Technical information

CHAPTER 2
Hardware Design
22
Video System
Video System 2
The DOS Compatibility Card includes a complete video system to support PC video.
The video system consists of a DRAM-based frame buffer and a VGA controller with
an integrated color lookup table (CLUT), triple digital-to-analog converter (DAC), and
clock generator.
Connecting a Monitor 2
Video output from the DOS Compatibility Card can be displayed either on a dedicated
monitor or on a monitor shared with the host Macintosh computer. When using a
dedicated monitor, the video-out end of the video adapter cable (a DB-15 socket) is
connected directly to the monitor. This mode of operation provides separate video from
the PC on the card and the Macintosh host computer.
When using a shared monitor, the video output from the DOS Compatibility Card is
connected to the same video cable as the video output from the Macintosh. The user can
switch from one video screen to the other using a programmable key sequence. When
the user selects Macintosh video, the software sets a bit in port A of the interface IC. This
bit is connected directly to the blanking input of the SynDAC IC (described on page 26)
and causes the PC’s video to be blanked (held at 0.0 V). The port A bit also controls a
multiplexer between the PC and Macintosh sync lines so that the video signal from the
Macintosh host computer is sent to the shared monitor.
When the user selects PC video, the software on the Macintosh host computer writes
black RGB values into all entries of the CLUT and sets the DC offset register in the DAC
to make the black and blank levels equal (0.0 V). The port A bit is then switched so that
the SynDAC unblanks the PC’s video signal and the video multiplexer sends the PC’s
video signal to the shared monitor.
To provide appropriate termination of the video signal, the DOS Compatibility Card
determines whether it is sharing the monitor with the Macintosh host computer. If the
monitor is being shared, the computer end of the video cable is terminated on the host
computer’s main logic board. If the DOS Compatibility Card is using a dedicated
monitor, then the card connects the proper 75-ohm termination resistor to its video
output line.
Monitors Supported 2
The DOS Compatibility Card has 512 KB of DRAM soldered on that provides all the
standard VGA modes and some extended SVGA modes. No video DRAM expansion is
provided because none is needed to meet full VGA compatibility. The VGA controller