Ax and ega video display apparatus utilizing a vga monitor

Abstract

A PC video adapter board for driving a VGA color monitor, the board including a VGA video color palette DAC for providing analog color video to the monitor. A VGA controller provides VGA digital color video signals to the DAC through a first multiplexer and timing signals to the monitor through a second multiplexer. The board also includes EGA video controllers and a Japanese EGA (JEGA). A read/write register is written from the computer to contain additional EGA digital color bits for supplementing the six JEGA bits so as to provide a full eight bits of digital video in the EGA modes. The register provides a control bit to switch the first and second multiplexers between the VGA and EGA digital color video and timing signals. A further control bit from the register controls the third multiplexer either to send the GS and RS video signals from the JEGA directly to the first multiplexer to operate in a limited six bit mode or to send the supplementary bits to the first multiplexer to operate in an eight bit video mode. The horizontal sync and blanking signals from the EGA controllers are delayed by two pixel clock times to center EGA screens on the VGA monitor and registers in the EGA controllers are programmed to adjust the EGA snychronization timing for compatibility with the VGA monitor.