Abstract:
An input circuit for an input component is disclosed for enabling the safe reading of input signals applied to the input component. The disclosed input circuit for an input component, particularly a bus input component, enables the safe reading of input signals applied to the input component, particularly according to the standards Category 4 DIN EN 954 and SIL 3 IEC 61508, which is characterized by at least one signal input circuit with a one-channel periphery connection for the connection of at least one signal generator, an evaluation circuit, which can be connected electrically to the signal input circuit via a first coupling element, and a comparison circuit, which is connected electrically to the signal input circuit, for setting a reference voltage for the activation and deactivation of the coupling element.

Description:
FIELD OF THE INVENTION 
   The invention relates to an input circuit for an input component, particularly a bus input component, for safe reading of input signals applied to the input component. 
   BACKGROUND OF THE INVENTION 
   Reading data from a process environment, such as, for example, from a field, via input signals that are provided by appropriately arranged signal generators, that is particularly by switches [and] sensors, puts demands on the requirements for error recognition and the failure behavior of a circuit constructed for this purpose as well as of the periphery. 
   Particularly in the case of safety-relevant data in the field of safety technology, these requirements must meet, for example, the standards according to DIN EN 954-1 or according to IEC 61508. To satisfy such requirements, complex, conventionally two-channel, structures of the input circuit are required, which entails high cost for producing the circuit and for providing the circuits for the periphery, that is, particularly with respect to the wiring to the signal generator, for example, to a safety switch. Such circuits consequently have a very complex structure and as a result are also cost intensive. Due to the increased need for component elements, these circuits are additionally also disadvantageous with regard to their availability. 
   SUMMARY OF THE INVENTION 
   A problem addressed by the invention is to provide a circuit for reading safety-relevant signals, which is novel and substantially simpler compared to the state of the art. 
   An input circuit according to the invention for an input component, particularly a bus input component, for the safe reading of input signals applied to the input component, which input circuit satisfies particularly standards Category 4 DIN EN 954 and SIL 3 IEC 61508, is characterized by at least one signal input circuit with a one-channel periphery connection for connecting at least one signal generator, as well as by an evaluation circuit that can be connected electrically to the signal input circuit via a first coupling element, and by a comparison circuit that is electrically connected to the signal input circuit for setting a reference voltage for the activation and deactivation of the coupling element. 
   For the implementation of such an input circuit according to the invention, the invention thus proposes, furthermore, a comparison circuit for a signal input circuit of an input component, particularly a bus input component, with a one-channel periphery connection for connecting at least one signal generator, where the signal input circuit can be connected electrically to an evaluation circuit for reading input signals applied to the input component via a first coupling element, and where the comparison circuit, to allow a safe reading of the input signals, comprises means for setting a reference voltage for the activation and deactivation of the coupling element. 
   The invention thus allows, by means of the comparison circuit, the application-specific setting of a switching threshold between the signal input circuit and the evaluation circuit, so that the safety requirements according to category 4 DIN EN 954 and SIL 3 IEC 61508 can also be satisfied, where the signal input circuit is nevertheless designed with one channel for a safe input switching to the periphery, and can thus be constructed with few construction parts in a cost efficient manner and with substantially improved times between the occurrence of errors, so-called MTBF (mean time between failures) times and consequently increased availability. 
   To further increase the safety, it is preferred for the comparison circuit to comprise a reference voltage source and a circuit for testing the reference voltage source. As a result, the comparison circuit itself can be tested, and the switching threshold of the input signals required for the necessary safety can be ensured and verified in a simple manner. 
   To further increase the safety with regard to component failures, the test circuit, in a more advantageous variant, presents two voltage dividers that are constructed from three resistors and are connected to a reference diode, to allow the setting of the reference voltage. 
   Furthermore, it is advantageous for each of the voltage dividers to be connected to a respective second coupling element to allow the individual superposition, in each case, of a current flow. As a result, substantially all the critical components of the comparison circuit and of the signal input circuit as well as the coupling element can be tested. 
   It is advantageous, in the practical implementation, to connect the reference diode to a supply voltage where, to limit the supply voltage for the reference diode, a resistor is arranged in the conductor between the reference diode and the supply voltage, and a Z diode is connected in parallel to the reference diode. 
   The input circuit advantageously comprises a Z diode that is connected in the signal input circuit to limit the input voltage, which is based on an applied input signal, to a maximum value. The subsequent components can thus be designed overall for lower voltages. 
   Furthermore, to ensure the ability to test the coupling element arranged between the signal input circuit and the evaluation circuit independently of the comparison circuit, which further increases the safety, a variant of the input circuit according to the invention comprises furthermore a test circuit that is electrically connected to the signal input circuit for testing the first coupling element. 
   In a particularly simple manner, the test circuit preferably presents, for the purpose of testing the first coupling element, a third coupling element through which a test voltage can be applied to the first coupling element in the signal input circuit. 
   To ensure the decoupling of different voltage ranges, in a preferred embodiment, at least the first coupling element is an optocoupler, and, particularly advantageously, all the coupling elements are optocouplers. 
   The safety is further increased if the evaluation circuit is constructed with two channels. 
   Furthermore, it is advantageous for redundant signal generators to be series connectable to the signal input circuit. Because the signal input circuit for the redundant channel is thus completely omitted, improvement of the availability and a further reduction of the wiring cost are achieved because, as a result, only one clamp continues to be needed even in the case of the use of redundant signal generators, such as, for example, safety switches. 
   An additional advantage is that a plurality of signal input circuits can be connected in parallel to the comparison circuit because one comparison circuit is thus sufficient for a plurality of input circuits. 
   Thus, it is preferred for the circuit according to the invention to be used in safety technology for reading safety-relevant sensor signals and/or to be integrated in a bus input component, which is part of a master, a safe control and/or a field unit. The circuit here guarantees a one-channel construction in the periphery area leading to a two-channel evaluation in the logic area, which results in a clear reduction of the manufacturing costs and the foot print. Furthermore, the complete circuit can be essentially tested with simple means, and the switching thresholds can be tested 100%. This circuit can be used not only for the acquisition of safe signals, but also for the acquisition of standard signals, particularly if the requirements are stringent. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in further detail below with the help of a preferred embodiment in reference to the attached drawing, in which 
       FIG. 1  represents an exemplary structure of a safe input circuit according to the invention. 
   

   DETAILED DESCRIPTION 
   The input circuit according to the invention according to  FIG. 1  comprises an input circuit  100 , which is essentially constructed from a signal input circuit  110  and an evaluation circuit  120  that can be connected electrically to the signal input circuit  110  via a coupling element IC 1 . The signal input circuit has a one-channel periphery connection “INO K 1 ” for connecting at least one signal generator, such as, for example, a sensor or switch. Furthermore, the input circuit comprises a comparison circuit  200  with a testable reference voltage source  210  and is connected electrically with the signal input circuit  100 , and a test circuit  300  for the coupling element IC 1 . 
   The connection between the comparison circuit  200  and the input circuit  100  is carried out via the signal input circuit  110  at a connection point marked “COMMON IN,” at which, through the comparison circuit  200 , a reference voltage can be set for the activation or deactivation of the coupling element IC 1 . 
   Because additional input circuits can be connected via the common connection point “COMMON IN,” a first essential characteristic of the input circuit according to the invention thus is that the comparison circuit  200  needs to be present only once, including for the operation of several input circuits. Because such additional input circuits are constructed advantageously according to the input circuit  100 , the following description is limited to the embodiment represented in  FIG. 1  and is thus applicable to such additional input circuits. 
   For the galvanic separation of the evaluation circuit  120  from the signal input circuit  110 , the coupling element IC 1  is an optocoupler, and the resistor R 1  is connected before the optocoupler on the primary side for current limitation. The represented signal input circuit  110  presents, for example, a voltage divider, constructed from the resistors R 2  and R 3 , for an input signal that is applied to the input of the periphery connection “INO K 1 ,” as well as two capacitors C 2  and C 3  for compensating for parasitic effects. It should be noted, in this regard, that another circuit like the one represented can in principle also be included. 
   Furthermore, the signal input circuit  110  includes a Z diode D 1  for the final limitation of a high state input voltage applied to the input of the periphery connection “INO K 1 ” at a predetermined maximum high state input voltage value, as well as a diode V 1 , which is connected after the optocoupler IC 1  on the primary side and is intended to protect the optocoupler against negative voltage. 
   The secondary side of the optocoupler IC 1  is connected to a supply voltage VCC and to the evaluation circuit  120 . In the example according to  FIG. 1 , the evaluation circuit  120 , for the additional logic area that is not represented, is divided into two channels “INO K 1  Logic K 1 ” and “INO K 1  LOGIC K 2 ,” so that the input signals, which are applied with one channel, can also be processed using two channels in the logic area, which increases the safety with regard to the evaluation, but does not have to be necessarily required in a specific application. The resistor that is assigned to the channel “INO K 1  LOGIC K 2 ,” is here represented as a coiled resistor, so that a decoupling of the two channels against overvoltage and thus back-coupling freedom is ensured. 
   The comparison circuit  200  represented according to  FIG. 1  comprises a test circuit  220 , which presents two voltage dividers constructed from three resistors R 21 , R 22 , R 23 , and which is connected to the reference diode IC 23  of a reference voltage source  210 . A resistor R 24 , which is arranged in the conductor between the reference diode IC 23  and a supply voltage US 1 , and a Z diode D 2 , which is connected in parallel to the reference diode, limit the supply voltage US 1  for the reference diode IC 23  to a predetermined value. The resistors R 21 , R 22 , R 23 , and R 24  are also represented as coiled resistors. 
   By means of the voltage dividers R 23  and R 21  or R 23  and R 22  at the reference diode IC 23  plus the forward bias of the diode V 1  and of the optocoupler IC 1 , a lower switching threshold of the input voltage is set by means of a reference voltage applied to the connection point “COMMON IN.” As a result, it is ensured that the input can be recognized as set only after an input voltage has been reached that is greater than the predetermined switching threshold voltage. 
   The test circuit  300  represented in  FIG. 1  and provided for testing the optocoupler IC 1  comprises an optocoupler IC 3  through which a test voltage can be applied to the optocoupler IC 1 . A test for checking the functional capacity of the optocoupler IC 1  is advantageously carried out if an input signal applied to “INO K 1 ” is in the high state or changes from the low state to the high state, the high state input voltage value thus is/becomes greater than the switching threshold voltage, and the evaluation circuit  120  consequently recognizes the input during normal operation as set. If a test signal “INO K 1  TEST” is applied to the optocoupler IC 3 , the optocoupler IC 1  is bridged according to the wiring represented in  FIG. 1 , and a test voltage is applied to the optocoupler IC 1  by using appropriate dimensions for the resistors R 5  and R 6 , respectively, which are connected before and after the optocoupler IC 3 , at which voltage the evaluation circuit  120  recognizes the input as not set in the case of error-free operation. 
   Furthermore, in the comparison circuit  200 , the resistors R 23 , R 21  and R 22  are particularly of special importance. Because the minimum input switching threshold is to be ensured through them, the input circuit is preferably constructed with redundancy at this location to increase the safety. Each of the voltage dividers consisting of R 23  and R 21  or R 23  and R 22  is connected via an optocoupler IC 21  or IC 22  for the corresponding individual superposition of a current flow, to a conductor path marked “COMMON IN TEST 1 ” or “COMMON IN TEST 2 .” As a result of the redundant design, the voltage dividers R 23  and R 21 , or R 23  and R 22 , the optocoupler IC 21 , IC 22  as well as the reference diode IC 23  can moreover be tested completely. The optocoupler IC 1  as well is testable. 
   Below, different states of the comparison circuit  200  and corresponding error detections during the performance of tests are described. 
   The tests are advantageously also carried out if the input signal applied to “INO K 1 ” is in the high state; the high state input voltage value is thus greater than the switching threshold voltage, and the evaluation circuit  120  consequently recognizes the input as set during normal operation. 
   The two voltage dividers R 23  and R 21 , and R 23  and R 22 , respectively, are connected consecutively, and the results on the logic side, i.e., the signals of the channels “INO K 1  Logic K 1 ” and “INO K 1  Logic K 2 ” of the evaluation circuit  120 , are compared. 
   In case of error-free operation, the predetermined switching threshold of the input circuit is then correctly set for the acquisition of the input in the high state, which is connected to the input clamp or the peripheral connection INO if only one of the two optocouplers IC 21  and IC 22  is set. 
   Then, the two optocouplers IC 21  and IC 22  are set, which results in raising the reference voltage at the connection point “COMMON IN” in case of error-free operation to a predetermined value above the maximum high state input voltage value, which is predetermined by the Z diode D 1 . Because the input voltage is thus limited by the Z diode D 1  to a value below the switching threshold voltage, current no longer flows through the optocoupler IC 1 . In this state then, if no error is present, the logic signals of the channels “INO K 1  Logic K 1 ” and “INO K 1  Logic K 2 ” of the evaluation circuit are in the low state. 
   If the Z diode D 1  limits, for example, the maximum high state input voltage value to 8.2 V, and, if only one of the optocouplers IC 21  and IC 22  is used, the set voltage is applied to the connection point “COMMON IN” at 4.85 V, then, if an input is in the high state, current flows through the optocoupler IC 1  with error-free operation, and the evaluation circuit  120  recognizes the input as set. The two possible states, namely when only one of the optocouplers IC 21  and IC 22  is set, are used furthermore to check the resistors R 23 , R 21  and R 22  and the optocouplers IC 21  and IC 22  by alternately controlling the optocouplers IC 21  and IC 22 . If both optocouplers IC 21  and IC 22  are set, then the set voltage in the connection point “COMMON IN” is 8.47 V, for example. This state is consequently also used particularly to check the Z diode D 1  and the back-coupling of the input. 
   Thus, with the input circuit according to the invention, the optocoupler IC 1  and the input circuit  100  are tested overall with two different processes, and, at the same time, the comparison circuit  200  is tested, and substantially all critical errors are detected; here the safe state of the circuit is the low state. 
   The input circuit of the invention according to  FIG. 1  thus offers a possibility, which can be implemented in a simple manner, to transfer with one channel the input data of an input circuit, which data are applied to the periphery connection “INO K 1 ,” to two channels for evaluation. Although the circuit in the periphery area may thus be implemented with only one channel, it meets the requirements for two-channel systems according to DIN EN 61508 and DIN EN 954. Consequently, it requires only one connection clamp for the connection of external sensors, such as, for example, safety switches, which reduces the wiring cost, where redundant switches/sensors, if needed, can be series connected, which again substantially reduces the wiring cost. The circuit according to the invention is thus suitable preferably for the acquisition of safety-relevant signals in an input component and it is, furthermore, preferably integrated within an input component, which is not represented in greater detail, particularly a bus input component, which, for example, can also be part of a master, a safe control and/or a field unit. However, the circuit according to the invention can be used furthermore for the acquisition of standard signals, particularly if the requirements are stringent.