System for testing digital circuits
Cross Reference to Related Application of the same applicants: Ser. No. 07/204,194, filed June 8, 1988, claiming priority of German Patent Application No. P 37 19 498.4 filed June 11, 1987. 
     The invention concerns a system for testing digital circuits, particularly in apparatus comprising circuit groups of digital signal-processing components operated in cascade. 
     It is known to test digital circuits with logic circuits and analysers which are able to show the nature of signals at various test points in the circuits. In modern test apparatus it is common to compare a reference test pattern which is derived from a properly operating functional group of components with a pattern from a component group just tested and to show only the differences in order to bring out disturbances in function. In these known methods, however, manual application of test clips to test points is necessary. It is further known that large functional units require a greater variety of test patterns than are needed in small functional units. The smaller functional units can be tested in isolation so that, in general, test operators manually separate functional units for test purposes for example by pulling out connection plugs and circuit boards. Furthermore the application of a suitable test pattern to the inputs of the separate functional units to be tested is often necessary. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a test system for digital circuits, for example digital video circuits, in which the running of a test will be as automatic as possible. 
     Briefly, test node circuits are interposed between successive circuit component groups that are normally operated in cascade. These test node circuits are connectable for test purposes to a test computer over a test bus. 
     The test node circuits are equipped to operate under control of the test computer to isolate for test purposes any intermediate portion of the circuit component group succession. The initial and final units of the succession can similarly be isolated by use of additional test nodes at the beginning and end of the succession. 
     The test node circuits are preferably equipped with certain additional components for participation in test operations, such as registers for storing signals for test evaluation by the test computer so that the test computer does not have to be connected in real time to everything to be tested in the apparatus. It is particularly useful for detection of an abnormal condition for a test node circuit to generate an interrupt signal which will result in the computer interrogating a register to identify an incipient failure. 
     For further details reference is made to the description given below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is further described by way of illustrative example with reference to the annexed drawings, in which: 
     FIG. 1 is a circuit block diagram providing an overall illustration of an embodiment of the invention; 
     FIG. 2 is a circuit block diagram of a test node; and 
     FIG. 3 is a circuit block diagram of part of a system according to the invention showing a more elaborately equipped test node. 
     The same parts and components are designated in the several figures with the same reference numerals. 
    
    
     DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS: 
     In the overall block diagram of FIG. 1 there are shown a number of signal processing circuit component groups 1, 2 and 3 which operate in cascade, the output of one being connected to the input of the next. The component groups 1, 2 and 3 can for example be parts of a digital video tape apparatus, or of a video camera or of a film scanner. Of course these component groups could also be parts of other digital electronic apparatus outside of the field of video technology. 
     Test nodes 4 and 5 are provided between the component groups 1, 2 and 3. In addition there is a further test node 6 between the input 7 of the apparatus and the first component group 1 and another test node 8 between the output of the last component group 3 and the output 9 of the apparatus as a whole. Two conductor groups 10, 11 form a test bus which is connectable to each of the test nodes. 
     In addition, the conductor group 11 is connected with the output of a test pattern generator 12, while the conductor group 10 is connected to the input of a test pattern evaluator 13. The conductor groups are, for example, each made up of 8 parallel conductors. 
     Test pattern generators and test pattern evacuators are known as such and do not need to be further explained in connection with this invention. A test computer 14 takes care of the control of the test pattern generator 12, the test pattern evaluator 13 and the test nodes 1, 2 and 3. It is also possible in the embodiment illustrated in FIG. 1, among other things, to supply a test pattern generated by the test pattern generator 12 through the test node 6 and the first component group 1 in place of the usual input signals of that component group. By corresponding switching of the test node 4, 5 and 8 the signals comprising the outputs of the component groups 1, 2 and 3 can be sequentially evaluated in the test pattern evaluator. In this fashion a simple localization of any possible errors can be accomplished. 
     Further details of the invention are particularly described with reference to FIG. 2. From the mechanical aspect of the test system, it should first be noted that both the circuit component groups and test node circuits that serve them may each be separate units, for example each being mounted on its own circuit board. The component group to be tested, however, is in general much more extensive than the test node relating to it. Furthermore, relatively many connections are needed between a component group and a test node, which is particularly the case for the example shown in FIG. 3. It will be therefore be useful in many cases to integrate the test node into the corresponding component group. This can be done by putting the test node on a common circuit board with the component group or by providing a so called pick-a-back plate for the test node. 
     FIG. 2 shows one of the test nodes in somewhat more detailed representation. The test node circuit includes essentially three unidirectional bus driving circuits 21, 22 and 23. The bus driver 21 connects the input 24 with the output 25 and thereby the output of the preceding component group with the input of the component group that follows the test node. In normal operation of the apparatus to be tested this bus driver is conducting and a corresponding control signal is provided from the input 26. 
     The bus driving circuit 22 is connected between the input 24 and the test bus 10. The output signals of the preceding module can therefore be supplied to the test bus 10 by applying a corresponding control signal through the input 27. 
     Finally the bus driving circuit 23 serves to lead signals from the test bus 11 through the input of the following component group for which purpose a control input of the bus driver 23 is connected to still another input 28. With test nodes 4, 5, 6 and 8 of FIG. 1 constructed in accordance with FIG. 2 it is therefore possible to interrupt the signal flow before and after any individual component group, to feed in test patterns and to supply the processed test patterns which have passed through one or more component groups to the test pattern evaluator 13. In order to lead the output signals of one of the component groups to a component group other than the one usually following it, it is provided in the example of FIG. 2 to insert one more bus driving circuit 29, as a switching device, between the test bus 11 and the test bus 10. 
     In the embodiment shown in FIG. 3 the test bus is subdivided into a data bus 31 and an address bus 32. The test node 33 is related to a component group 34, the output 35 of which is connected with the input of the test node 36. In order to indicate that the circuits described regarding the component group 34 represent only a part of a possibly much more extensive component group, only a part of a circuit board is illustrated holding the components shown. 
     For explaining the function of the test node shown in FIG. 3 it may be for example assume that the component group 34 includes among other things some kind of signal processing circuit 37, an error recognition circuit 38 and an error correction circuit 39 which is constituted as an exclusive-OR circuit. The error corrected signals proceed from the output 35 of the component group 34 to further processing equipment. 
     A bus system 40 of the test node 33 can be connected with the data bus 31 through a bidirectional bus driver 41. The bidirectional bus driver is activated by an address decoder 42 which is activated by the test computer 14 by means of the address designating that particular decoder, when the data bus 31 is to be connected with the bus system 40 through the bidirectional bus driver 41, therefore when the test node 33 is to be connected to the test bus. 
     As in the case of the test node according to FIG. 2 the signals supplied to the input 36 can be supplied to the test system in the case of the test node 33. For this operation the bus driver 43 is switched through in addition to the bidirectional bus driver 41. In many different applications of the test system of the invention, signals of very high data rates are processed by the component groups. It can therefore be useful to write the signals to be checked in a test system memory first for a short period of time and then to check and evaluate the signals at lower speed with the help of the test computer. In the embodiment of FIG. 3 accordingly, a read-write memory (RAM) 45 is connected to the data bus through a bus driver 44. The latter is switched through simultaneously with the bus drivers 41 and 43, so that the prescribed time-section of the signals to be checked will be written into the read-write memory 45. The test computer can then have access to the data stored in the read-write memory 45 and check these out at reduced speed. 
     The test system according to the invention is suitable both for testing digital circuits during operation and for testing them in a special test mode of operation of the circuits. For monitoring digital operation two sensors 46 and 47 are provided in the component group 34 which respectively provide signals when the operating voltage gets outside of a prescribed tolerance region and when the clock signals drop out. These sensors are connected with respective inputs of a status register 48 to which still other sensors can be connected. In the case of an error signal a data word designating the failure is written into the status register 48. In addition an interrupt signal will be given to the test computer over the conductor 49 in response to which the test computer will interrogate the test node connected to the status register and determine the place and nature of the failure. 
     Another register 50 has its inputs connected to the bus system 40 and also can receive a control signal from the address decoder 42. Switching signals can be written into the register 50 by the test computer 14 which can produce various changes in the component group 34. Thus the component group can be switched over from normal operation into a test operation. In the embodiment of FIG. 3, there can be performed for a test operation, the furnishing of correction signals out of the error recognition circuit 38 to the error correction circuit 39 in response to a switching control signal supplied over the conductor 51, so that in test operation the output signals of the signal processing circuit 37 can be checked without actually correcting an error. 
     In order to support a test of the component group 34 in accordance with the signature analysis procedure that is known under that name, an EPROM 52 and a register 53 are provided. In the EPROM 52 reference signatures for various test points in the circuit of the component group 34 are stored. 
     For testing, the test pattern is applied to an input of the component group. With assistance of a pick-up probe and the signature analysis system, the signatures are obtained from the signals at the test points. In this operation the sequence of test points is given to the test operator from the signature analysis system, for example by indication on a picture screen, and out of the EPROM 52 the corresponding reference signatures are called out and compared to the signatures obtained with the probe. The storage of the reference signatures in the test node has the advantage that in the event of replacement of the component group the test node will likewise be replaced. In this way the result is obtained that the stored reference signatures will always correspond to the state of development of the component group. 
     Commercially supplied components are available for the bus driver and registers. The status register 48 and the address decoder can be very simply provided with programmable array logic circuits (PAL). 
     Although the invention has been described with reference to a particular illustrative example, it will be recognized that variations and modifications are possible within the inventive concept.