Service Manual
TANDY 6000/6000-HD
TAN D Y COMPUTER PRODUCTS ----------------------------------------------------------------------
7.5 Video/Keyboard Interface Board
The Video/Keyboard Interface board has two major functions.
One is to control a built-in, high-resolution video monitor
capable of displaying 24 lines of either 80 normal
characters or 40 expanded characters in both upper and lower
cases.
The other is to control a keyboard that includes the
normal set of key functions plus eight software-programmable
function keys.
Figure 7-10 shows a block diagram of this PCB assembly.
It
includes a high speed oscillator (Ul) wnose frequency
(12.48Hz) is the rate at which information is shifted to the
CRT and video dots are written, and a CRT controller
providing diversified functions such as video timing and
display refresh memory addressing.
A multiplexer switches
the control of the Display RAM address source to either the
CRTC or the CPU, depending on the RAM/Video RAM Logic
Selection.
The block diagram also shows a video-board
select logic which controls a 3-state buffer, the keyboard
control, and enables the video and Real Time Clock.
Data is
latched from the Display RAM into a ROM character generator,
then shifted to the video output to finally appear on the
display monitor.
Figure 7-10 includes as many blocks as possible for clarity.
For example, the pulse-width adjuster block in the block
diagram is simply an MSI monostable multivibrator with
Schmitt-trigger inputs.
Its main function is to provide
noise immunity and pulse width stability to the horizontal
sync signal, which is one of the CRTC ouputs.
Also, the
vertical sync goes through the RTC & NMIRQ* Logic to
generate a Real Time Clock signal (30 or 60 Hz) and a
non-maskable interrupt request signal.
All the different blocks describing the control system
(Video/Keyboard) are described in the following sections.
7.5.1.
Theory of Operation
1.
High Speed Timing (refer to Video/Keyboard schematic)
The -timing for the system is derived from a crystal
oscillator.
This oscillator is shown in the upper LH corner
of the schematic.
It consists of a 12.48 MHz
fundamental-cut crystal in a parallel resonant circuit that
is composed of two inverters (part of Ul) that are biased
into their linear region by resistors R2 and R3 (470 ohms
e a c h ) .
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Summary of Contents for 6000 HD
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Page 279: ...FIGURE 3 2 CONTROL AND DATA TIMING REQUIREMENTS 3 7...
Page 283: ...FIGURE 3 3 LSI CIRCUIT BOARD ASSEMBLY 3 11...
Page 289: ...FIGURE 3 5 DISKETTE CARE AND HANDLING 3 17...
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Page 294: ...FIGURE 4 4 INTERCONNECT BLOCK DIAGRAM 4 4...
Page 295: ...FIGURE 4 6 TRACK O TIMING 4 5...
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