Abstract:
A device for identifying a wire break between an electrical connection of a digital output and a load includes an evaluation module connected to the electrical connection and a voltage limiter module also connected to the electrical connection for co-determining a voltage value which is present at the evaluation module in the event of the wire break.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to the European application No. 03027733.9, filed Dec. 2, 2003 and which is incorporated by reference herein in its entirety. 
   FIELD OF INVENTION 
   The present invention relates to a device and a method for error diagnosis at digital outputs of a control module. In particular the present invention relates to a device and a method for identifying a wire break between an electrical connection at a digital output of the module and a load connected thereto. 
   BACKGROUND OF INVENTION 
   Many variants of control modules with digital outputs are already known. A digital output can thereby have one of the two digital states “digital ONE” or “digital ZERO”. The control module controls the load connected to the digital output depending on the state of said digital output. 
   In automation systems such control modules are used for example to activate a very wide range of actuators, e.g. electromagnetic components and in particular also signal generators. Such actuators have a specific load resistance and are in some instances connected together with a current-limiting series resistor via the corresponding electrical connection to one of the digital outputs of the control module. 
   If such signal generators are used in automation systems for monitoring purposes for example, the digital output will have one of the two digital states depending on operating state. For example in the case of a fire alarm in normal operation of the automation system, the output of the control module will have the “digital ZERO” state, while in the event of a failure, the “digital ONE” state is present at the output and the signal generator is therefore activated. 
   If an interruption now occurs between the electrical connection and the signal generator due to a wire break, the actuator can no longer be controlled by the control module and a malfunction results with the failure of the automation system. If, as in the case of the signal generator in the form of a fire alarm, the “digital ZERO” state corresponds to the normal operating state and the “digital ONE” state occurs only rarely, such a malfunction is not identified for quite a long time without additional monitoring. In order to establish such a malfunction promptly, it would be conceivable to send an additional test signal to the electrical connection at regular intervals to test whether the load is still securely connected to the electrical connection. 
   SUMMARY OF INVENTION 
   The object of the present invention is to specify a device and a method for error diagnosis at digital outputs of a control module, with which a cable break between an electrical connection at a digital output of a control module and the load connected thereto in particular can be identified in a simple manner in “digital ZERO” state. 
   This object is achieved by the claims. 
   By connecting a voltage limiter module and an evaluation module to the electrical connection of a digital output such that, in the event of a wire break between the electrical connection and the load connected thereto, a voltage value also defined by the voltage limiter module is present at the evaluation module, when the digital output has a first “digital ZERO” state, this voltage value can then be compared in the evaluation module with a reference voltage value, whereby a wire break can be identified, diagnosed and reported. Both the inventive device and the inventive method can therefore be used to identify a cable break, without an additional test signal even being required for this. The present invention therefore allows error diagnosis at digital outputs of control modules in the simplest manner. 
   Further advantageous embodiments and preferred developments of the invention will emerge from the dependent claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention and advantageous embodiments of the same are described in more detail using examples with reference to the Figures below, in which: 
       FIG. 1  shows the essential structure of the inventive device, 
       FIG. 2  shows a first example of a circuit arrangement for implementing the inventive method, 
       FIG. 3  shows a second example of a circuit arrangement for implementing the inventive method. 
   

   DETAILED DESCRIPTION OF INVENTION 
     FIG. 1  shows the state of the digital output D 1  of the control module IC 1  by means of the switch connected to the voltage Vcc, with a closed switch indicating the “digital ONE” state and an open switch indicating the “digital mere ZERO” state of the digital output D 1 . In the alternative circuit diagram for this switch the digital output D 1  in the. “digital ONE” state is connected via a resistor Ron and in the “digital ZERO” state via a resistor Roff to the supply voltage Vcc, whereby Ron is smaller than Roff. An electrical connection K 11  is located at the digital output D 1  of the control module IC 1 , said connection being configured as a terminal for example with the load L to be controlled by the control module IC 1  connected to it. This load L can be simulated by a load resistor RL, which, as shown here, is a purely ohmic load but generally also has inductive and capacitive components. The load L, which can for example be a signal generator acting as an actuator, is therefore connected on the one hand by means of the electrical connection K 11  to the digital output D and on the other hand via a further electrical connection K 12  to earth Gnd. According to the invention a voltage limiter module V 1  and an evaluation unit A are now connected between the digital output D and the electrical connection K 11 . The voltage limiter module V 1  is thereby preferably configured as a voltage divider comprising a resistor R 11  and a Zener diode ZD 1  between the supply voltage Vcc and earth Gnd. This voltage divider is connected to the digital output D 1  and the electrical connection K 11  via a second resistor R 12 . The Zener diode ZD 1  causes a certain voltage value to be present at the node between R 11 , R 12  and ZD 1  depending on the Zener voltage predefined by it. The evaluation unit A serves to measure the voltage value U 1 a present at the node between the digital output D 1  and the electrical connection K 11  and to compare it with a reference volt-age U 1 ref 1 . If with the digital output D 1  in “digital ZERO” state a cable break or some other sort of interruption occurs between the electrical connections K 11  and K 12 , a voltage is injected at said node via the second resistor R 12  of the current-limiting circuit V 1 , so that a voltage value U 1 a is present between the digital output D 1  and the electrical connection K 11  and therefore also at the evaluation unit A, which is essentially defined by the Zener voltage of the Zener diode ZD 1  of the voltage limiter module V. A comparator M 1  in the evaluation unit A compares this voltage value U 1 a with the preset first voltage reference value U 1 ref 1 . If the voltage value U 1 a is greater than or equal to the voltage reference value U 1 ref 1 , the signal UM 1  at the output of the comparator M 1  will change and thereby indicate the presence of a cable break. 
     FIG. 2  shows an example of a possible embodiment of a circuit arrangement for implementing the inventive method. The corresponding dimensioning of the individual components of the modules V and A for a control module IC 1  of type BTS 4141D with a supply voltage of Vcc=24V is also shown here. The actuator L 1 , which can be connected to the digital output D 1  should thereby have a resistance value in the 47 Ohm to 4K Ohm range and an inductivity of up to 1.1 Henry. The circuit arrangement shown in  FIG. 2  is thereby designed such that in addition to a cable break, other possible error states, in particular short circuit to supply voltage Vcc and to earth Gnd can be identified and diagnosed between the electrical connection K 11  and the load L 1 . The evaluation unit A has three comparators M 1 , M 2  and M 3  for this purpose. The voltage limiter module V 1 , as already shown in  FIG. 1 , has a Zener diode ZD 1 , a first (R 11 ) and a second (R 12 ) resistor. The Zener diode ZD 1  is designed such that a Zener Voltage of 8.2V can occur at it. If a cable break should occur between the load L 1  and the electri-cal connection K 11 , or even between the load L 1  and a further terminal K 12  connecting the load L 1  to earth Gnd, with the digital output D 1  in the “digital ZERO” state, a voltage value U 1 a will be established between the electrical connection K 1  and the digital output D 1 , which is defined by the Zener voltage of 8.2V and the voltage drop across the second resistor R 12 . In the present example this voltage value U 1 a is compared in the evaluation module A using the comparator M 1  of type LM2901 with a preset reference voltage value of U 1 ref 1 =5V. As the voltage value U 1 a is greater than the reference voltage value U 1 ref 1 , an output signal UM 1  is generated by the comparator K 1  to signal the error state cable break, whereupon the cable break can be identified and diagnosed. 
   In the exemplary embodiment shown in  FIG. 2  the two other comparators M 2  and M 3  are provided, in order to be able to detect and therefore diagnose the error states short circuit to earth (Gnd) and to supply voltage (Vcc). In the case of a short circuit to earth error, a voltage value U 1 a of only a few millivolts (approx. 5 mV) will now be present between the digital output D 1  and the electrical connection K 11 . This voltage value U 1 a is compared in the evaluation unit A of the comparator M 2  with a preset reference voltage value of U 1 ref 2 =11 mV. If the voltage value U 1 a is smaller than this reference voltage value U 1 ref 2 , the comparator M 2  generates an output signal UM 2  signaling the error state short circuit to earth. Correspondingly in the case of a short circuit to supply voltage Vcc error, a voltage value U 1 a will correspondingly be present between the digital output D 1  and the electrical connection K 11 , which corresponds approximately to the value of the supply voltage of Vcc=24V. This voltage value is compared in the evaluation module A of a comparator M 3  with a preset reference voltage value of U 1 ref 3 =15V. As in this case the voltage value U 1   a  is greater than the reference voltage value U 1 ref 3 , the comparator M 3  generates an output signal UM 3  signaling the error state short circuit to supply voltage. For completeness it should be pointed out that with the comparators M 1  to M 3 , and the additional components shown in  FIG. 2 , i.e. the laboratory resistor RM and the operational amplifier  10 , error states can also be determined when the digital output D 1  is in the “digital ONE” state. The circuit arrangement shown in  FIG. 2  therefore offers the option of identifying and diagnosing all possible errors relating to short circuit and cable break at the interface of a load L 1  with the electrical connections K 11  and K 12  and for both digital states at the digital output D 1 . 
     FIG. 3  shows a further exemplary embodiment of a possible circuit arrangement for implementing the inventive method, in which in particular a plurality of digital outputs can be monitored by a single common evaluation module A. The evaluation mod-ule A here comprises an integrated differential multiplexer  20  with a downstream operational amplifier  10 , an electric changeover switch  30 , an analog-digital converter  40  and a controller  50 . The voltage values U 1 a, U 2 a, . . . present at the individual electrical connections K 11 , K 21 , . . . of then control modules are read by the differential multiplexer  20  at its inputs  41 , 42 , . . . ,  4 n. To verify the possible errors in the “digital ZERO” state, the operational amplifier  10  is bridged as shown in the FIG. by the sub-sequent electric changeover switch  30 , so that the read-in voltage values U 1 a, U 2 a, . . . can be forwarded from the differential multiplexer  20  directly via the analog-digital converter to a subsequent controller for evaluation. The voltage values are then corn-pared in this controller with the reference voltage values stored there and an output signal is generated correspondingly, as in the exemplary embodiments shown in  FIGS. 1 and 2 , signaling an error state. If the electric changeover switch  30  is in the other position (not shown), error states at “digital one”, of the output of the control modules IC 1 , IC 2  can also be identified by means of the operational amplifier  10 , as shown in  FIG. 2 . This circuit arrangement offers the advantage in particular for error diagnosis with a plurality n of digital outputs D 1  ,D 2 ,. . . that error diagnosis can be carried out at all the digital outputs D 1 , D 2 , . . . with the minimum outlay. The digital outputs to be monitored can thereby all belong to a single control module or as shown in  FIG. 3  to n control modules. It is also possible for the digital outputs D 1 , D 2 , . . . to have the same output value or even different output values or for the connected actuators L 1  ,L 2 ,. . . to have different specifications. This circuit arrangement is therefore characterized in particular by the small component outlay and the high level of flexibility when monitoring a plurality n of digital outputs.