Vector 4 - Page 82

VECTOR 4
TECHNICAL INFORMATION
B. Gray Scale Graphic Modes
This Graphics Mode can be divided into two categories. These are the 160 and
320 modes which were shown pictorially in Exhibit II-9. Both of these modes
use the address circuitry (from the Video Controller to the input of Graphic
Mode Shift Registers) described in the previous section. However during this
mode the Shift Register outputs are sent to a different MUX.
In the 160 mode the bits are sent to the Video Bit MUX in two groups of 4.
From this point they travel to a lateh (<KC9>) where they are clocked by the
decoded output of the Gray/Color Subsystem Clock. The resultant 4-bits are
then routed to two different locations:
~ External RGB Connector (J16).
- 4-bit Digital/Analog Subsystem.
Section 3.3-C desceribes how the 4-bits are transferred to the External RGB
Connector.
The 4-bit value that is sent to the Digital/Analog Subsystem is converted into
an analog value. This value is then coupled with the output of the 7407
Drivers (Ul116). If the Drivers are "logically OFF" the data will not be
displayed on the screen. i.e. Dark screen during retrace. Otherwise the
analog value is used by the internal CRT System to create one pixel position.
In the 320 mode 1 byte of data (from Dynamic RAM) is divided into four, 2-bit
groups. Each of these 2-bit values is sent to a Mapping RAM where it selects
one of four, 4-bit registers. Exhibit JI-13 shows how the values in these
registers can be changed for the Gray and Color Scale 320 Modes.
The Mapping RAM routes the 4-bit data to the Video Bit MUX where the
SELECT Line (pin 1) determines the correct transmitting frequency (comes from
U74). Pin 15 is used to determine the horizontal and vertical blanking periods.
The 4-bit value is sent to a latch and then routed through the same channels as
was the 4-bit value in the 160 Gray Scale Mode.
Section IV has a complete description (including a pin-out) of the video
connector used for the internal CRT.
C. Color Scale Graphic Modes |
As with the Gray Scale this graphic mode can be divided into 160 and 320
modes. The hardware protocol is basically the same for these modes as it was
for the Gray Scale Graphic Modes. The primary difference is found in how the
"eolor palette" is used.
09-01-82 7200-0001 II 3-19