WO1999031588A1

WO1999031588A1 - Method, apparatus and device for digital testing and diagnostics of pc boards

Info

Publication number: WO1999031588A1
Application number: PCT/IL1997/000407
Authority: WO
Inventors: Reuben Schrift
Original assignee: E & M Engineering (Y.G.R.) Ltd.
Priority date: 1997-12-14
Filing date: 1997-12-14
Publication date: 1999-06-24
Also published as: AU5234798A

Abstract

A method, apparatus and device for detecting faulty buses and for diagnosing faulty devices, open pins and high impedance shorts on a faulty bus within circuit sections of a printed circuit board by digital means. A series of digital pulses is applied to one of the circuit sections within the board under test in diverse enabled-disabled states of the bus devices connected to the said circuit section. The current pulse drawn by the circuit section in each of said enabled-disabled states of the said devices is picked up and transformed into a voltage pulse by a current sensor means connected to the said circuit section while differentiation of the voltage pulse is performed by the said current sensor means whereby measuring integrity is enhanced. The resulting voltage pulse is amplified, integrated, clamped and shaped and the voltage pulses received from different circuit sections via different sensor means are multiplexed by an addressable multiplexer card or by an inventive circuit design comprising a wired/OR gate. The value of each of the voltage pulses at the measured strobe is compared with a predetermined threshold value for the respective enabled-disabled state of the bus devices connected to the circuit section.

Description

Method Apparatus and Device for Digital Testing and Diagnostics of PC Boards

Field of the Invention

One of the most important tools in the circuit board industry is a fast, precise and efficient testing equipment. Production costs and production rate as well as the quality of the finished product largely depend on the performance of the tester applied in the production line of boards for different products such as cellular phones, PCs, laptop computers etc..

In this respect, the manufacturing defect analyzers (MDAs) and in circuit testers (ICT) of the prior art have the disadvantage that they do not provide a fully automatic, reliable and time saving means for testing printed circuit boards.

For a long time now the industry has been in want of a tester that detects stuck bus lines in printed circuit boards and, having located a stuck bus is then capable of automatically, efficiently and reliably identifying the individual device that causes disfunction of the defective bus .

There is also a long felt want for a testing means that detects open pins on bus devices as well as open pins on output pins of other digital components. Finally there is a need for a means for the detection of high impedance shorts on bus lines.

One testing approach known in the an is based on the detection of microvolt changes on the stuck node while turning chip enable on and off separately for each of the devices within the bus. Due to its major disadvantage of being highly susceptible to noise this method is very rarely applied.

Another method involves direct use of a current tracer. According to this latter method an AC signal is applied to the stuck bus and the current tracer is brought in contact with the pins of each of the suspect devices in sequence whereby a high current entering one of the pins may be easil - detected. It is an obvious disadvantage of this method that it does not lend itself to automation and that it is very sensitive to the precise placing of the tracing instrument over the device pins. A manual test of this kind is time consuming and it would be impractical to apply as part of the production line where it would be apt to create a bottle neck.

A method for automatic diagnosis of a failing bus in a printed circuit board is disclosed in US 4,459,693. The method detects stuck bus lines by disabling all bus devices, pulling each line to a high state and then to a low state, and measuring the bus line voltage level at each state respectively. In this method the bus line is declared stuck if the measured bus line voltage does not follow the pull state.

US 4,459,693 also teaches a method for stuck bus diagnostics that involves measuring the stuck bus line voltage and applying an opposite or same static voltage level. The bus devices are then disabled one by one and direct current measurements taken for each state respectively. The current measured on the common bus node in a state of total disablement is compared to the respective current values measured on the common bus node during enablement of individual devices and the bad device is determined on the basis of the current change with reference to the all disable state.

US 4,459,693 does not solve the problem of detecting a bus device open pin fault, nor does it provide a solution for the detection of high impedance shorts faults.

There is a need for a method that provides a greater diversity of tests for a PC board while at the same time being safer, faster, more sensitive and more reliable than the methods of the prior art.

Summary of the Invention

It is the aim of the present invention to provide a quick and efficient method for testing faulty digital devices and circuit sections in PC boards by digital means, that enables stuck bus detection and diagnostics as well as detection of open pins on bus devices or other faulty digital devices and detection of high impedance shorts on bus lines. The invention further provides an apparatus for digital testing of circuit sections in a PC board and a novel sensor device that is implemented in the said apparatus for carrying out the said method. In accordance with one aspect of the novel method, apparatus and device of the invention, a vector technique is employed wherein a dynamic pulse train is applied to a circuit section such as a bus line of the PC board under testing at high speed, in different enabled-disabled states of the devices comprised in the said circuit section. In accordance with another aspect of the invention the current pulse responses of the said circuit section in the said different states are picked up and amplified by specially designed current sensor devices comprised in a PC board tester apparatus. In accordance with yet another aspect of the invention measuring circuits and computer programs are used to dynamically compare the said current pulse responses to predetermined threshold levels.

It is one advantage of the invention that it uses dynamic digital vectors for driving the circuit section under test as well as for measuring responses from the said circuit section whereby test time is faster and backdrive period is shorter by orders of magnitude than the test time required for the equipment of the prior art, thus considerably increasing safety for board and components under testing.

It is a further advantage of the invention that specially designed current sensors and amplifier circuits are applied that perform a differentiation followed by an integration function. As a result measurements are much less sensitive to noise, voltage polarity, current drift and temperature changes than the methods of the prior art and signal integrity is considerably enhanced. In accordance with one aspect of the invention stuck bus detection is achieved by disabling all bus devices, applying a dynamic pulse train to the bus lines and measuring the current pulse response via the inventive current sensor means. A bus line is declared stuck when the voltage pulse output from the said current sensor means is found to be of a magnitude that exceeds a pre-determined threshold level.

In accordance with another aspect of the invention stuck bus diagnosis is performed by applying a train of dynamic pulses to a stuck bus in which all devices have been disabled, enabling the devices one at a time and monitoring the current pulse response for each device respectively by the inventive current sensor means. The bad device is pinpointed by a voltage pulse output from the said current sensor means that does not exceed a predetermined threshold. In accordance with yet another aspect of the invention open pin faults are detected by measuring the current pulse response on the line to which the pin is connected under application of a train of dynamic pulses to the said line while the devices are enabled one at a time. Where the voltage pulse output from the said current sensor means does not exceed the pre-determined threshold level for an enabled device, the corresponding bus line is declared open. The same method is applicable to detection of faults in output pins of non tristatable devices. It will be understood that in the latter case no enabling is required.

In accordance with a further aspect of the invention, high impedance shorts faults in buses are detected by applying a train of pulses and consecutively pulsing each bus line one at a time, while keeping all other bus lines at low state and all devices of the bus under testing in a disabled state. The current pulse responses are monitored by the inventive sensor means and the voltage pulse outputs of the said sensor means are compared to predetermined threshold levels. Adjacent bus lines for which the sensor voltage pulse outputs exceed the predetermined threshold level are declared shorted.

In accordance with yet another aspect of the invention it is suggested to perform multiplexing of the signal outputs from the said current sensor means by conducting the said signals outputs through circuits that shape them into all positive or all negative going signals to a wiring/OR gate that allows a signal from any single sensor means to reach the measuring point without disturbance from the remaining sensor means. It will be appreciated that such arrangement eliminates the need for a multiplexing and addressing scheme.

Brief Description of the Drawings

Fig. l is a diagrammatic presentation of a CLIPS sensor means

Fig. 2 is a diagram of a CLIPS sensor means with a pulse shaping and amplifying circuit

Fig. 3 is a basic diagram of a test cycle for a stuck low node

Fig. 4 is a basic diagram of a test cycle for a stuck high node

Fig. 5 is a schematic drawing of an in circuit tester apparatus with CLIPS sensor means on the fixture wires

Fig. 6 is a schematic drawing of an in circuit tester apparatus with CLIPS sensor means on the multiplexing - amplifier board Fig. 7 is a schematic drawing of an in circuit tester apparatus with a CLIPS sensor means integrated into the driver card and measurement circuitry of the tester

Fig. 8 is a schematic drawing of a multiplexing means with a wiring/OR gate for a tester apparatus

Detailed Description of a Preferred Embodiment

The inventive method and device will be described hereinbelow in respect of a preferred embodiment. It will be understood that the invention is not limited to this embodiment and many other ways of constructing, implementing and using the invention are envisaged that remain within the scope of the description and the claims.

The invention teaches a method and device for testing a printed circuit board that enables the detection and diagnostics of failing digital devices and circuit sections such as stuck bus nodes, open pins on bus devices or other faulty digital devices as well as detection of high impedance shorts on bus lines.

In accordance with one aspect of the invention a vector technique is employed wherein a dynamic pulse train is applied to the bus lines of the PC board under testing at high speed. The current pulse responses of the components are picked up, transformed and amplified by novel current sensor means, that will be designated hereinbelow as CLIPS (Current Line Pulse Sensing) means. In accordance with another aspect of the invention measuring circuits and computer programs are used to dynamically compare the current pulse responses to predetermined threshold levels. A diagrammatic presentation of a CLIPS sensor means is shown in Fig. 1. The CLIPS sensor means, designated S, comprises a pickup coil with windings C , to which a resistor R is connected in parallel.

A fixture wire W connects the pin driver P to the node N such that the test pulses are carried to the said node N that connects to the board under test and the current pulse on the node N is sensed by the said CLIPS sensor means S. The CLIPS sensor means S is attached on the fixture wire W and the fixture wire W is wound once around the said CLIPS sensor means S creating loop Wl. According to known principles of electronics in the case of two adjacent lines a current change in one line will induce a current pulse in the other, adjacent line and the longer the two lines run parallel to each other, the stronger the effect will be. Similarly, a current change in the loop Wl will induce a current pulse in the windings C which in turn will be transformed to a voltage pulse on the resistor R and the longer the windings, the stronger the effect will be. It will be understood to those versed in the art that instead of a single loop Wl two or more loops may be made by winding the fixture wire W around the CLIPS sensor means S. However such an arrangement may affect signal integrity. The number and diameter of the windings C is calculated according to the conditions in the testing apparatus. Thus for example a CLIPS sensor means with 50 windings and having a perimeter of 30 mm would emulate about 5 feet of parallel running wire. Experiments demonstrate that such a device would produce a 300 milliVolt pulse over a resistor R of 200 Ohm, corresponding to a 300 milliamper pulse in the fixture wire W.

In the preferred embodiment no core is provided within the inventive CLIPS sensor means S so as to avoid affecting either the signals fed to the board under test via the fixture wire W or the output signals measured on the said fixture wire W.

It will be understood that a core may be comprised in the CLIPS sensor means S whereby signal integrity may be affected.

In accordance with another embodiment of the invention, the CLIP S sensor means S could assume the form of a miniature transformer (SMD) with a one or more loop primary connected in series to the fixture wire and a secondary serving as the sense line, again without a core.

According to another aspect of the invention, since the CLIPS sensor means S performs a differentiation function the voltage pulse output on the CLIPS sensor means S corresponds to current changes only whereby the inventive device and method may be equally implemented with positive or negative currents for stuck high or stuck low conditions respectively. According to yet another aspect of the invention, The CLIPS sensor means S may be applied for bipolar or negative logic applications.

Fig. 2 is a diagrammatical drawing of a preferred embodiment of an electric circuit that enables amplification and shaping of the pulse output of a CLIPS sensor means S whereby a high resolution stable current pulse measurement of the circuit sections in the board under test is provided. The invention is not limited to the circuit design of Fig. 2 and diverse circuits for amplifying and shaping the measured pulse may be employed as known in the art. Referring now to Fig 2, a current pulse is fed into a node N within a board under test and the current change on the fixture wire W that connects the pin driver P to the said node N within the board under test is picked up, transformed and differentiated by the CLIPS sensor means S via the induction or crosstalk effect between the loop Wl and the coil C as described hereinabove with regard to Fig 1 and then transmitted to the measurement point M via amplifier means A and diode means D

Gain values for the amplifier means A are determined by resistor means Rl, R2 and R3 which may be set in accordance with sensitivity and current change requirements It will be appreciated that a number of resistors other than three may be used.

The diode means D. jointly with resistor means R4 and amplifier means A serves as integrator of the measured pulse while the said diode means D, resistor means R4 and amplifier means A also provide for clamping of the signal such that only positive going pulses may reach the measurement point M It will be appreciated that polarity is reversed only for the sake of convenience so that only positively going pulses are received after amplification by amplifier means A It will be further understood that a different amplifier circuit, that does not reverse polarity, may be employed without significantly altering the method and device of the invention

Fig. 3 shows by way of example a basic timing diagram of the wave forms measured at various points of the circuit of Fig 2 or a similar circuit that enables amplifying shaping and clamping of the voltage pulse output of the CLIPS sensor means S. during pulse testing of a stuck low node. In Fig 3 I is the pulse driven into the stuck low node by the pin driver P of Fig.2, J is the current flowing through the node N of a bus within the board under testing, K is the sensor response pulse measured on the CLIPS sensor means S of Fig 2, L is the amplified positively going pulse measured as an output of the amplifier means A of Fig 2, and N is the measure strobe that is received on measurement point M of Fig 2 after shaping by diode means D and resistor means (Rl, R2) R3 of Fig. 2

Fig 4 shows by way of example a basic timing diagram of the wave forms measured at various points of the circuit of Fig. 2 or a similar circuit that enables amplifying shaping and clamping of the current pulse output of the CLIPS sensor means S. during pulse testing of a stuck high node. In Fig 4 II is the pulse driven into the stuck high node by the pin driver means P of Fig.2, Jl is the current flowing through the node N of a bus within the board under testing,

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SUBST1TUTE SHEET (RULE 26) Kl is the sensor response pulse measured on the CLIPS sensor means S of Fig. 2; LI is the amplified positively going pulse measured as an output of the amplifier means A of Fig. 2 ;and Nl is the measure strobe that is received on measurement point M of Fig. 2 after shaping by diode means D and resistor means (Rl, R2) R3 of Fig. 2.

It will be understood that since the voltage pulse measured at measurement point M of Fig. 2 corresponds to current changes on the fixture wire W only, the resulting measurements in accordance with the inventive testing method do not depend on the polarity of the tested device or circuit section or the logic applied but only on the output impedance of the said device or circuit section.

In order to clarify the inventive method a test cycle is suggested by way of example that consists of 7 clock cycles with a duration of 2.5 microseconds for each cycle whereby the test is allowed to run at a speed of 400khz such speed falling well within the capabilities of state of the art In-Circuit (ICT) testers on the market. The first 5 cycles serve to initialize the sensor circuit to the DC current drawn by the node. These are followed by a high and a low to establish the drive pulse. Such initializing is important when stuck high nodes are tested, regardless of whether they are positive logic, negative logic or bipolar.

The CLIPS sensor means of the invention may be applied in a PC board tester apparatus-such as an in circuit tester in many different ways. Three alternative modes of applying the CLIPS sensor means in an ICT tester-apparatus will be described hereinbelow by way of example.

A preferred embodiment of the invention in a typical ICT tester apparatus is shown in Fig. 5. The ICT tester apparatus comprises a bed of nails a with an array of pins 1,2 and 3, connected to a fixture interface b via fixture wires 1.1, 2.1., 3.1 and connection means 1.2, 2.2 and 3.2 respectively. It will be appreciated that the actual number of pins comprised in the bed of nails a may be much larger as known in the art. Fig. 5 further shows a printed circuit board c comprising a multiplicity of varied devices and circuit components. The printed circuit board c is placed on the ICT tester such that contact is established with the array of pins 1,2 and 3. The tester further comprises other elements, not shown in Fig.5, such as a Driver Receiver card, measuring circuits, computer and computer programs as known in the art for the feeding of pulses into the circuit sections comprised in the board c and for the processing and presentation of the output data received from the said circuit sections. On wire 3.1 a CLIPS sensor means S is attached When a train of pulses is sent to a circuit section within the board under test c via the fixture wire 3.1 and the nail 1, the said CLIPS sensor means S is induced by the pulse current flowing through the said fixture wire 3.1. The CLIPS sensor means S is routed to a multiplexer and amplifier board d via a sensor wire 4 and the voltage pulse output of CLIPS sensor means S is transmitted to the said multiplexer and amplifier board d to be amplified and shaped by circuits such as described in respect of Fig. 2 hereinabove or other suitable circuits.

The multiplexer-amplifier board d may also perform the necessary addressing of the current pulse outputs from the CLIPS sensor means S. In accordance with a further aspect of the invention, described hereinbelow, the multiplexer amplifier means d may comprise wired OR connection means that obviate the need for a multiplexing and addressing scheme.

It will be understood that where a wireless in circuit testing apparatus is used, the CLIPS sensor means S may be attached directly to the fixture pins.

Fig. 6 shows another preferred embodiment of the application of the CLIPS sensor means of the invention in an ICT tester apparatus that comprises a bed of nails a with an array of pins 1,2 and 3, connected to a fixture interface b via fixture wires 1.1, 2.1., 3.1 and connection means 1.2, 2.2 and 3.2. It will be appreciated that the actual number of pins comprised in the bed of nails a may be much larger, as known in the art. Similarly to the method described hereinabove in respect of Fig. 5, a printed circuit board c comprising a multiplicity of varied devices and circuit components is placed on the ICT tester apparatus such that contact is established with the array of pins 1,2 and 3 and the tester further comprises other elements, not shown in Fig.6, such as a Driver/Receiver card, measuring circuits, computer and computer programs as known in the an for the feeding of pulses into the circuit sections comprised in the board c and for the processing and presentation of the output data received from the said circuit sections

In the ICT tester apparatus of Fig 6 the CLIPS sensor means S is built into a multiplexing- amplifying board e, the fixture wire 1.1 is routed to the said multiplexing-amplifying board e, contact is established between the pickup coil of the CLIPS sensor S and the fixture wire 1.1 by winding the said fixture wire 1.1 around the said pickup coil as described in respect of Fig. 1 hereinabove and the current pulse on fixture wire 1.1 during testing is sensed by the said CLIPS sensor means S The pulse picked up by the CLIPS sensor means S is then amplified and shaped by circuits within the multiplexing-amplifying board e such as the circuit described in respect of Fig 2 hereinabove or other suitable circuits The pulse is then conducted from the multiplexing-amplifying board e via the Driver/Receiver card of the in circuit tester apparatus to the measuring circuits of the tester

In accordance with yet another embodiment of the invention, shown in Fig 7, the CLIPS sensor means is designed into the Driver/Receiver card of the tester and routed internally to the measuring circuits

Referring now to Fig 7, an ICT tester apparatus is shown comprising a bed of nails a with an array of pins 1,2 and 3, connected to a fixture interface b via fixture wires 1.1, 2.1., 3.1 and connection means 1.2, 2.2 and 3.2 It will be appreciated that the actual number of pins comprised in the bed of nails a may be much larger as known in the art A printed circuit board c comprising a multiplicity of varied devices and circuit components is placed on the ICT tester such that contact is established with the array of pins 1,2 and 3 and the tester further comprises other elements, not shown in Fig.7 and delineated by dotted lines il, i2, such as a Driver/Receiver card, measuring circuits, computer and computer programs as known in the art for the feeding of pulses into the circuit sections comprised in the board c and for the processing and presentation of the output data received from the said circuit sections and devices

In the ICT tester apparatus of Fig 7 the CLIPS sensor means S is designed into the said measuring circuits As shown schematically in Fig 7, the CLIPS sensor S is attached on a wire 4 that connects the connecting means 2.2 to the driving circuits and specifically to a driver h Contact is established between the pickup coil of the CLIPS sensor S and the said wire 4 by winding the said wire 4 around the said pickup coil as described in respect of Fig 1 hereinabove and the current pulse on wire 4 during testing is sensed by the said CLIPS sensor means S The pulse current output of the CLIPS sensor means S is then amplified and shaped by circuits such as described in respect of Fig 2 hereinabove or other suitable circuits and the resulting pulse is measured and compared to a preset threshold value by the said measuring circuits In accordance with a characteristic advantage of the preferred embodiment of the inventive method that the measured current pulse may be controlled to be always positive going due to the above described differentiation and integration action.

Since the pulses are always positive going, the CLIPS sensor means can be connected via multiplexer and amplifier circuits such as described in Fig. 2 in a wired OR connection to a single amplifier means. Such connection performs the same function as a multiplexer amplifier board while eliminating the need for a multiplexing-addressing scheme and allowing for random wiring of the sensor means to the measuring circuits.

Referring again to Fig. 6, it shows an in circuit test apparatus with a multiplexing amplifying board e on which the CLIPS sensor means are connected. Refeπing now to Fig. 8, it shows a preferred embodiment of the connections comprised in an inventive multiplexing amplifying board that will be called a CLIPS board means. While in Fig. 8 connections to the measuring means are shown for three CLIPS sensor means only, it will be appreciated that a large number of CLIPS sensor means may be similarly connected to the measuring means. As seen in Fig. 8, each of the CLIPS sensor means within the CLIPS board means is connected to a node that leads to a circuit section in a board under test such that it may pick up current pulses from the said node and it is also connected to a preamplifier means. Referring again to Fig. 8, each of nodes Nl N2 and N3 connect to a circuit section within the board under test. In accordance with the inventive test method current pulses are driven into nodes Nl N2 and N3 by drivers PI P2 and P3 via wires Wl, W2 and W3 respectively and the current pulses on the said wires are picked up and transformed by CLIPS sensor means SI S2 and S3 respectively as described hereinabove with regard to Fig. 1. The voltage pulse output of each of CLIPS sensor means S1,S2 and S3 is amplified and shaped by the preamplifier means Al with resistor means R1,R2,R3, preamplifier means A2 with resistor means R4,R5,R6 and preamplifying means A3 with resistor means R7,R8,R9 respectively. Each of the preamplifier means Al A2 and A3 is connected via respective diode means D1.D2 and D3 to a common bus (wired OR connection means) CB.

The said common bus means CB is connected to a unit gain amplifier means A4 that serves as an output buffer and the output pulse of the said amplifier means is transmitted to the measuring circuits of the in circuit test apparatus. In an inactive state each of preamplifiers A1,A2 and A3 has a zero Volt output. Therefore the Common Bus means CB is at zero volt as well. When the sensor SI picks up a current pulse from the node Nl, the preamplifier means Al produces a positive going pulse output. Such positive going pulse will be allowed through the diode means Dl and cause a positive going pulse on the common bus means CB without interference from the preamplifier means A2 and A3. The positive going pulse on the common bus means CB will then be transmitted to the measuring circuits via amplifier means A4. In a similar manner, when a current pulse appears on the node N2 it is picked up by the sensor means S2 and transmitted via the preamplifier means A2 and diode means D2 with the result of a positive going pulse on the said common bus means CB that is then transmitted to the measuring circuits via amplifier means A4. Finally a current pulse on the node N3 that is picked up by sensor means S3 will be transmitted via the preamplifier means A3 and diode means D3 to appear on common bus means CB as a positively going pulse that is then transmitted to the measuring circuits via the unit gain amplifier means A4.

It will be understood that the above described CLIPS board means enables testing a PC board in accordance with the inventive method without the need for a multiplexing addressing scheme, resulting in considerable saving of time and costs.

It will be further understood that the inventive CLIPS board means could also be implemented with a negative going pulse output, and with other circuit designs or a different number of resistors while still enabling testing without the need for a multiplexing addressing scheme. In accordance with another preferred embodiment, the inventive CLIPS sensor means are not comprised in the CLIPS board means but rather attached on the fixture wires as shown and described with regard to Fig. 5. Only the respective preamplifier means, resistor means, and diode means for each CLIPS sensor means will be attached on a common multiplexer-amplifier means. Such an arrangement also enables the appearance of positive going pulses on the common bus means (wired OR connection means) and the transmission of the said pulses to the measuring ciruits of the in circuit testing apparatus via a unit gain amplifier without the need for a multiplexing addressing scheme thus saving considerable time and costs. In this embodiment, similarly to the above described embodiment of Fig. 8, the said common bus CB is connected to a unit gain amplifier means A4 that serves as an output buffer and the output pulse of the said amplifier means is transmitted to the measuring circuits of the in circuit test apparatus. It is an advantage of the invention that the novel sensor means may be implemented in an existing ICT tester with or without the novel inventive CLIPS board means.

In accordance with one aspect of the invention stuck bus detection is achieved by disabling all bus devices, applying a dynamic pulse train to the bus lines and measuring the current pulse response via the inventive current sensor means. A bus line is declared stuck when the voltage pulse response from the said current sensor means is found to be of a magnitude that exceeds a predetermined threshold level.

In accordance with another aspect of the invention stuck bus diagnosis is performed by applying a train of dynamic pulses to a stuck bus in which all devices have been disabled, enabling the devices one at a time and monitoring the current pulse response for each device respectively by the inventive current sensor means. The bad device is pinpointed by a voltage pulse output from the said current sensor means that does not exceed a predetermined threshold.

In accordance with yet another aspect of the invention open pin faults are detected by measuring the current pulse response on the line to which the pin is connected under application of a train of dynamic pulses to the said line while the devices are enabled one at a time. Where the voltage pulse output from the said current sensor means does not exceed the pre-determined threshold level for an enabled device, the corresponding bus line is declared open. The same method is applicable to detection of faults in output pins of non tristatable devices. It will be understood that in the latter case no disabling or enabling is required.

Claims

1. A method for detecting faulty buses and for diagnosing faulty devices, open pins and high impedance shorts on a faulty bus within circuit sections of a printed circuit board by digital means comprising the following steps: applying a series of digital pulses to one of the said circuit sections within the board under test in diverse enabled-disabled states of the bus devices connected to the said circuit section; picking up the current pulse drawn by the said circuit section in each of said enabled-disabled states of the said devices and transforming the said current pulse into a voltage pulse by a current sensor means connected to the said circuit section while performing a differentiation of the said pulse by the said current sensor means to enhance measuring integrity; amplifying the said voltage pulse by amplifying means; integrating, clamping and shaping the said voltage pulse; multiplexing the voltage pulses received from different circuit sections via different sensor means ; and comparing the value of each of the said voltage pulses at the measured strobe with a predetermined threshold value for the respective enabled-disabled state of the said bus devices connected to the said circuit section.

2. A method according to claim 1 wherein the said circuit section comprises a bus line, a series of digital pulses are applied to the said bus line in a state of disablement for all devices; and the said bus line is declared faulty when the value at the measured strobe of the voltage pulse received from the cunent sensor means connected to the said bus line is found to exceed a pre-determined threshold level.

3. A method according to claim 1 wherein the said circuit section is a faulty bus line, in the said diverse states all devices connected to the said faulty bus line are disabled and then each device is enabled one at a time, and a bad device is detected when the value at the measured strobe of the voltage pulse received from the current sensor means connected to the said faulty bus line during enablement of the said bad device does not exceed a predetermined threshold

4 A method according to claim 1 wherein the said circuit section a faulty bus line, in the said diverse states first all devices connected to the said faulty bus line are disabled and then each device is enabled one at a time, and an open pin on a line leading to one of the said devices is detected when the value at the measured strobe of the voltage pulse received from the current sensor means connected to the said faulty bus line during enablement of the said bad device does not exceed the predetermined threshold level.

5. The method according to claim 1 wherein the said circuit section contains non tristatable devices and an open output pin on a line leading to one of the said devices is detected when the value at the measured strobe of the voltage pulse received from the current sensor means connected to the said line does not exceed the predetermined threshold level.

6. The method according to claim 1 wherein the said circuit section is a bus, the said series of digital pulses are applied to each of the lines of the said bus consecutively and the said diverse states consist of keeping all bus devices in a disabled state and all bus lines at a low state except the line that is being pulsed, wherein a high impedance short fault is detected when the value at the measured strobe of the voltage pulse received from each of two current sensor means connected to one of a pair of adjacent bus lines respectively exceeds a predetermined threshold level.

7. A method according to any of the preceding claims wherein the need multiplexing or addressing of the said voltage pulses received from a multiplicity of the said current sensor means is eliminated by connecting the outputs of the said cunent sensor means after amplifying, integrating, shaping, and clamping of the said outputs via diode means to a common bus line means such that all voltage pulses reaching the said common bus line means have the same polarity, and whereby voltage pulses from any of the said current sensor means will reach the measuring line without disturbance from the other sensor means and random connecting of the said sensor means to the said circuit sections or devices within the said printed circuit board is enabled

8. Apparatus for detecting faulty buses and for diagnosing faulty devices, open pins and high impedance shorts on a faulty bus within circuit sections of a printed circuit board by digital means, comprising : means for enabling and disabling the said devices. driving means for applying a digital pulse to the said circuit sections; a multiplicity of current sensor means each for picking up a current pulse from a line connecting to one of the said circuit sections respectively and transforming the said current pulse into a voltage pulse while performing a differentiation of the said current pulse; means for amplifying the said voltage pulse; pulse shaping means for integrating, clamping and shaping the said voltage pulse ; means for multiplexing the voltage pulses received from several current sensor means; measuring and processing means for comparing a voltage pulse at the measured strobe with a predetermined threshold value for the respective enabled-disabled state.

9. Apparatus for detecting faulty buses and for diagnosing faulty devices, open pins and high impedance shorts on a faulty bus within circuit sections of a printed circuit board by digital means according to claim 7 wherein the said current sensor means consists of a pickup coil, one or more loops of a wire conducting to one of the said circuit sections are wrapped around the said pickup coil and the pickup coil is connected to an amplifying circuit such that the said current pulse drawn by the said circuit section is picked up, transformed and differentiated by the said pickup coil and the voltage pulse from the said current sensor means is amplified by the said amplifier circuit whereby measuring integrity is enhanced.

10. Apparatus for detecting faulty buses and for diagnosing faulty devices, open pins and high impedance shorts on a faulty bus within circuit sections of a printed circuit board by digital means according to claim 8 wherein the said current sensor means are installed on the said wires conducting to the said circuit sections, each of the said sensor means is assembled together with the said amplifying circuit means and the said pulse shaping means are attached on a multiplexing amplifying board comprised in the said apparatus.

11. Apparatus for detecting faulty buses and for diagnosing faulty devices, open pins and high impedance shorts on a faulty bus within circuit sections of a printed circuit board by digital means according to claim 8 wherein the said current sensor means, the said amplifying circuit means and the said pulse shaping means are attached on a multiplexer amplifier board within the said apparatus and the wires conducting to the said circuit sections are routed to the said multiplexer - amplifier board

12. Apparatus for detecting faulty buses and for diagnosing faulty digital devices, open pins and high impedance shorts on a faulty bus within circuit sections of a printed circuit board by digital means according to claim 8 wherein the said sensor means are designed into the said driving and measuring means.

13. Apparatus according to claim 11 wherein the said sensor means are attached on the wires leading from the said driving and measuring means to the said circuit sections. And the voltage pulse output of the said current sensor means is directed internally into the said driving and measuring means.

14. Apparatus according to any of the preceding claims wherein the said current sensor means are randomly connected to the said circuit sections or devices within the said printed circuit board, the said multiplexing means comprise a diode means connected to each of the said voltage pulse amplifier means respectively and the said amplifier means and the said diode means are connected to a common bus line that connects to a measuring line such that all voltage pulses reaching the said common bus line means have the same polarity, whereby voltage pulses from any of the said current sensor means will reach the measuring line without disturbance from the other sensor means.

15. A current sensor device for picking up and amplifying the current pulse drawn by one of the said circuit sections of a printed circuit board under test that enables testing the said circuit board by digital means in an in circuit tester apparatus, comprising a pickup coil wherein one or more loops of a wire conducting to the said circuit section are wrapped around the said pickup coil and the pickup coil is connected to an amplifying circuit such that the said current pulse drawn by the said circuit section is picked up, transformed and differentiated by the said pickup coil and integrated by the said amplifier circuit whereby measuring integrity is enhanced.

16. A current sensor device according to claim 13 wherein a core is comprised in the said pickup coil.

17. A sensor device for picking up, transforming and amplifying the current pulse drawn by a digital device or by a bus or by a bus line comprised in a circuit section of a printed circuit board that enables testing the said circuit board by digital means in an in circuit tester apparatus, comprising a one or more loop primary coil connected in series to a fixture wire of the said apparatus and a secondary coil without a core whereby a current pulse on the said fixture wire is picked up, differentiated and amplified by the said sensor device and amplifier means.