The 98760A display receives data from the CPU, buffers it in RAM, and displays it on the CRT screen.
The display is a memory mapped, raster scan CRT. A total of 256 characters can be displayed, including all 128 ASCII characters. The display uses either the English or Japanese Katakana character set, depending on which ROM is selected and inserted at the factory. Character display matrix is 720 dots (horizontal) by 390 dots (vertical).
The bottom of the screen can be set up to display in half-bright, inverse video, on a line-by-line basis. This feature visually separates softkey labels from the remainder of the screen.
The CRT display consists of these assemblies:
• Alpha/Interface Board
• Graphics/Digital Video Board
• Video Board
• Sweep Board
• CRT N oke Assembly
• Display Power Supply
Alpha/Interface Board
The alpha/interface board is mounted to the bottom of the display to interface the lOP bus from the processor with the graphics/digital video board in the display (Figure 2-21). Connectors P1 and P2 interface the two boards. Connectors P3 and P4 interface the alpha/interface board to the compu-ter mainframe. The connectors are as follows:
P1 - 40-pin; data/control to graphics/digital video board P2 - 50-pin; data/control to graphics/digital video board P3 - 50-pin; power to alpha/interface board
P4 - 50-pin; lOP bus to alpha/interface board
The alpha/interface board receives data from the lOP via data lines 1000 through 10015. It buffers this data, then either stores it in the alpha memory or passes it on to the graphics/digital video board. At the appropriate time, data stored in the RAM is read out and sent to the character and attribute ROMs on the graphics/digital video board.
The alpha/interface board functions are controlled by a 6845 CRT alpha controller chip. Functions include interface logic, timing, and control logic. Alpha timing signals are provided to the graphics/
digital video board.
A 50/60 Hz select switch is mounted on the alpha/interface board and can be accessed through a hole in the display casting. This sliding switch should be preset at the factory as appropriate.
P4
Graphics/Digital Video Board
The graphics/digital video board has an attribute ROM and an English or Katakana character ROM (Figure 2-22). The board receives the alpha signals from the alpha/interface board, generates appropriate characters and attributes (blinking, underline, and inverse), and produces the red, green, and blue analog video streams for the video board. Alpha characters have I-of -8 colors.
The graphics/digital video board has a graphics display controller which receives graphics data from the alpha/interface board and shifts it into graphics memory. At the appropriate time, data stored in the RAM is read out and sent to the color map. The color map has 16 locations of 12 bits each, four bits per gun. Therefore, at any given time, the color map can store 16 of a possible 4096 colors (16 red by 16 green by 16 blue).
The graphics/digital video board also has a 51 MHz crystal oscillator which provides timing signals to both the alpha/interface and graphics/digital video boards. The board has a sync generator circuit which provides the vertical and horizontal sync signals and the vertical blanking signal to the video board.
Three digital-to-analog converters on the graphics/digital video board convert the 12-bit digital output of the grahics color map, and the red, green, and blue alpha video streams which are not colormapped, into red, green, and blue analog signals suitable for the video board.
LD
Multiplexer Memory State Machine 12
12SK Bytes
+ I
V
Character and Shift Register Alpha!
~ ) ~ )
Figure 2-22. Graphics/Digital Video Board Block Diagram
Red
Video Board
The video board receives red, green, and blue video streams from the graphics/digital video board.
It contains three identical video amps which amplify each video stream to the voltage level required to drive a CRT color gun.
Adjustments on the video board include black level and white level for each of the video streams:
red, green, and blue.
Sweep Board
The sweep board receives horizontal and vertical sync signals and vertical blanking from the graphics/digital video board, and produces the current required for horizontal and vertical deflec-tion of the electron beam.
The sweep board generates the voltages required by the CRT control grids and for accelerating and focusing the electron beam. A flyback transformer is utilized for this function.
Potentiometers on the sweep board control the focus, intensity, vertical centering and height, and horizontal centering of the CRT display. A coil on the sweep board controls the horizontal width.
CRTNoke Assembly
The CRT/yoke assembly displays the video signals. The resulting display is 25 lines by 80 charac-ters per line. The graphics is 512 dots horizontal by 390 dots vertical.
The CRT/yoke assembly has adjusting magnets for pincushioning. They are set at the factory and normally do not need adjusting.
Display Power Supply
The function of the power supply is to change 100-240V ac into filtered, regulated dc levels of
+
5V,+
12V,+
75V, and - 12V, and to provide overcurrent and overvoltage protection.The display has an ac fan which runs off the power transformer primary.
The display power supply remains turned off until it senses the