Image display device for displaying an identical image on its display panel and an external display unit, but capable of freezing and erasing an image on the latter

Claim 1

An image display device which is connectable with an external display means such that an identical image can be simultaneously displayed on the external display means as well as on its own display means, comprising: a central processing unit; a display control circuit for outputting image data corresponding to an image to be displayed and a synchronous signal under control of the central processing unit, the image data and the synchronous signal constituting display data; a first display means for displaying an image based on the display data output from the display control circuit; a first interface connected with the first display means; a second interface connectable with a second external display means; a memory circuit connected between the display control circuit and the second interface and having an internal memory to and from which display data received from the display control circuit is written and read out, the memory circuit having a write mode wherein display data in the internal memory is rewritten and the rewritten display data is output from the memory circuit, and a write inhibit mode wherein rewriting of display data in the internal memory is inhibited such that the display data stored in the internal memory without being rewritten is read out and output from the memory circuit; a disable circuit connected between the memory circuit and the second interface for, when in operation, inhibiting display data received from the memory circuit from being output to the second interface, and outputting monochromatic display data instead of the display data received from the memory circuit such that a screen of the second external display means exhibits a single color; a sign pattern generator for, on receiving the synchronous signal from the display control circuit, outputting pattern data corresponding to a sign indicating display mode conditions of the second external display means; a synthesize circuit for synthesizing display data received from the display control circuit and pattern data received from the sign pattern generator and outputting the synthesized display data to the first display means through the first interface; a control circuit for controlling the memory circuit and the disable circuit under control of the central processing unit such that: during a normal mode of the image display device wherein an identical image is displayed on the first display means and the second external display means at the same time, the memory circuit assumes the write mode and the disable circuit does not operate; during a freeze mode of the image display device wherein an image on the screen of the second external display means is frozen, the memory circuit assumes the write inhibit mode, and the disable circuit does not operate; and during an erase mode of the image display device wherein an image is erased from a screen of the second external display means, the disable circuit operates.