GB2287813A - Display apparatus with lockable screen - Google Patents

Abstract

A display device comprises a display screen. A drive circuit (200) generates an image on the display screen (210) in response to an input video signal (R, G, B). Stand-by means (SL) disables the drive circuit (200) in response to a stand-by signal. Reset means (RL) restores operation of the drive circuit (200) on receipt of a predetermined security code from push-buttons on a user control panel (260). The invention prevents unauthorised access to data when the user of a computer system forgets to log-off after the display automatically enters stand-by mode. <IMAGE>

Description

DISPLAY APPARATUS WITH LOCKABLE SCREEN The present invention relates to display apparatus having means for locking the display screen to prevent unauthorised access to data.

Some information handling systems such as personal computer systems for example include energy saving or "power management" mechanisms to reduce power consumption during dominant periods of operation. Such systems typically each include a display device having its own power management mechanism. An example of such a display device includes a display screen such as a cathode ray tube or liquid crystal display panel for example. A drive circuit for receiving an input video signal is connected to the display screen to generate an image on the display screen in response to the input video signal. To provide power management function in the display, a timer generates a count in response to a clock signal, and a detector resets the timer on detection of a change in image content between successive frames of the input video signal. Control means is connected to the timer and to the drive circuit for at least partially disabling the drive circuit in response to the count stored in the timer exceeding a predetermined threshold. Usually, the control means is configured to progressively shut down the drive circuit as the period of inactivity increases until, eventually, the display device is left operating in a stand-by mode with the detector, timer and control means remaining operational. The display device is only restored to normal operation from stand-by mode in response to detection of a change in image content by the detector. In general, the timer, detector and control means are at least partially implemented by a microprocessor.

This arrangement permits power management function to be provided in the display device independently of any host computer system. Power management can therefore be effected in the display device regardless of the system unit to which the display device is attached.

In another example of a display device having power management function, normal operation is restored from stand-by mode on detection of the close proximity of a user. In a further example of a display device having power management function, normal operation is restored from stand-by mode on detection of a keyboard and/or mouse entry.

In the display devices described above, when normal operation is restored from standby mode, the operator gains immediate access data stored in the system unit. Because such display devices enter standby mode automatically, the previous operator can easily forget to reactivate the display and log off any computer systems he or she has made connection to. This represents a potential security exposure.

In accordance with the present invention, there is now provided a display device comprising a display screen; a drive circuit for generating an image on the display screen in response to an input video signal (R,G,B); and stand-by means for disabling the drive circuit in response to a stand-by signal; characterised in that the device includes reset means for restoring operation of the drive circuit on receipt of a predetermined security code.

Because, in accordance with the present invention, the drive circuit can only be restored to normal operation by providing the display device with an appropriate security code, data stored in the host computer system can only be presented on the display screen to a user who knows the appropriate code. The present invention thus solves the problem of preventing unauthorised access to data that could otherwise be accessed via a display device from a host computer system to which the display device is connected.

Preferably the display device includes entry means for entering the security code. In a preferred embodiment of the present invention, the entry means includes a key pad having a plurality of push-buttons. The reset means preferably includes a processor having a plurality of interrupt inputs for receiving interrupts from the push-buttons of the key-pad. The processor allows the security code to programmed and reprogrammed by the user.

The processor is preferably configured to control the drive circuit to change one or more image parameters of the image displayed on the display screen in response to an interrupt from one or more of the push buttons. The processor can thus be configured to provide the key-pad with two functions. Firstly, the key pad can be used to adjust the parameters of the image displayed on the display screen according to personal preference when the display device is operating normally. Secondly, when the display device is in standby-mode, the key-pad can be used to enter the security code to the display device to restore normal operation.

Preferred embodiments of the present invention include a lock circuit for configuring the standby means to disable the drive circuit, the reset means disabling the lock circuit to restore operation of the drive circuit on receipt of the security code. Preferably, the standby means comprises an output port of the processor, and the lock circuit comprises a memory element connected to an input port of the procesaor for storing the state of the output port. The reset means preferably resets the contents of the memory element on receipt of the security code. The memory element preferably comprises a capacitor.

The present invention also contemplates a computer system comprising a display device as described above, a system unit for generating the input video signal to generate an image on the screen of the display device, and data input means for inputting data and commands to the system unit.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a block diagram of a computer system; Figure 2 is a block diagram of a display device for the computer system; Figure 3 is a block diagram of a user control panel and display processor of the display device; and Figure 4 is a diagram of a mechanical security feature for the display device.

Referring first to Figure 1, a computer system comprises a system unit 5 including a random access memory (RAM) 10, a read only store (ROS) 20, a central processing unit (CPU) 30, a communication adaptor 40, a display adaptor 70, a pointing device adaptor 80, a keyboard adaptor 90, and a mass storage device 100 as a hard disk drive or tape streamer for example, all interconnected by a bus architecture 60. System unit 5 is connected via adaptor 90 to a keyboard 110. A pointing device 120 such as a touch screen, a tablet, or a mouse is connected to system unit 5 via adaptor 80. System unit 5 is also connected via adaptor 70 and an interface cable 135 to a display 130 such as a cathode ray tube (CRT) display or a liquid crystal display for example. A network 50 of other system units is connected to system unit 5 via communication adaptor 40.

In operation, CPU 30 processes data stored in a combination of RAM 10 and mass storage device 100 under the control of computer program code stored in a combination of PS- 20, RAM 10, and mass storage device 150.

Communication adaptor 40 controls transfer of data and computer program code between system unit 5 and other system units in network 50 through communication adaptor 40. Keyboard and mouse adaptors 90 and 80 permit data and instructions to be manually entered into system unit 5 from keyboard 110 and pointing device 120 respectively. Display adaptor 70 translates output data from system unit 5 into video signals, R, G and B, and horizontal and vertical picture synchronisation (sync) signals, H and V, for configuring display 130 to generate a visual data output. Bus architecture 60 coordinates data transfer between RAM 10, ROS 20, CPU 30, storage device 100, and adaptors 40, 90, 80 and 70.

Referring now to Figure 2, display 130 comprises a display screen 210 in the form of a colour cathode ray display tube (CRT) connected to display drive circuitry 200. Display drive circuitry 200 comprises an Extra High Tension (EHT) generator 230 and a video amplifier 250 connected to display screen 210. Line and frame deflection coils 290 and 280 are disposed around the neck of the CRT. Deflection coils 290 and 280 are connected to line and frame scan circuits 220 and 240 respectively.

Line scan circuit 220 and EHT generator 230 may each be in the form of a flyback circuit, the operation of which is well known by those skilled in the art. Furthermore, as is also well-known in the art, EHT generator 230 and line scan circuit 220 may be integrated in a single flyback circuit.

A power supply (not shown) is connected via power supply rails (not shown) to EHT generator 230, video amplifier 250, and line and frame scan circuits 220 and 240. In use, the power supply provides electrical power on the supply rails from Line and Neutral connections (not shown) to the domestic electricity mains supply. The power supply may be in the form of a switch mode power supply, the operation of which is well-understood by those skilled in the art.

EHT generator 230, video amplifier 250, and line and frame scan circuits 220 and 240 are each connected to a display processor 270.

Display processor 270 comprises processor logic preferably in the form of a microcomputer of the kind including microprocessor and accompanying memory. Drive circuitry 200 includes a user control panel 260 connected to key-pad interrupt lines of display processor 270. Control panel 260 comprises a plurality of manual operable switches.

In operation, EHT generator 230 generates an electric field within CRT 210 for accelerating electrons in beams corresponding to the primary colours of red, green and blue towards the screen of CRT. Line and frame scan circuits 220 and 240 generate line and frame scan currents in deflection coils 290 and 280. The line and frame scan currents are in the form of ramp signals to produce time-varying magnetic fields that scan the electron beams across the screen of CRT 210 in a raster pattern. The line and frame scan signals are synchronised by line and frame scan circuits to input line and frame synchronisation (sync) signals H and V generated by video adaptor 70. Video amplifier 250 modulates the red, green and blue electron beams to produce an output display on CRT 210 as a function of corresponding red, green and blue input video signals R, G and B also generated by adaptor 70. Line and frame sync signals H and V and video signals R, G and B are supplied to display 130 from adaptor 70 along corresponding signal lines in interface cable 135. The signal lines of interface cable 135 terminate at the end remote from display device 130 in a connector (not shown) for detachably connecting the signal lines to adaptor 70. For compatibility, the connector is preferably a 15 pin D type connector although other connectors may be used.

Display processor 270 is configured to control the outputs of EHT generator 230, video amplifier 250, and line and frame scan circuits 220 and 240 via control links 275 as functions of preprogrammed display mode data and inputs from user control 260. The display mode data includes sets of preset image parameter values each corresponding to a different popular display mode such as, for example, 1024 X 768 pixels, 640 X 480 pixels, or 1280 X 1024 pixels. Each set of image display parameter values includes height and centring values for setting the output of frame scan circuit 240; and width and centring values for controlling line scan circuit 220. In addition, the display mode data includes common preset image parameter values for controlling the gain and cut-off of each of the red, green and blue channels of video amplifier 250; and preset control values for controlling the outputs of ERT generator 240. The image parameter values are selected by display processor 270 in response to input mode information from adaptor 70. The mode information is delivered from adaptor 70 to display processor 270 via line and frame sync lines H and V. Display processor 270 processes the selected image parameter values to generate analog control levels on the control links.

A user may also manually adjust the control levels controlling red green and blue video gains and cutoffs at video amplifier 250; and image width, height, and centring at line and flame scan circuits 220 and 240 via the user control panel 260. User control panel 260 includes a set of up/down control keys for each of image height, centring, width, brightness and contrast.

Referring now to Figure 3, the control keys are preferably in the form of push-buttons connected to key-pad interrupt inputs 320 to display processor 270. When, for example, the width up key is depressed, user control panel 260 issues a corresponding interrupt to display processor 270. The source of the interrupt is determined by display processor 270 via an interrupt polling routine. In response to the interrupt from the width key, display processor 270 progressively increases the corresponding analog control level sent to line scan circuit 220. The width of the image progressively increases. When the desired width is reached, the user releases the key. The removal of the interrupt is detected by display processor 270, and the digital value setting the width control level is retained. The height, centring, brightness and contrast setting can be adjusted by the user in similar fashion. User control panel 260 preferably further includes a store key. When the user depresses the store key, an interrupt is produced to which display processor 270 responds by storing in memory parameter values corresponding the current settings of the digital outputs to D to A convertor as a preferred display format. The user can thus programme into display 130 specific display image parameters according to personal preference.

Remaining with Figure 3, to provide power management function in display device 130, display processor 270, under the control of a microcode instruction program, maintains a count of pulses of a clock signal.

Display processor 270 resets the count on detection of a change in image content between successive frames of the input video signal. In the absence of a change in image content, display processor 270 continues to increment the count on each pulse of the clock signal. Display processor 270 begins disabling drive circuit 200 in response to the count stored in the timer exceeding a first predetermined threshold. Initially display processor 270 disables only high power portions of drive circuit 200 such as line scan circuit 220 and EHT generator 230. Subsequently however, as the count successively exceeds further thresholds progressively higher than the first threshold, lower power portions of driver circuit 200 such as frame scan generator 240 and video amplifier 250 are disabled in turn.

Eventually, only the low power portion of the power supply is left operational, to provide power to display processor 270 on low voltage supply line Vs (typically SV). Display processor 270 has a power supply control port SL connected to the power supply of display device 130 for selectively disabling and enabling the higher power portions of the power supply. To disable the higher power portions of the power supply, display processor 270 sets control port SL high. Display device is now operating in stand-by mode.

In accordance with the present invention, display device 130 comprises a lock circuit 300 connected to display processor 270. Lock circuit 300 comprises two field effect transistor switches Q1 and Q2. The drain of Q2 is connected via a resistor R1 (typically 10 kilo-ohms) to the low voltage power supply line Vs. The source of Q2 is connected to circuit ground 0V. The drain of Q2 is also connected to a lock sense input port LO of display processor 270. The gate of Q2 is connected via a diode D1 to power supply control output port SL of display processor 270. A capacitor C1 of typically 0.5uF is connected across the gate of Q2 and system ground 0V. The drain of Q1 is connected to the gate of QZ. The source of Q1 is connected to system ground 0V. The gate of Q1 is connected to reset output port SR of display processor 270. Transistors Q1 and Q2 are preferably insulated gate field effect transistors (IGFETs) because such transistors typically have a large but very low leakage gate capacitance compared with other types of transistors. Similarly, capacitor C1 is preferably a polycarbonate capacitor because typically such capacitors have a very high insulation resistance compared with other types of capacitor. For example, a luF polycarbonate capacitor for C1 may provide a hold time in excess of 24 hours.

In operation, when control port SL goes high to disable the higher voltage portion of the power supply, C1 charges to approximately Vs less one diode drop from D1 (eg: 5V less 0.7V). Q2 therefore turns on. If control port SL now goes low, D1 becomes reverse-biased. Therefore the voltage on the gate of Q2 remains even if the power is removed from display processor 270. When Q2 turns on, the drain of Q2 goes low.

Therefore lockout input LO to display processor 270 goes low. Display processor 270 polls lockout input port LO in response to any attempt to bring display device 130 out of stanclby mode. If polling indicates that input port LO is low, display processor 270 continues to disable drive circuit 200. The low charge leakage characteristics of the preferred implementations of Ql, Q2, and Cl rlolong the period or "hold time" for which input port LO can be held low by lock circuit 300 following removal of power from display device 130. The presence of C1 may increase the rise time of output port SL. This may produce an unacceptable delay in switching the operation of display device 130 to standby mode. This problem is solved in preferred embodiments of present invention by introducing a buffer at output port SL. The buffer also permits a higher voltage to be applied to Cl to either increase the hold time or, alternatively, to permit implementation of C1 by a smaller value capacitor.

In accordance with the present invention, to bring display device 130 out of stand-by mode, the user has to enter a security code into display processor 270. When display processor 270 recognises the correct security code, it pulses reset output port RL high. Ql turns on in response to the pulse on output port RL. C1 therefore discharges trough Q1. The voltage on the gate of Q2 therefore falls and Q2 turns off.

Lockout input LO therefore returns to a high state. Display device 130 is therefore unlocked. Display processor enables drive circuit 200 to display data on screen 210 via control links 275.

The security code is preferably entered by pressing the button of user control panel 260 in a predetermined sequence. In other words, display processor 270 only pulses reset output port RL high if it detects a predetermined sequence of interrupts on key-pad interrupt inputs 320.

User control 260 preferably further comprises a separate push-button 310 to permit a new security code to be programmed into display processor 270. The security code entered by first depressing button 310 and then entering the new desired sequence before releasing button 310. Display processor 270 detects the interrupt corresponding to button 310 and records the sequence of interrupts from the other button received while the interrupt from button 310 is present as the new security code.

With display device 130 locked in stand-by mode by lock circuit 300, an unauthorised user can only gain access to visual information from the computer system by interchanging display device 130 with a new display device which itself must be unlocked from stand-by mode. This makes an attempt to gain such access very obvious to observers. Referring now to Figure 4B, to obviate this possibility in particularly preferred embodiments of the present invention, a mechanical security feature 400 is provided with display device 130. In use, security feature 400 prevents interface cable 135 from being detached form system unit 5 without a key. Referring to Figure 4A, interface cable 135 of display device 130 is fitted with a multi-pin connector for releasably connecting display device 130 to display adaptor 70 of system unit 5. Connector 410 comprises a pair of knurled screws 420. In use, screws 420 mate with threaded receptacles on the system unit housing and are tightened to secure connector 410 in place. Returning to Figure 4B, mechanical security feature 400 preferably comprises a shroud having two parts, 430 and 440 adapted to fit together in clam-shell formation. Parts 430 and 450 are preferably moulded from plastics material. A key-operated lock mechanism 450 is provided to lock the two parts 440 and 430 together. In use, with connector 410 secured to system unit 5, the two parts 440 and 430 are brought together around interface cable 135 to encase connector 410 (shown in phantom in Figure 4B). Lock mechanism is actuated as the two parts 430 and 440 are brought together. Thereafter, the two parts 430 and 440 cannot be separated without unlocking lock mechanism 450 with an appropriate key. Release of connector 410 from system unit 5 is prevented because access to screws 420 is obscured by the shroud.

In the preferred embodiments of the present invention hereinbefore described, lock circuit 300 included memory means in the form of capacitor C1 to provide a limited hold time. However, in other embodiments of the present invention, lock circuit 300 may include other memory means, such as, for example, a non-volatile memory, to extend the hold time indefinitely.

Preferred embodiments of the present invention have been hereinbefore described with reference to a CRT display device. However, it will be appreciated that the present invention is equally applicable to other types of display device such as, for example, flat panel display devices including liquid crystal displays and the like.

Claims (12)

1. A display device comprising: a display screen (210); a drive circuit (200) for generating an image on the display screen (210) in response to an input video signal (R,G,B); and stand-by means (270,SL) for disabling the drive circuit (200) in response to a stand-by signal; characterised in that the device includes reset means (270,RL) for restoring operation of the drive circuit (200) on receipt of a predetermined security code.

2 A display device as claimed in claim 1, including entry means (260,320) for entering the security code.

3. A display device as claimed in claim 2, wherein the entry means (260,320) includes a key pad (260) having a plurality of push-buttons.

4. A display device as claimed in claim 3, wherein the reset means (270,RL) includes a processor (270) having a plurality of interrupt inputs (320) for receiving interrupts from the push-buttons of the keypad (260).

5. A display device as claimed in claim 4, wherein the processor (270) is configured to control the drive circuit (200) to change one or more image parameters of the image displayed on the display screen (210) in response to an interrupt from one or more of the push buttons.

6. A display device as claimed in claim 5, comprising a lock circuit (300) for configuring the standby means (270,SL) to disable the drive circuit (200), the reset means (270,RL) disabling the lock circuit (300) to restore operation of the drive circuit (200) on receipt of the security code.

7. A display device as claimed in claim 6, wherein the standby means (270,SL) comprises an output port (SL) cf the processor (270), and the lock circuit (300) comprises a memory element (C1,Q2) connected to an input port (LO) of the processor (270) for storing the state of the output port (SL).

8. A display device as claimed in claim 7, wherein the reset means (270,RL) resets the contents of the memory element (C1,Q2) on receipt of the security code.

9. A display device as claimed in claim 8, wherein the memory element comprises a capacitor (C1).

10. A display device as claimed in any preceding claim, comprising key operable means (Figure 4B) for preventing disconnection of the display device from a computer system unit.

11. A computer system comprising a display device (130) as claimed in any preceding claim, a system unit (5) for generating the input video signal to generate an image on the screen of the display device, and data input means (110,120) for inputting data and commands to the system unit (5).

12. Display apparatus comprising a display device as claimed in claim 1 and entry means (260) for entering a security code.