 as seen in the “actual value direction” must transmit the telegrams of the “previously” situated slave units SL 2 to SLn, that is to say n−1 telegrams. By way of example, if a sporadic interruption U of a line m causes failure of cyclic telegrams from the master unit M in the desired value direction between the first slave unit SL 1 and the second slave unit SL 2 , all the subscribers SL 2 to SLn situated downstream of the disturbed location U report, in the next cycle, correspondingly fewer telegrams (n-n) transmitted in the desired value direction. This relationship is illustrated in FIG. 6 , which shows the number of cyclic telegrams in the event of a temporary line disturbance between the first slave unit SL 1 and the second slave unit SL 2 for a serial communication system according to the invention with a return line L&plus;. If the disturbance concerns the actual value direction with the failure of k of the telegrams sent from the slave units SL 2 to SLn to the master unit M, then this can be determined from the telegram number reported by the subscribers—in this case slave unit SL 1 —upstream of the disturbed location U. The situation shown in FIG. 6 illustrates these relationships assuming that, at the disturbed location U, owing to a sporadic line interference, m cyclic telegrams fail in the desired value direction and k cyclic telegrams fail in the actual value direction.