WO2000002057A1

WO2000002057A1 - Apparatus for detecting electrical shorts and opens on circuit boards having a plurality of bus lines

Info

Publication number: WO2000002057A1
Application number: PCT/US1999/014550
Authority: WO
Inventors: Mahesh P. Parshotam; Alan D. Howard
Original assignee: Huntron Instruments, Inc.
Priority date: 1998-07-02
Filing date: 1999-06-28
Publication date: 2000-01-13
Also published as: AU4837599A

Abstract

Short circuits between bus lines or open circuits in bus lines in an electrical circuit, such as a microprocessor, are detected by applying a current-limited sine wave signal to each bus line connection point in sequence, when the electrical circuit board is in a power-off condition, and measuring the voltage between the power supply line and ground as the sine wave signal is applied to each bus connection in turn. The measured voltages are then compared against voltages from a circuit which is known to be good, and any voltage values which are outside of a preselected tolerance are stored and then displayed. Those out-of-tolerance voltage values are indicative of a short between bus lines or an open bus line associated with the out-of-tolerance condition.

Description

APPARATUS FOR DETECTING ELECTRICAL SHORTS AND OPENS ON CIRCUIT BOARDS HAVING A PLURALITY OF BUS LINES

Technical Field

This invention relates generally to troubleshooting apparatus for printed circuit boards, and more specifically concerns such apparatus for detecting shorts and opens on bus lines on the circuit board.

Background of the Invention Analog signature analysis is a technique which is useful for finding faults on a printed circuit board. With this technique, the board is in an unpowered condition. A current-limited sine wave is applied between two selected test points on the circuit board. The result of the applied signal is a waveform, referred to as a "signature", displayed typically on a CRT monitor, which is then compared against a signature for a board which is known to be good. Many different types of faults can be readily determined using this technique. However, for microprocessor circuit boards, which typically include a number of bus lines, used both for data communication and control, the conventional signature comparison cannot be used reliably for finding certain faults, such as for instance, shorts and opens. A short, for instance, might result from two bus lines (typically but not necessarily adjacent) being inadvertently connected by virtue of a solder splash. Since conventional signature analysis is typically unable to identify such a problem, the circuit board is placed in service in a selected apparatus, which can result in a significant number of malfunctions in the apparatus. Other conventional test arrangements, such as those using a bed of nails" fixture in combination with functional testing adapted for that particular board, may be able to determine the presence of such faults; however, such fixtures are designed for a particular board. What is lacking is a universal or general approach which can be used with as many different boards as possible.

Hence, it is desirable to have a test technique for circuit boards which is relatively simple and easy to implement, while at the same time being reliable in its detection of short and open conditions .

Disclosure of the Invention Accordingly, the present invention is a system for detecting electrical shorts in circuits which include a plurality of bus lines, comprising: means for applying an interrogating signal to a first bus line connection to an electrical circuit, such as a microprocessor, for producing a signature signal for said connection, when the circuit is in a power-off condition; means for measuring a test voltage between a power supply line to the circuit and ground when said interrogating signal is applied to the circuit connection for the first bus line; means for applying said interrogating signal to a second bus line connection to the circuit; means for measuring a test voltage between said power supply line and ground when said interrogating signal is applied to the circuit connection for the second bus line; means for comparing said test voltage values for said first and second bus lines against standard voltage values from a circuit which is known to be good, and means for determining which corrections have test voltage values which are beyond a preselected threshold difference relative to said standard voltage values.

Brief Description of the Drawings

Figure 1 is a diagram showing the hardware organization of the system of the present invention.

Figure 2 is a software flowchart diagram for the software used in the system of the present invention.

Figures 3A-3C are diagrams showing three examples of the use of the apparatus of Figure 1. Best Mode for Carrying Out the Invention

Figure 1 shows the system of the present invention. A circuit board under test is shown generally at 10. Typically, circuit board 10 will be a microprocessor circuit board having a number of individual bus lines connected to its various pins. The bus lines can be data lines, such as an address and data bus, from the central processing unit (CPU) of the microprocessor, or communication lines which permit the CPU to communicate with external devices. These are of course only representative examples of various bus lines connected to„ the various pins of the circuit board. In Figure 1, three bus lines are shown, referred to at 12, 14 and 16. This is provided as an example only, as a typical board will usually have many more than three bus lines. In the embodiment shown, a short is shown between bus lines 14 and 16. As indicated above, however, conventional signature analysis which will detect many other board faults cannot be used to detect such a short, since the short will not affect the appearance of the characteristic signatures for either of those bus lines. The present system does include, however, a signature analysis apparatus, shown generally at 20. As indicated above, such an analog signature analysis device generates a current-limited sine wave which is applied between two test points on the circuit. The voltage applied to the circuit and the resulting current flowing through the circuit produces a characteristic signature on a CRT monitor. The CRT is shown generally at 22 in Figure 1. The signature analyzer 20 can take various forms; it can include a simple display device with the operator making a determination as to the correctness of the signature, or it can be more sophisticated, in which the signature is obtained, stored and then compared against stored values from circuits known to be good, with an indication of whether or not the circuit test point is good produced by the apparatus. One example of such an apparatus is the ProTrack_® instrument manufactured by Huntron Instruments, Inc. of Mill Creek, Washington, the assignee of the present invention. One lead 24 from the signature analyzer is applied to a ground connection on the circuit board, while the other test lead 26 is applied to a selected test point, i.e. a bus line connection point. This will result in a signature displayed on CRT 22 for that particular test point or node.

A voltage measurement is then made between the circuit board's power supply line (+V_CC) using a separate voltmeter or a voltmeter capability within the signature analyzer itself, as illustrated, using leads 24 and 27. The voltage value will be different for two shorted buses than the voltage value for the same buses which are not shorted. Hence, the system of the present invention requires first that voltage values between the power supply line (+V_CC) and ground be obtained for each of the pins on a circuit board which is known to be good (no shorts) . These are referred to herein as the standard voltage values. This information is then stored, so that test results from subsequent boards can be compared against the standard values. Those test points (pins) which differ by a preselected amount from the standard values are then identified.

As indicated above, the voltage determination can be made by a conventional voltmeter. The voltmeter, as well as the storage and retrieval of voltage information, can be either separate from the signature analyzer, an add-on feature to certain existing analog signature analyzers, or they can be incorporated into a signature analyzer.

Figure 2 shows a software flowchart for the system of the present invention. The first part of the flowchart concerns the measurement and recording of voltage values for each pin for circuit boards which are known to be good, as indicated by block 30.

The first step, as shown at 32, is to establish the test parameters for the signature analyzer for the testing of boards known to be good. This requires selecting the particular test ranges of the analyzer which will produce a voltage difference between the power supply line and ground for a short condition relative to a non-short condition. A typical signature analyzer will have a number of different operating ranges, each with different voltage and resistance settings for the analyzer signal. Typically, the selection of a particular range or ranges is achieved by comparing voltage readings for the power supply line to ground for two selected bus lines at their pin connection points when they are not shorted to voltage readings when one of the busses is artificially shorted. This is done for the various ranges available for the signature analyzer.

If the two voltage readings are different beyond a particular threshold value, i.e. five percent, then that particular test range is used for that board. Typically, it has been found that a medium range (for example, 15V, 1.2K ohms) or a low range (for example, 10V, 54 ohms) results in the desired difference in voltage between shorted and non- shorted conditions.

After the desired test range has been determined for a particular board, the voltage values (unshorted) for each pin are then read, as shown at 34. The values obtained at step 34 may then be either individually stored, as determined at 36, and then stored at 38, or the values may be used to establish a range (max-min) of values at 40, with the newly read value being merged with the established range of values, at 42. This sequence of reading and storing individual or merged voltage values continues for each pin on the circuit board for a selected range, or if more than one range is appropriate, for each range in turn for each pin in the sequence, as shown at 44 and 46. When the last pin and/or range has been tested, then the test sequence for a good board is complete. The standard values have been obtained and stored.

The next portion of the flowchart, represented generally by block 50, involves the measurement of voltage values on those boards which are to be tested (not known to be good) and comparing these values against the standard values which have been previously obtained and stored. The test parameters are first established for the board to be tested, the parameters being the same as for the board known to be good, as shown at 52. The voltage values are then read, at 54, for each pin in sequence for a particular range or for each range in turn for each pin. The voltage values are then compared at 56 to individual values (42) or merged (range) values at 58. Following the comparison, those pin numbers for which voltage differences were determined to be outside of the specified tolerance (typically, greater than five percent) are stored at 60. The tolerance value can be varied. However, it has been determined that a five percent threshold tolerance value will in fact provide an accurate indication of a bus short. It will also indicate a possible open. In the event of an open, the signature will also change significantly, so an inspection of the signature, in addition to a determination of voltage change beyond the established threshold is necessary to conclude that an open exists. The test process continues until each pin in a particular range has been tested and/or every range in turn for every pin has been tested, as represented at 62 and 64. When the comparisons are complete, then those pins having voltage differences beyond the tolerance value will be shown in a priority order, i.e. from greatest difference to least, as shown in 66.

Three specific examples of circuit testing are shown in Figures 3A-3C. Figure 3A is for pins 12 and 13 of one microprocessor on a Cyrix 486 DLC motherboard with CPU and no memory. The signatures for pins 12 and 13 (non-shorted) are shown for the medium range and the low range of the signature analyzer. In the medium range, the signal applied to the circuit is 15 volts and 1.25K ohms, while the low range is 10 volts and 54 ohms. When pins 12 and 13 are shorted, no change in the signatures is noted. However, in the medium range, a change in the voltage from 158mv to 202mv occurs. This indicates a short between those pins, which is appropriately noted and stored.

The test results from another microprocessor from the 486 motherboard is shown in Figure 3B. This figure shows only the signatures for the medium range, with pins 6 and 7 shorted. Again, there are no differences between the signatures, but there is a significant difference in the voltage measurement between the power supply line and ground, indicating a short between those pins .

Figure 3C shows several ranges for still another microprocessor on the motherboard. This figure shows pins 8 and 9 when they are non-shorted and when they are shorted, for four different operating ranges of the signature analyzer. The signatures are all substantially identical, but there is a difference in the voltage measurement of approximately seven percent for the low range. Hence, it is important to identify the correct range to determine shorted bus lines.

All of the above examples show changes in the measured voltage between the power supply line and ground when bus lines on a microprocessor are shorted together.

Hence, a system has been disclosed which provides reliable indication of a short between two bus lines on a printed circuit board. The system can also detect opens as well.

Although a preferred embodiment of the invention has been disclosed herein for illustration, it should be understood that various changes, modifications and substitutions may be incorporated in such embodiment without departing from the spirit of the invention, which is defined by the claims as follows.

What is claimed is :

Claims

1. A system for detecting electrical shorts in circuits which include a plurality of bus lines comprising, means for applying an interrogating signal to a first bus line connection to an electrical circuit, for producing a signature signal for said connection, when the circuit is in a power-off condition; means for measuring a test voltage between a power supply line to the circuit and ground when said interrogating signal is applied to the circuit connection for the first bus line; means for applying said interrogating signal to a second bus line connection to the circuit; means for measuring a test voltage between said power supply line and ground when said interrogating signal is applied to the circuit connection for the second bus line; means for comparing said test voltage values for said first and second bus lines, against standard voltage values from a circuit which is known to be good; and means for determining which connections have test voltage values which are beyond a preselected threshold difference relative to said standard voltage value.

2. A system of claim 1, wherein the preselected threshold difference is five percent.

3. A system of claim 1, wherein the interrogating signal is a current-limited sine wave signal.

4. A system of claim 1, including means for storing the test voltages and means for storing voltage differences which are beyond the preselected threshold difference value.

5. A system of claim 3, including means for displaying all of the connections on the circuit having voltage differences beyond the preselected difference value.

6. A system of claim 1, further including means for analyzing differences in signature signals to determine existence of an open on said bus lines.

7. A system of claim 1, including means for comparing test voltage values against standard voltage values for each bus line on the electrical circuit in turn and for determining which connections have test voltage values which are beyond said threshold difference.

8. A system of claim 3, wherein the means for supplying the sine wave signal includes means supplying signals having more than one range, each range having a different output voltage and resistance, and wherein voltage values are obtained for each connection for each signal range.

9. A system of claim 1, including means for obtaining and storing standard voltage values for each connection from a circuit known to be good.

10. A system for detecting electrical shorts on bus lines in an electrical circuit such as a microprocessor comprising: means for applying an interrogating signal in turn to a plurality of bus line connections to an electrical circuit, the interrogating signal producing a signature signal for said connections, when the circuit is in a power-off condition; means for measuring test voltage values between a power supply line and ground when the interrogating signal is applied to each bus line connection in turn; means for comparing said test voltage values against standard voltage values for each of the bus line connections from a circuit which is known to be good; and means for determining which connections have test voltage values which are beyond a preselected threshold difference relative to said standard voltage values.

11. A system of claim 10, wherein the preselected threshold difference is five percent.

12. A method for detecting electrical shorts on bus lines in an electrical circuit such as a microprocessor, comprising the steps of: applying an interrogating signal in turn to a plurality of bus line connections to an electrical circuit, the interrogating signal producing a signature signal at each of said connections in turn, when the circuit is in a power- off condition; measuring test voltage values between a power supply line and ground when the interrogating signal is applied to each bus line connection in turn; comparing said test voltage values against standard voltage values for each of the bus line connections from a circuit which is known to be good; and determining which connections have test voltage values which are beyond a preselected threshold difference relative to said standard voltage values.