Tandy 1000 HX Technical Reference Manual page 44

• TANDY COMPUTER PRODUCTS
Video System Logic
A major block of the Tandy 1000 HX is the video interface
circuitry. A block diagram of the video controller
circuit is shown in Figure 14. This custom part contains
all of the logic necessary to generate an IBM compatible
color video display. The video interface logic
consists of the 84 pin custom video circuit, 8 - 64K X 4
RAMs, a 74LS244 buffer, and associated circuitry for
generation of composite video.
The Tandy 1000 HX video interface circuitry controls 256K of
memory. See Video System Memory Map, Figure 15. This RAM is
shared by the CPU and the video. Normally, the video only
requires 16K or 32K for the video screen and the remainder
of the 256K is available for system memory uses.
The Tandy 1000 HX video interface custom circuit is composed
of a 6845 equivalent design, dynamic RAM address generation/
timing (see Figure 16.), and video attribute controller
logic.
Normal functioning of the video interface custom circuit is
as follows: After the 6845 is programmed with a correct; set
of operating values (see Table 2 ) , the address inputs to the
dynamic RAMs are generated by a 4:1 multiplexer. This MUX
switches between video (6845) addresses and CPU addresses as
well as between row and column addresses. In addition, the
video interface chip provides the RAM timing signals and
generates a wait signal to CPU for proper synchronization
with the video RAM access cycles.
The outputs from the RAM chips are only connected to the
video interface custom circuit, so all CPU read/write
operations are buffered by this part. During a normal
display cycle, video data from the RAM chips is first
latched in the Video Attribute latch and the Video Character
latch. The video interface requires a memory organization
of 64K X 16 and will latch 16 bits of memory during each
access to RAM. From the output of the two latches, the
data is supplied to the character ROM for the Alpha modes
or to the shift registers for graphics modes. A final 2:1
MUX is used to switch between foreground or background in
the alpha mode.
From the 2:1 MUX, the RGBI data is combined with the PC
color select data and latched in the Pre-Palette latch.
This latch synchronizes the RGBI data before it is used to
address the palette.
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