Not to be caught sleeping, the semiconductor industry has carefully watched the way the computer manufacturers build video displays. Using the theory that anything done in LSI is a better deal than the same thing done with random logic, American Microsystems has developed an incredible video display generator LSI chip called the 68047 vdg.


Fig. 2-12. A simple eight-color display.

This is one of the most sophisticated chips on the computer market—generating 64 character ASCII alphanumeric characters with an internal ROM, uppercase and lowercase ASCII via external ROM, two semigraphics-mode block graphics in a 64×32 or 64×48 matrix with up to eight colors, and eight full graphics modes from 64×64 in four colors all the way up to 256×192 in one color. This chip generates a composite video display on standard NTSC compatible black-and-white or color television. Fig. 2-13 shows a block diagram of the 40-pin 68047 chip and Fig. 2-14 shows what the display looks like for the eight-color semigraphics mode.

As you can see in the block diagram of Fig. 2-13 the chip contains everything needed for a full-color display in one package. A full-blown graphics computer built around this chip is shown in Fig. 2-15. Here we see that a 6800 microprocessor is used as the main processor and a 6820 PIA (peripheral interface adapter) is used to control and access the 68047 vdg chip. A 9K dynamic RAM memory is used as display memory and an external RAM is used to hold an extra character set. An rf modulator allows going into the tv antenna terminals directly.

Note the FS output on the 68047. This is a signal that indicates the chip is in the vertical retrace period. This is coupled into the IRQ interrupt input on the 6800 microprocessor so that once every vertical retrace period the 6800 microprocessor is interrupted to update the display RAM with new information or to compute new data. When the display is busy it scans the display RAM and sends the information in it to the tv.


Fig. 2-13. A 68047 video display generator.


Fig. 2-14. “Honest Abe” was produced with the AMI 68047 video display generator chip in the eight-color semigraphics mode.

Motorola makes a similar chip called the MC6847Y (interlaced) or the MC6847 (noninterlaced). An evaluation board, containing all the necessary hardware to build a complete color graphics display for the 6800 series of microprocessors, is available from Motorola, Inc. It is called the Micro Chroma 68 and is a good way to go if you are interested in designing and experimenting with your own color graphics hardware.

A color graphics board based on the 68047 chip from AMI that is designed to work with any S-100 bus computer is available from Biotech Electronics in Ben Lomond, California. It's called the BCG-800. An intelligent color graphics board based on the Motorola MC6847 (called the CGS-808) is also available from Biotech for S-100 owners. It has firmware packs that allow several graphics subroutines to be executed, including point plot, line draw, cursor read, direct memory transfer, etc. All data transfers are through two i/o ports, allowing fast parallel processing of display and computer.

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