The block diagram showing the connection of 6845. CRTC along with supporting components is shown in figure.
1) Whenever any key is pressed then ASCII code pressed key character is stored in display RAM by MP. Similarly attribute code containing different information like blinking, blanking underlining etc is also stored by MP into attribute memory.
2) To start CRT scanning from upper left corner of screen initially.
a) ROW COUNTER = 0
b) CHARACTER COUNTER = 0
c) LINE COUNTER = 0
The value of row counter and character counter is transferred through memory address pins MA13 – MA0 to address pins of display RAM and attribute RAM. So 8 bit ASCII code and attribute code of first character is transferred into corresponding octal latch.
3) The value of line counter = 0 is transferred on raster address line RA4 – RA0 bits. 8 bit ASCII code in octal latch with RA4 – RA0 bits are used to select one memory location of character generated ROM. This memory location will contain dot patterns of first line of first character. This dot patterns is converted into serial form and transferred on video signal pin which will control ON and OFF electron beam.
4) After scanning first line of first character counter is increased by 1 and same process is repeated. In this way first line of 80 characters of first row are scanned. This is called Horizontal Trace.
5) Now, character counter is again made 0 but line counter is increased by 1 and same process is repeated for second line.
6) After scanning all 25 rows 6845 gives logic 1 pulse on VSYNC (Vertical Synchronous Pin) which is used to energize an electromagnet for vertical retrace. This complete operation is called 1 frame and no. of frames in one second is 60 only one data is made to glow at a time but due to persistence of vision all characters in CRT screen can be seen together.
Explain the functioning of color graphic adapter in detail
The figure shows the block diagram of IBM PC Color graphic adapter (CGA) board. This board again uses MOTOROLA 6845 CRT Controller device to do the overall display control 6845 produces the sequential address required for display refresh RAM, the horizontal SYNC pulses and vertical SYNC pulses. The adapter board is designed to drive either a monitor with separate Red, Green and Blue inputs which has all the required signals combined on a single line. The 16 HB display refresh RAM on the CGA board is dual ported so that it can be accessed by either the system processor r CRT controller. The adapter board can operate in either character mode or a graphic mode. In the character mode it uses a character generates ROM and shift registers (alpha serializes) to produce serial dot information for the RGB outputs. In character on alpha-numeric mode each character is represented by two bytes in the display refresh RAM in the format shown in figure.
The lower 4 bits of attribute bytes use the codes shown in figure to specify the color of display character.
I R G B Color
0 0 0 0 Black
0 0 0 1 Blue
0 0 1 0 Green
0 0 1 1 Cyan
0 1 0 0 Red
0 1 0 1 Magenta
0 1 1 0 Brown
0 1 1 1 White
1 0 0 0 Gray
1 0 0 1 Light Blue
1 0 1 0 Light Green
1 0 1 1 Light Cyan
1 1 0 0 Light Red
1 1 0 1 Light Magenta
1 1 1 0 Yellow
1 1 1 1 High Intensity White
The frame buffer in CGA system starts at absolute address B8000H and 2 bytes are required to hold character code and attribute code for each character.
When operating in a color graphic mode a CGA board uses separate shift register to produce the dot information for each of color guns and for overall intensity.
CGA adapted board can be adopted operated in low resolution, medium resolution or high resolution mode. The low resolution mode is not of much interest because the display has only 100 rows of palette with 160 palettes in each row. The high resolution mode display 200 rows with 640 palettes in each rows but it can produce monochrome graphic display. In medium resolution mode the display consist of 200 rows of palettes with 320 palettes in each row or total of 64000 pixels.
Explain the following adapters in brief
→ EGA (Enhanced Graphic Adapter):
The Enhanced Graphic Adapter has a programmable CRT controller which allows you to set the display mode much as you do with the MC 6848 on a CGA board. However the key parts of EGA hardware that we need to talk about the either frame buffer or palette register. The figure shows how they function for the most commonly used format a 640 x 350 pixel display with 16 colors from a palette of 64.
In this mode the frame buffer memory is configured as four planes. Each plane hold one of the 4 bits required to specify the colors of each pixel. A 4 bit value read from the four planes is used to address one of 16 palette registers. The lowest six bits from 8 bit each.
The addressed palette registers are O/P to color monitor.
There are 64 possible combinations for six bit value in each palette register. But since there are only 16 palette registers only 16 of 64 possible combinations can be stored at a time. The 16 value in palette registers at any particular time then specify the 16 colors during palette of 64.
Raster Scan Character Displays:
A CRT is basically a large bottle shaped vacuum tube. An electron gun at the rear of the tube produces a beam of electron which is directed towards the front of the tube by a high voltage. The inside surface of the front of the tube is coated with a phosphor substance which gives off light when it is struck by electron. The color of lights given off is determined by the particulars phosphor used.
The most common method of producing images on a CRT screen is to sweep the electron beam back and forth from left to right across the screen. When the beam reaches the right side of the screen it is turned off and retraced rapidly back to the left side of the screen to start over. If the beam is slowly swept from top of the screen to the bottom of the screen as it is swept back and forth horizontally, the entire screen appear lighted. When the beam reached the bottom of the screen it is blank and rapidly retraced back to the top to start over.
A display produced in this way is referred to as Raster Scan Display. To produce and image an electron beam is turned on or off as it sweeps across the screen. The trick here is to get the beam intensity or video information synchronized with the horizontal and vertical sweeping so the display is stable.
For a first example the following figure shows the scanning used to produce pictures on TV sets and displays on some computer monitor.
IBM PC Monochrome Adapter
The diagram shows IBM PC monochrome display adapter board. The circuit has following parts:-
1) CRT Controller
2) Character Generator
3) DOT Shift Registers
4) 2H byte memory where ASCII codes of character to be displayed are stored. To the right of this memory is another 2H byte memory used to store attribute codes for each character.
An attribute code specifies whether a character is to be displayed normally with an underlined with increased or decreased intensity blinking etc.
We may observe for example that it is common practice to display a screen menu at reduced intensity.
ASCII code from display fresh RAM go to character generators. Also going to character generator are the four address lines which specify the dot line of the character scanned. The counter which generates this address is contained in MC 6845 CRT controller device. Output from character generator goes to a shift register to be converted to serial form for the video amplifier.
The most common method of producing images is shown in figure. As shown electron beam is swap back and forth from left to right across the screen. When beam reaches right side of screen, the beam is turned off and horizontally retraced rapidly back to left hand side of the screen to start new one. Thus beam swept left to right with some angle. Therefore it also slowly goes down to the bottom of the screen. Thus full screen is lightened up when beam reaches to the bottom of the screen it is blank and rapidly retraced back to top to start over.
This retraced is referred as vertical retraced again while retracing beam is off a beam produced in this way is referred as raster scan display. To produce an image an electron beam is turned on or off as it sweeps across the screen. The important point is to get beam intensity or video information synchronized with the horizontal or vertical sweeping so that display is stable on a standard monochrome monitor the electron beam scans the screen 50 times per second. The phosphor has persistence which means it keeps radiating light after electron beam has moved on.
Under Raster Scan we have two techniques of scanning:-
1) Non inter-latched scanning
2) Inter-latched scanning