Self-diagnostic circuit for a video display and method of implementing the same

A circuit for a video display device having a cable connector connecting to a cable of an external system, wherein a video signal applied at an output terminal of said cable connector is supplied to a cathode ray tube via a video preamplifier and phase-locked loop circuit and a video main amplifier connected in series, a horizontal/vertical synchronization signal applied at said output terminal of said cable connector is supplied to said cathode ray tube via a horizontal oscillation and phase-locked loop circuit and a horizontal output circuit connected in series, and a vertical oscillation and phase-locked loop circuit and a vertical output circuit connected in series, respectively, a power supply applied from an external power source is supplied to respective circuits of said video display device at a predetermined voltage level by an operation of voltage drop in a switching mode power supply to energize said respective circuits, said circuit comprising:

• a first signal waveforn shaping portion for shaping a signal output from said video main amplifier, and then feeding back the shaped signal to an input terminal of said video preamplifier and phase-looked loop circuit thereby enabling said shaped signal to establish a comparison with a video signal input from said cable connector;
• a second signal waveform shaping portion for shaping a signal output from said horizontal output circuit, and then feeding back the shaped signal to an input terminal of said horizontal oscillation and phase-locked loop circuit thereby enabling said shaped signal to establish a comparison with a horizontal synchronization signal -input from said cable connector;
• a third signal waveform shaping portion for shaping a signal output from said vertical output circuit, and then feeding back the shaped signal to an input terminal of said vertical oscillation and phase-locked loop circuit thereby enabling said shaped signal to establish a comparison with a vertical synchronization signal input from said cable connector;
• a fourth signal waveform shaping portion for shaping respective voltage waveforms supplied from said switching mode power supply;
• and
• a microprocessor for demanding information for output gain and receiving said information and a phase difference signal from respective said video preamplifier and phase-locked loop circuit, said horizontal oscillation and phase-locked loop circuit, and said vertical oscillation and phase-locked loop circuit, for determining an operational status of respective said video preamplifier and phase-locked loop circuit, said horizontal oscillation and phase-locked loop circuit, and said vertical oscillation and phase-locked loop circuit, and for detecting a power supply signal input via said fourth signal waveform shaping portion, and then generating the result to said external system via said cable connector.

A method for self-diagnosis testing one of a random-access-memory and an electrically erasable programmable read-only-memory built-in a microprocessor for controlling operation of a video display device, said method comprising the steps of:

• a first process for recording preset data in an address location of a reserved area of a selected one of said random-access-memory and said electrically erasable programmable read-only-memory, reading out stored data from said address location, and making a comparison of said stored data with said preset data to determine whether said selected one of said random-access-memory and said electrically erasable programmable read-only-memory is normal;
• a second process for repeatedly performing said first process by incrementing by each address location until a last address location of said reserved area in said selected one of said random-access-memory and said electrically erasable programmable read-only-memory;
• a third process for determining that said reserved area of said selected one of said random-access-memory and said electrically erasable programmable read-only-memory is normal when said comparison indicates that said stored data read from said address location corresponds to said preset data during a time period of said second process, and then outputting a data signal to a host computer system to inform an operator of such normality of said selected one of said random-access-memory and said electrically erasable programmable read-only-memory;
• and
• a fourth process for determining that said reserved area of said selected one of said random-access-memory and said electrically erasable programmable read-only-memory is abnormal when said comparison indicates that said stored data read from said address location does not correspond to said preset data during said time period of said second process, and then outputting a different data signal to said host computer system to inform the operator of such abnormality of said selected one of said random-access-memory and said electrically erasable programmable read-only-memory.