Abstract:
In order, in the case of a programmable logic controller having a modular structure, to be able to insert and withdraw assemblies even in the course of operation, without disturbing the data traffic taking place via the bus of the programmable logic controller, provision is made for arranging an evaluation circuit in a bus segment or a bus access circuit. The evaluation circuit establishes whether an assembly is connected to the bus and accordingly controls a variable resistor, which is arranged in one of the supply lines for the assembly, to have a high resistance or a low resistance.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a bus segment and bus access circuit for connecting an assembly of a programmable logic controller to a bus. 
     BACKGROUND INFORMATION 
     International Patent Application No. WO 93/15459 describes a bus access circuit. German Patent Nos. DE 36 03 750 and DE 36 03 751 describe bus segments and bus access circuits that are frequently used in programmable logic controllers having a modular structure. 
     In the case of such a programmable logic controller having a modular structure, it may be necessary to insert or withdraw assemblies in the course of operation. However, the insertion or withdrawal may give rise to the occurrence of feedback to the bus which connects the assemblies to one another, which feedback disturbs the bus traffic. In order to avoid such disturbances, provision is customarily made in the prior art for providing leading and/or lagging contacts on the assembly, which is provided with an evaluation circuit and a variable resistor (typically a MOSFET), with the result that the assembly is electrically connected to the bus only after complete insertion and is already electrically isolated from the bus before complete withdrawal. 
     This procedure is disadvantageous insofar as at least the evaluation circuit must be supplied with power from the outset. However, even the connection of the evaluation circuit to the power supply can already bring about disturbances to the voltage supply of the bus, to be precise particularly when a fault occurs in the evaluation circuit or the voltage supply of the evaluation circuit. A short circuit of the two supply contacts, for example, of the assembly, by means of which the evaluation circuit is supplied with power, would ineluctably lead to a complete collapse of the power supply of the entire bus. The same consequence would emerge if other, external effects on the plug-in location for the assembly caused the supply contacts of the plug-in location to be short-circuited to one another. This case may occur particularly when an assembly which, although it can be inserted mechanically into the plug-in location, is not, however, electrically and functionally intended for this plug-in location is inserted into the plug-in location. 
     SUMMARY OF THE INVENTION 
     The object of the present invention consists in providing a bus segment and a bus access circuit by means of which negative feedback to the bus is avoided under all circumstances. 
     A bus segment and a bus access circuit ensure, in particular, that an inserted assembly is supplied with power only when all the contacts, i.e., the signal contacts as well, are inserted into the plug-in location and the contacts thereof, and, furthermore, the assembly is isolated from the bus again if it is operating defectively or is not intended for this plug-in location. 
     The measure yields particularly smooth and hence disturbance-free coupling of the assembly to the bus. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows a modular programmable logic controller. 
     FIG. 2 shows the connection of an assembly of the programmable logic controller to the bus. 
     FIG. 3 shows the voltage characteristic at the test contact. 
     FIG. 4 shows the resistance characteristic of the variable resistor. 
     FIG. 5 shows the voltage characteristic at one of the supply contacts. 
    
    
     DETAILED DESCRIPTION 
     In accordance with FIG. 1, a modular programmable logic controller comprises a power supply assembly  1 , a central processing unit  2  and peripheral assemblies  3 . The peripheral assemblies  3  may be, for example, digital or analog input and/or output assemblies. Furthermore, the peripheral assemblies may also be hybrid input/output assemblies or intelligent functional modules. Via the peripheral assemblies  3 , the central processing unit  2  controls and monitors a technical process  6 , for example a chemical installation or a hydraulic press. 
     The central processing unit  2  and the assemblies  3  are for this purpose connected to one another in terms of data technology via the control bus  4 . The central processing unit  2  and the peripheral assemblies  3  are furthermore supplied with electrical energy via the supply lines  4 ′,  4 ″. A common earth connection is established via the supply line  4 ″; the supply line  4 ′ usually carries a potential of +5 volts. The control bus  4  and supply lines  4 ′,  4 ″ together form the backplane bus  5  of the programmable logic controller. 
     In the example above, only two peripheral assemblies  3  are illustrated. Alternatively, the programmable logic controller could have more assemblies, for example  5 ,  8 ,  10  . . . . Moreover, the control bus  4  may be designed as required. 
     Thus, the control bus  4  may be, for example, a serial bus comprising just one clock line and one data line. However, the control bus  4  could also be a parallel bus comprising a multiplicity of address, data and control lines. The concrete structure of the control bus  4  is of secondary importance within the context of the present invention. In every case, however, the lines of the control bus  4  form the signal lines of the present invention. 
     FIG. 2 now shows the connection of the central processing unit  2  or of the peripheral assemblies  3  to a bus segment of the bus  5 . As is immediately evident from FIG. 2, the assembly  2 ,  3  connected to the bus  5  has an internal circuit  7 . In the case of a peripheral assembly, the said circuit  7  is, moreover, connected to the technical process  6  via the process lines  8 . In every case, however, the assembly is inserted into a plug-in location  10  via the plug-in connection  9  and connected to the bus  5  in this way. 
     As is furthermore immediately evident from FIG. 2, the assembly  2 ,  3  is coupled in terms of data technology to the bus  5  directly via the assembly signal contacts  9 - 1  to  9 -n and the plug-in location signal contacts  10 - 1  to  10 -n, which are connected via the signal spur lines  11 - 1  to  11 -n to the signal lines  4 - 1  to  4 -n of the control bus  4 . 
     The power supply of the assembly  2 ,  3 , on the other hand, takes place via the assembly supply contacts  9 ′,  9 ″, which are inserted into the plug-in location supply contacts  10 ′,  10 ″. The plug-in location supply contacts  10 ′,  10 ″ are connected via the supply spur lines  11 ′,  11 ″ to the supply lines  4 ′,  4 ″. A MOSFET  12  is in this case arranged in the supply spur line  11 ′. The volume resistance of the MOSFET  12  is, as is known, variable and adjustable within wide limits. The MOSFET  12  therefore represents the variable resistor of the present invention. 
     The MOSFET  12  is controlled to have a high resistance or low resistance by the evaluation circuit  13 , which is likewise connected to the supply lines  4 ′,  4 ″, in accordance with the signals which are fed to the evaluation circuit  13 . 
     As is furthermore evident from FIG. 2, the plug-in location  10  has a test contact  14 , which is connected via the pull-up resistor  15  to the supply line  4 ′. The potential present at the test contact  14  is fed to the evaluation circuit  13  via the test contact line  16 . Furthermore, the potential present at the supply contact  10 ′ is additionally fed to the evaluation circuit  13  via the supply contact line  17 . 
     The present invention is based on the fact that the potential of the supply line  4 ′ is present at the test contact  14 , on account of the pull-up resistor  15 , if the assembly  2 ,  3  has not been inserted into the plug-in location  10 . If, on the other hand, the assembly  2 ,  3  has been inserted into the plug-in location  10 , the test contact  14  is connected directly to the supply line  4 ″ via the plug-in location supply contact  10 ″, the assembly supply contact  9 ″ and the mating test contact  18  directly connected to the latter, with the result that in this case the other supply potential is present at the evaluation circuit  13 . 
     As a result, the evaluation circuit  13  can evaluate the potential present at the test contact  14  and control the variable resistor  12  to have a low resistance or high resistance, depending on whether or not the assembly  2 ,  3  has been inserted into the plug-in location  10 . Furthermore, by evaluating the potential present on the supply contact line  17 , it is possible to establish whether the supply potential is collapsing on account of the insertion of the assembly  2 ,  3 . In this case, the MOSFET  12  is then controlled to have a high resistance again, with the result that the assembly  2 ,  3  is isolated from the bus  5 . As a result, the other assemblies connected to the bus  5  can continue to be operated. 
     The exact method of operation of the evaluation circuit  13  will now be explained below with reference to FIGS. 3 to  5 . 
     On the ordinate, the voltage U p  present at the test contact is plotted in FIG. 3, the resistance R of the MOSFET  12  is plotted in FIG.  4  and the voltage U of the supply contact  10 ′ is plotted in FIG.  5 . The time t is plotted in each case on the abscissa. 
     As long as the potential U p , which is fed to the evaluation circuit  13  via the test contact line  16 , of the test contact  14  corresponds, in accordance with FIG. 3, to the supply voltage U 0  of, for example, 5 volts, the MOSFET  12  is driven by the evaluation circuit  13  in such a way that it has a high resistance, as illustrated in FIG.  4 . When the potential momentarily drops to 0 volts at the instant t 1  and rises to 5 volts again at the instant t 2 , the MOSFET  12  likewise continues to have a high resistance, since the MOSFET is controlled to have a low resistance only when the potential Up present at the test contact  14  approximately corresponds to the (earth) potential of the supply line  4 ″ in an uninterrupted manner during a previously determined waiting time T 1 . 
     Such a brief drop in the potential U p  present at the test contact  14  can happen, for example, due to so-called contact bounce when the assembly  2 ,  3  is inserted. 
     Although, in accordance with FIG. 5, the potential U transmitted via the supply contact line  17  at this instant likewise collapses, this is irrelevant for the driving of the MOSFET  12  at this instant. 
     At a later instant t 3 , the assembly  2 ,  3  is definitively inserted into the plug-in location  10 . As a result, the running of the waiting time T 1  is initiated. After this waiting time T 1  has elapsed, the MOSFET  12  is, as illustrated in FIG. 4, gradually controlled to have a low resistance. Accordingly, the resistance of the MOSFET  12  decreases from the instant t 4 , and the voltage U present at the supply contact  10 ′ rises. 
     At an instant t 5 , the evaluation circuit  13  tests whether the potential U present at the supply contact  10 ′ approximately corresponds to the supply potential U 0  of the supply line  4 ′. In the present case, it is tested whether the potential present at the supply contact  10 ′ deviates by a maximum of 0.5 volts above or below the desired potential U 0 =5 volts. If the measured potential U lies within this value range, the resistor  12  continues to have a low resistance. It acquires a high resistance again (at the instant t 6 ) only when the test potential U p  rises again to the supply potential U 0 . 
     If, on the other hand, as illustrated by dashed lines in FIGS. 4 and 5, the potential U present at the supply contact  10 ′ lies outside the predetermined value range (in the present case, for example, outside the range of 4.5 to 5.5 volts) at the instant t 5 , this is assessed as a defective connection of the assembly  2 ,  3  to the bus  5 . In this case, the evaluation circuit  13  immediately controls the resistor  12  to have a high resistance again (see dashed line in FIG.  4 ). A flag is in this case set in the evaluation circuit  13  and prevents renewed controlling of the MOSFET  12  to have a low resistance until the assembly  2 ,  3  is withdrawn again from the plug-in location  10 . The flag is therefore reset only at the instant t 6 . 
     The time interval between t 5  and t 4  is the so-called stabilization time T 2 . 
     In accordance with the circuit illustrated in FIG. 2, the plug-in location  10 , the evaluation circuit  13 , the MOSFET  12  and the pull-up resistor  15  and the corresponding lines are part of the backplane bus  5 . In particular, the evaluation circuit  13 , the MOSFET  12  and the resistor  15  are assigned to the bus segment in which the plug-in location  10  is arranged. The backplane bus  5  then has a plurality of such circuits, namely one circuit per plug-in location  10 . 
     As an alternative, the backplane bus  5  itself could also have a modular structure, with the result that it comprises individual bus segments which can be connected to one another. This is indicated in FIG. 2 by virtue of the fact that plug-in connections  19  drawn with dashed lines are indicated. 
     It would equally be possible to arrange the evaluation circuit  13 , the MOSFET  12  and the resistor  15  together with associated interconnections and the plug-in location  10  in a dedicated bus access circuit  20 , which can be plugged onto the bus  5 , with the result that the access circuit is arranged between the assembly  2 ,  3  and the bus  5 . This is also illustrated diagrammatically in FIG.  2 . In this case, the bus access circuit  20  also has, in addition to the plug-in location  10  for the assembly  2 ,  3 , a bus connection  21  with contacts  21 ′,  21 ″,  21 - 1  to  21 -n. The contacts  21 ′ and  10 ′ are then again connected to one another via the connection line  11 ′; the same applies analogously to the contacts  21 ″ and  10 ″ and the contacts  21 - 1  and  10 - 1  to  21 - 1  and  10 -n. 
     However, the functionality of the circuit always remains the same, irrespective of the concrete structure.