GB2070256A - Fault finding in electronic circuits - Google Patents

Abstract

The present invention provides what is essentially a switch (10) that may detachably be connected to a chosen part of an electronic circuit to be tested in which a fault is to be produced, the switch being activated-closed-to induce the test fault, and then being removed after the test has been run. The switch itself is essentially a pair of contacts (15) supported on a substrate (13) which can be removably mounted on a predetermined circuit component (11) so that the switch is linked across two of the component's connector elements (12) and so shorts the component out when the two contacts are allowed to close following withdrawal of separator means (23) previously keeping the contacts apart. This method allows the fault to be simply induced in a circuit which is in operation in a cramped environment. <IMAGE>

Description

SPECIFICATION Fault finding in electronic circuits This invention concerns fault finding in electronic circuits, and relates in particular to apparatus for temporarily causing a fault in an electronic circuit so that there may be tested the procedure for finding that fault.

It is often necessary to be able at will to cause a temporary fault to occur in electronic apparatus in order to test the procedure for finding and dealing with that fault. This is particularly so when the apparatus in question is both extremely complicated (as in, for example, a computer) and under normal circumstances must not be out of service for any significant length of time (thus, a computer running a microwave communications network). However, there are certain problems associated with the deliberate production of test faults.

Because it is a requirement that the test fault be induced on otherwise fault-free equip- ment (which equipment is either actually in, or will soon be placed in, a real in-use environment) it is important that the methods used to induce the test fault and then to remove it should not result in any real fault being occasioned. Most test faults are induced by the shorting-out of one or more of the circuit components in the equipment, and this is normally effected simply by soldering a conductive connector to the component carrier-which these days is commonly a printed circuit board-across the component(s) to be shorted.Unfortunately, the very act of soldering (and afterwards unsoldering) the connector can cause physical damage to the carrier, and this is particularly so where the carrier is a modern printed circuit board, for these tend to have very fine circuit interconnector elements which are easily separated from the board and then broken.

Additionally, it is usually desirable to be able to make the test fault first appear only when the equipment is actually in operation (powered up) rather than when it is not (powered down), not only because this will the more accurately reflect what really happens in a practical situation but because in certain types of electronic apparatus, specifically those incorporating logic circuits and an inbuilt fault-finding ability, the operational "awakening" of the equipment may never reach the point at which the fault is "found" if it was present even before the equipment was powered up (an analogous situation would be that of testing the ability of a car's tyres and suspension to cope with a 100 mph blow-out; there is little point in bursting a tyre while stationary, and then accelerating to 100 mph!).

Finally, because the test fault is to be induced while the circuitry concerned is in situ, and because modern electronic equipment is usually packed at a very high density (five 20 X 20 cm printed circuit boards might be contained side-by-side in a box no more than 10 cm across, giving each board a slot no greater than 2 cm), so that it istwery difficult to get into the equipment without dismantling it, it is an important requirement that the physical means of actuating test fault induction be suited to a really rather cramped environment.

The present invention, at least in its preferred aspect, seeks to provide the means whereby the problems occasioned by real equipment, powered up circuitry and a cramped environment are all at least substantially eliminated.

In one aspect, therefore, this invention provides a test fault module for enabling the induction of a test fault into electronic circuitry to which the module is attached, which module comprises: (a) a substrate which (i) is shaped for removably mounting on the top of a chosen circuit component, (ii) bears at least two terminals thereon each so positioned as to make electrical connection with one of the connector elements of the chosen component when the substrate is mounted thereon, and (iii) also bears a pair of contacts electrically connected to the pair of component terminals so as in operation electrically to link one terminal, and thus one component connector element, to the other; and (b) removable contact separating means for placing between the two contacts of each contact pair whereby the two contacts may be maintained apart until the separating means is withdrawn.

It is the purpose of the invention to provide what is essentially a switch that may detachably be connected to a chosen part of the electronic circuit in which the fault is to be produced, the switch being activated--clo- sed-to induce the test fault, and then being removed after the test has been run. As will already be appreciated, the switch itself is essentially a pair of contacts supported on a substrate which can be mounted on a predetermined circuit component so that the switch is linked across two of the component's connector elements and so shorts the component out when the two contacts are allowed to close following withdrawal of the separator means.

The module substrate, which is conveniently made entirely of any suitable insulator material, is so shaped that it may be removably mounted on the top of the chosen circuit component. Most conveniently, this shaping is such as to achieve a friction fit-a press fit-of the substrate onto the component, the two then being held together by sufficient friction to keep the combination secure until they are deliberately parted. The shaping of the substrate for this purpose is conveniently such as to cause the substrate to bear on its lower (reverse) side a hole~a socket-match- ing in shape and size the component it is to fit.Naturally, the actual shape and size of the hole will therefore depend upon the particular component upon which the substrate is to be mounted; as discussed hereinafter, the component is very preferably an integrated circuit (which commonly takes the form of an oblong block with a multiplicity of connector elements down each of two opposed long sides), and so the substrate very preferably has an oblong~ i.e., integrated-circuit-shaped#ole.

The substrate bears at least two terminals thereon each positioned so as in use to connect electrically with one of the component's connector elements. There may of course be more than two terminals (and not every one of these need contact a component connector element), so that if the component has several connector elements (such aS an integrated circuit does) it may be "shorted out" in one or more ways in order to produce different test faults, or sets of faults, at different times.

Though in practice it is unlikely that one would wish to short out any particular component in more than one way at one time, nevertheless, it is convenient (for manufacturing purposes) to make the substrate so that it has as many terminals as the component it is to fit has connector elements, so that thereafter there may be chosen any pair (or pairs) of terminals, and thus of connector elements, to be linked. This is especially so when the component is an integrated circuit; these are all more or less identical on the outside but vary tremendously on the inside (and so require to be "shorted out" in different ways).

The terminals on the substrate are so positioned as to make contact with the relevant connector elements of the chosen component when mounted thereon. The actual positioning of the terminals therefore depends to a very large extent on the location, and availability, of the component's connector elements when the component is itself mounted on the circuit carrier. Accordingly, if the carrier is a printed circuit board, the components thereon all being mounted tight against the board's surface (as is the present fashion), there may well be some types of components-such as simple semiconductor devi ces-where the connector elements are hidden; these components cannot easily be used with a test fault module of the invention.

Other components such as resistors and capacitors-are often placed upon the board on their side with their connector elements extending outwards at either end; these components can more easily be used with a test fault module of the invention. However, it is very much preferred that the chosen component the module is designed to fit be an integrated circuit, for (as mentioned above) these have their connector elements (or "pins") projecting outwardly and downwardly along the two opposed long sides of the component, and it is a simple matter to so place the module's terminals on the substrate that they contact the pins when the module is mounted upon the integrated circuit.Indeed, for a module intended to be mounted upon an integrated circuit, it is preferred that the terminals project through the substrate from the top (where, as described hereinafter, they are connected to the contacts) to the bottom (where they make the required physical and electrical connection with the pins). Indeed, it is particularly preferred that the terminals of an integratedcircuit-fitting module project (as aforementioned) down into, and at the two opposed long sides of the integrated-circuit-shaped hole in the substrate's under (reverse) surface; such a module is described in more detail hereinafter with reference to the accompanying drawings.

The substrate also bears a pair of contacts electrically connected to the pair of terminals (if there are two or more pairs of terminals to be connected then there is a corresponding number of contact pairs, but generally there will only be one pair of terminals to be connected, and thus only one pair of contacts). The contacts-which may in themselves be quite conventional (thus, two copper strips bearing platinum contact elements and being urged into contact with each others are (as intimated above) most conveniently placed on top of the substrate (on its obverse side). They may be secured to the substrate in any suitable fashion, and may be connected to the terminal pair also in any suitable fashion (say, by a short length of wire, or a passive component such as a resistor or capacitor, soldered between contact and terminal).

The inventive module is associated with contact separating means which can be placed between the contacts of the pair of contacts so as to keep them physically and electrically apart until it is removed (whereupon the contacts close, the two appropriate component connector elements are electrically linked, and the component is shorted out in the predetermined manner). The separating means conveniently takes the form of a small, thin piecc a flake, or a short strips insu- lating material, for example a polyester like MELINEX (Registered Trade Mark) in film form.

It is a very much preferred requirement that the separating means can be withdrawn, and the associated contacts allowed to close, when the module is mounted upon a component on a carrier in situ in the apparatus, despite the high packing density of the equipment. This can easily be achieved if the separating means is attached to thread-like pull means (a nylon thread, for example) which can be lead out from the innards of the apparatus through the availabic but small-gaps between its vari ous parts.

The module as a whole is, of course, of a size such that it can be mounted upon a component on a carrier in situ without fouling any neighbouring equipment.

As can doubtless be appreciated, the use of the module of the invention in, for example, an exercise to test a Repair Engineer's ability to find the printed circuit board bearing the faulty component, is as follows:~ (1) Unbeknownst to the Engineer, the test fault module is mounted on the appropriate component on the selected board, the board is replaced in the equipment with the module's contact separating means pull means projecting therefrom, and the equipment is powered up.

(2) When power up is completed, the pull means is pulled, causing the contact separating means to be withdrawn from the contacts and thus the test fault to be induced (the pull means and attached separating means are pulled out of the equipment so as to leave no externally visible trace as to which board is faulty.

(3) The Engineeer is then observed as he runs through the laid-down "Find-the-Fault" procedure, and timed up to when he locates and removes the faulty board.

An embodiment of the invention is now described, though only by way of illustration, with reference to the accompanying drawings in which: Figure 1 shows a diagrammatic, exploded and part see-through perspective view of a test fault module of the invention together with an integrated circuit; Figure 2 shows a diagrammatic side elevation of the module and integrated circuit of Fig. 1; and Figure 3 shows a diagrammatic end view of the module and integrated circuit of Fig. 1.

Where possible, the same parts have the same reference number.

The inventive module (generally 10) is one designed for use with an integrated circuit (11) of the type having four pins (as 12) projecting outwardly and downwardly of each of its two opposed long sides. The module 10 comprises a substrate (13) with a corresponding number of terminals (as 14) thereon, together with a pair of contacts (generally 15 in Fig. 1) mounted ( by means not shown) on its top surface. At the underneath of the substrate 13, and extending into the interior thereof, there is a space (16) of such a size and shape that the integrated circuit 11 is a press fit therein.The terminals 14 extend from above the substrate's top surface, through the substrate, and into the space 16, where they lie "on" the space's side walls (as 17; this is best seen in Fig. 3) in such a way that when the module is mounted on the integrated circuit the upper portions of the pins 12 of the latter contact the lower ends of the terminals 14 of the former.

The contacts 15 secured on the top of the substrate 13 are two springy strips (18, 19) each bearing a contact element (26) at its free end and facing the other strip. Each strip 18, 19 is connected to a chosen terminal 14 by a short length of wire (21, 22) soldered on at each end. Between the contact elements 20, and separating them, is a small piece of insulating film (23) to which is attached a long nylon pull thread (24) with a ring (25) at the free end (not shown in Fig. 3); if the thread 24 is pulled to withdraw the separator 23 from between the contact elements 20, then the latter will touch, electrically connecting the two terminals 14 linked by the wires 21,22 to the contact strips 18, 19~and thus, when the module is mounted on the integrated circuit 11, the appropriate two pins 12 will be linked so as to "short out" the integrated circuit and induce the required test fault.

Claims (10)

1. A test fault module for enabling the induction of a test fault into electronic circuitry to which the module is attached, which module comprises: (a) a substrate which (i) is shaped for removably mounting on the top of a chosen circuit component, (ii) bears at least two terminals thereon each so positioned as to make electrical connection with one of the connector elements of the chosen component when the substrate is mounted thereon, and (iii) also bears a pair of contacts electrically connected to the pair of component terminals so as in operation electrically to link one terminal, and thus one component connector element, to the other; and (b) removable contact separating means for placing between the two contacts of each contact pair whereby the two contacts may be maintained apart until the separating means is withdrawn.

2. A module as claimed in claim 1, wherein the substrate bears on its lower (reverse) side a socket matching in shape and size the component it is to fit, so as to achieve a friction fit therewith.

3. A module as claimed in either of the preceding claims, wherein the particular component upon which the substrate is to be mounted is an integrated circuit in the form of an oblong block with a multiplicity of connector elements down each of two opposed long sides.

4. A module as claimed in any of the preceding claims, wherein the substrate has as many terminals as the component it is to fit has connector elements.

5. A module as claimed in any of the preceding claims, which module is intended to be mounted upon an integrated circuit, wherein the terminals on the substrate project through the substrate from the top to the bottom.

6. A module as claimed in claim 5, the substrate of which module has a suitably shaped socket in its under (reverse) surface, wherein the terminals project down into, and at the two opposed long sides of, the socket.

7. A module as claimed in any of the preceding claims, wherein the contacts are placed on top of the substrate (on its obverse side).

8. A module as claimed in any of the preceding claims, wherein the contact separat-- ing means is a short strip of insulating material.

9. A module as claimed in any of the preceding claims, wherein the contact separating means is attached to thread-like pull means.

10. A test fault module as claimed in any of the preceding claims and substantially as described hereinbefore.