DIAGNOSTIC SYSTEM FOR A RASTER SCAN TYPE DISPLAY DEVICE

Abstract: A diagnostic system for use in a raster scan type cathode-ray tube (CRT) display device is described in which a predetermined portion of a video output signal is sampled in timed relationship with multiple reference timing signals. Known character and character-attribute information is stored in a random access memory (RAM) under diagnostic program control at a location corresponding to a character location on the display screen. The data stored in the RAM is output to the display circuitry in response to the reference timing signals. The video output signal of the display circuit is coupled to a data latch that samples the video signal corres­ ponding to the location of the character and character-­ attribute information stored in the RAM. The sampled data is coupled to a micro-processor where a comparison is made btween the sampled data and a predetermined value to verify the proper operation of the circuit.

Description: DIAGNOSTIC SYSTEM FOR A RASTER SCAN TYPE DISPLAY DEVICE Background of the Invention The present invention relates to a system for diagnosing a video output of a raster scan type display device by sampling a predetermined portion of the video output signal in timed relationship with multiple reference timing signals. Raster scan type cathode-ray tube (CRT) display devices produce flicker free displays containing a great deal of information composed of many kinds of character and images. These devices also produce various video modes, such as normal video (i.e. intensified characters on a dark background), reverse video (i.e. dark characters on an intensified background), or highlighting (i.e. half intensity shading of characters). Raster scan types CRT displays that are used for displaying alphanumeric information and various patterns, such as portions of logic timing waveforms, cursors, and the like, are generally divided into a matrix of M rows and N columns. Each coordinate of the M X N display matrix defines a character location on the display and is subdivided into a p X q character matrix that defines pixel locations on the display. A character read only memory (ROM) stores a plurality of electrical patterns, representative of character information, in p' X q' matrices corresponding in size to the p X q matrices of the M X N display matrix. A display random access memory (RAM) stores characteraddress and character-attribute information corresponding to the coordinates of the M X N display matrix in separate memory locations. In the above type display, the character-attribute information defines the various video-modes. To produce a display on the CRT, the display RAM outputs character-address and video-mode information in synchronization with the raster scan operation of the display device. When addressed by the display RAM, the character ROM outputs a corresponding electrical pattern, representative of character information, to the video display circuitry. Concurrently, the videomode information is coupled to the video display circuitry to enable the appropriate video mode circuits. The output of the video display circuitry is coupled to the Z-axis control grid of the CRT to produce the output display. Diagnostic self-test systems have been used in raster scan type CRT display devices for verifying the proper operations of the circuits within the device before use. Diagnostic self-test systems generally have a diagnostic program, stored in a read only memory (ROM), a microprocessor, and readback latches placed at strategic points in the circuit. Each readback latch monitors the output data from that portion of the circuit to be tested and couples that data to the microprocessor via the data bus. During a diagnostic test of the display circuitry, for example, checking. the display RAM, the microprocessor writes known data valves to the RAM under diagnostic program contro. As the data is output from the RAM, a data latch monitors the output and coupled the information to the microprocessor. The microprocessor compares the data from the latch with predetermined data to verify the operation of the RAM and the associated input and output circuitry. For diagnosing the character ROM, the contents of the ROM is output under program control and monitored by another data latch. The latched data is coupled to the microprocessor where a check sum is calculated for the entire contents of the ROM and compared with a pre determined value. In all cases, the data latches monitoring the output of the circuits under test must be connected to the bus of the microprocessor. Raster scan type CRT display devices are increasingly being designed with more "user-friendly" features, such as horizontal and vertical smooth scrolling, cursors, positioning, parameter readouts, and the like. A conventional diagnostic self-test system for devices with large feature sets would require a great deal of additional circuitry for monitoring the various circuits. The cost of such a system would become prohibitively expensive. A diagnostic self-test system is needed that verifies the proper operation of the display circuitry of a raster scan type CRT display device having a large feature set without the use of many readback latches and the associated circuitry required to couple the monitored data to the data bus of the microprocessor. Summary of the Invention The present invention is directed toward a diagnostic system for use in a raster scan type CRT display device where a predetermined portion of a video output signal is sampled in timed relationship with multiple reference timing signals. A microprocessor, under diagnostic program control, stores known character-address and character-attribute information in a random access memory (RAM) at a location corresponding to a verification window. In the preferred embodiment, the verification window corresponds to the last eight pixel bits of the two hundredth raster line of the display. The stored character-address information is output from the RAM to a character read only memory (ROM) in timed relationship with the production of raster lines on a CRT type display and a character frequency reference signal. The character ROM outputs corresponding electrical patterns representative of the character information on a raster line base to the video display circuitry. Concurrently, the characterattribute information is coupled to an attribute circuit that enables the appropriate character-attribute circuit, such as normal or reverse video or highlighting, in the video display circuitry. The video output from the display circuitry is coupled to data latch that samples a portion of the video signal corresponding to the verification window and couples the data back to the microprocessor via a data bus where a comparison is made with known value. By incorporating horizontal and vertical smooth scrolling functions in the display device, each pixel row of the electrical pattern, representive of the character information, can be sampled by the data latch. Since the video output signal is checked, the present invention can verify the proper operation of all data paths of the display circuitry of the display device. The present invention requires only a single sampling circuit as hardware. A more complete understanding of the present invention and its various features, advantages, and objectives may be had by referring to the following detailed description and the accompanying drawings. Brief Description of the Drawings FIG. 1 illustrates a block diagram of a preferred embodiment of the present invention; FIG. 2 illustrates a display screen for explaining the operation of the present invention. FIG. 3A illustrates a magnified view of a part of the display screen for explaining the operation of the present invention; FIG. 3B illustrates the contents of a memory for displaying the video signal of FIG. 3A; and FIG. 4 illustrates a time chart for explaining the operation of a portion of the present invention. Detailed Description of the Invention Referring to Fig. 1, there is shown a block diagram of a preferred embodiment of the present invention as used in a raster scan type CRT display device. Timing block 10 produces timing signals G and S that are respectively the pixel and character frequencies. In this embodiment, the pixel frequency is 9.6 MHZ and the character frequency is 1.2 MHZ. Timing block 10 also produces vertical and horizontal timing pulses, V and H respectively, that are in timed relationship with timing signals G and S. The V and H timing pulses are coupled to vertical and horizontal drivers 12 and 14 and to counter 22. Drivers 12 and 14 generate vertical and horizontal raster scan signals in response to timing pulses V and H. The vertical and horizontal raster scan signal are coupled to deflection yokes 16 and 18 respectively on CRT 20 for the production of raster lines on the screen of the CRT. Counter 22 is coupled to an AND gate 52 and address generator 24. The address generator 24 is also coupled to receive the 1.2 MHZ character frequency S timing signal and data input from data bus 26. The outputs of address generator 24 are coupled to a display random access memory (RAM) 28 and a character read only memory (ROM) 34. The display RAM 28 is also coupled to data bus 26 along with a central processing unit 30, such as an 8080 or Z-80A microprocessor, a program read only memory (ROM) 32, which contains a diagnostic self test program, the character ROM 34, an attribute circuit 36 and exclusive OR gate 44. The output of character ROM 34 is coupled to a parallel-to-serial shift register 38 that has its clock input coupled to receive the 9.6 MHZ G signal. The output of shift register 38 is coupled to one input of exclusive OR gate 40. The second input of exclusive OR gate 40 is coupled to the character attribute circuit 36. The output of exclusive OR gate 40 is coupled to one input of OR gate 42 that has its second input coupled to the output of AND gate 46. One input of AND gate 46 is coupled to character attribute circuit 36 while the other input is coupled to the output of exclusive OR gate 44. As previously mentioned, one input of exclusive OR gate 44 is coupled to data bus 26 while the other input is coupled to receive the 9.6 MHZ G signal. The output of OR gate 42 is coupled to buffer amplifier 54 which is coupled to the Z-axis grid of CRT 20 and to serial-to-parallel shift register 48 that has a clock input coupled to receive the 9.6 MHZ G signal. The output of shift register 48 is coupled to a data latch 50 that has a clock input coupled to AND gate 52. The output of data latch 50 is coupled to data bus 26. During normal display mode operation, character information and character-attribute information, defining various video modes, is input to display RAM 28 via data bus 26 and display data lines 56. The character and video mode information is stored at locations in display RAM 28 corresponding to character locations on the screen of CRT 20. The stored data in RAM 28 is output in response to address signals from address generator 24. The Lo - L2 and X6 - X10 output of address generator 24 correspond to the raster line count from counter 22, and the XO - X5 output corresponds to the 1.2 MHZ character frequency S signal that defines the character boundaries. The character and video-mode information is concurrently output to character ROM 34 and attribute circuit 36, respectively. ROM 34 contains a plurality of electrical patterns, corresponding to alphanumeric characters that are addressed by the display RAM 28 and the Lo - L2 raster line information. The parallel output of ROM 34 is coupled to parallel-to-serial shift register 38 that converts the parallel data to serial data at a 9.6 MHZ pixel rate of signal G. The serial data is coupled to one input exclusive OR gate 40. The other input of OR gate 40 is coupled attribute circuit 36. The attribute circuit controls the outputs of exclusive OR gate 40 and AND gate 46 so as to produce video effects such as normal video, reverse video, and highlighting. When the signal output of attribute circuit 36 to exclusive OR gate 40 is a logic low, the output of gate 40 is not inverted, thus producing normal video. When the signal output is a logic high, the output of gate 40 is inverted with respect to the output of register 38 and the video is reversed. When the output signal of attribute circuit 36 to AND gate 46 is a logic low, the 9.6 MHZ G signal from exclusive OR gate 44 is prevented from reaching OR gate 42. When the output signal is a logic high the 9.6 MHZ signal is coupled to OR gate 42 to produce highlighting. The video output from OR gate 42, which is composed of pixel patterns corresponding to alphanumeric characters and the superimposed video mode information, is coupled to a buffer amplifier 54 which amplifies the signal and couples it to the Z-axis grid of CRT 20. The video output signal is also coupled to a serial-to-parallel shift register that is clocked at the 9.6 MHZ G signal. The resultant parallel output is coupled to a data latch 50 that has its clock input coupled to the output of AND gate 52. In this embodi ment, a clock pulse is produced by AND gate 52 when the raster line counter increments to line 200. The data from latch 50 is coupled to the data bus 26. In order to more easily understand the diagnostic system of the present invention, the following description will be referenced to a video display on CRT 20. Referring to FIG. 2, there is shown the display screen of CRT 20 having a dashed line representative of raster line 200. A verification window, shown as block 58, corresponds to the last character column position on the CRT display and contains the last eight pixel bits of raster line 200. During a diagnostic self-check, the microprocessor under program control, stores known character and video mode information at a location in RAM 28 corresponding to the verification window 58. As previously described, the contents of RAM 28 is output in response to address signals from address generator 24. The output of RAM 28 addresses ROM 34 and attribute circuit 36, which produce the combined video signal at the output of OR gate 42. The serial video output is continuously converted to parallel data by shift register 48 and coupled to data latch 50. When the inputs to AND gate 52 and all logic highs, representative of the end of the two-hundredth raster line, the output of AND gate 52 goes to a logic high causing the data latch 50 to sample the data that is present on its input of that time. The sampled data represents the last 8 pixel bits of raster line 200 corresponding to the verification window. This data is coupled to the microprocessor 30 via data bus 26 where the data is compared with a known value to establish verification of circuit operation. By incorporating vertical and horizontal smooth scrolling in the display device, the diagnostic system can incrementally check the video output on a raster line and pixel basis. As an example, Figs. 3A and 3B shows an electrical pattern from ROM 34 representative of the letter "T". As previously described, ROM 34 outputs the character data on a raster line basis. Initially, the first raster line of the character "T" is positioned within window 58 as shown in FIG. 3A. Data latch 50 samples the first line of data "01111100" ("7C" in hexadecimal), and couples the data via data bus 26 to microprocessor 30 where the data is stored in a stack memory. The microprocessor increments the raster line count in address generator 24 by one via data bus 26. This scrolls the data one raster line. The second line of the character "T" is now positioned within window 58, and the second line of data "0001000" ("10" in hexadecimal) is sampled and coupled to microprocessor 30. Microprocessor 30 computes the sum of the first and second lines of data and stores the result in a stack memory. The abovedescribed operation is repeated until all eight lines of data are sampled and a computed checksum is produced. This value is compared with a predetermined checksum value for the character. If the checksums are equal, the contents of ROM 34 corresponding to the character "T" are correct and the video processing circuitry is operating properly. Each electrical pattern in ROM 34 can be checked in this manner and a checksum can be computed for each character. It is also possible to compute a total checksum for all the characters in ROM 34 and compare the results with a predetermined value corresponding to the total checksum of the characters. To perform a diagnostic check of the reverse video mode, the microprocessor stores a value in RAM 28 corresponding to the verification window that produces a blank character from ROM 34. The corresponding video mode information stored in RAM 28 will cause a logic high output from the attribute circuit 36 to exclusive OR gate 40. The logic low output from ROM 34 will be inverted to a logic high output by exclusive OR gate 40. As previously described, the video output is sampled by data latch 50 and coupled to microprocessor 30 where a comparison is made between the sampled value and a known value corresponding to reversed blank character. If the comparison is true, then the reverse video mode is operating properly. Referring now to FIG. 4, there is shown a timing diagram for explaining the diagnostic check of the highlighting mode. Signal A represents the 9.6 MHZ G signal corresponding to the pixel frequency. Signals B and D represent the signal input to exclusive OR gate 44 from data bus 26. Signal C and E represent the output from exclusive OR gate 44 when the signal input from data bus 26 is respectively low and high. To perform a diagnostic check of the highlighting mode, the microprocessor stores a value in RAM 28 corresponding to the verification window that produces a blank character from ROM 34. The corresponding video mode information stored in RAM 28 will cause a logic high output from attribute circuit 36 to AND gate 46. The logic high on AND gate 46 enable the 9.6 MHZ output from exclusive OR gate to be coupled to OR gate 42. The output of OR gate 42 is coupled to shift register 48. When the input to exclusive OR gate 44 from data bus 26 is low, the 9.6 MHZ signal is not inverted and is in phase with the 9.6 MHZ clock signal to shift register 48. Since the signals are in phase, the output of shift register 48 will be a series of logical highs. This output is sampled by data latch 50 and coupled to microprocessor 30 where a comparison is made with a predetermined known value corresponding to a video signal of logical highs. When the input to exclusive OR gate 44 from data bus 26 is high, the 9.6 MHZ signal is inverted and is 1800 out of phase with the 9.6 MHZ clock signal to shift register 48. This causes the shift register to output series of logical lows. This output is sampled by data latch 50 and coupled to the microprocessor 50 where a comparison is made with a predetermined value corresponding to video signal of logical lows. If both the logical high and low comparison are true, then the highlighting video-mode is operating properly. It will be obvious to those having ordinary skill in the art that many changes may be made in the above-described details of the preferred embodiment of the present invention. For example, the verification window may be positioned at any desired part of the display screen, e.g., the initial eight pixel bits of the first raster line. Additionally, the number of pixel bits sampled by data latch 50 is determined by the number of pixels contained in the character segment width. Therefore, the scope of the present invention should be determined only by the following claims.

Claims: CLAIM: A signal monitoring circuit comprising: means for producing reference timing signals;and signal producing means coupled to receive said reference timing signals for producing multiple input signals; signal processing means coupled to receive said multiple input signals and said reference timing signals for producing an output signal in timed relationship with said multiple input signals; sampling means coupled to receive said output signal and at least one of said reference timing signals for monitoring a predetermined portion of said output signal corresponding to said input signals.

Inventors: OLIN, DANIEL C.
Applicants: TEKTRONIX, INC.
Application year: 1984
Main classification: G01R35
IPC codes: H04N17, G01R35, G09G3, G09G5