Texas Instruments 990 Manual page 18

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945424-9701
There are two VDT controllers available with the Model 911 VDT. The United States and European
controller is part number 946076; the Japanese Katakana controller is part number 2263490.
The VDT controller performs the following functions:
•
Provides interfaces between keyboard and computer,
monitor.
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• Generates (via ROMs) character codes for 96 standard display symbols, including upper-
case and lowercase Gothic alphabet characters, numeric characters, and special symbols
(+, -, ;,
etc.). An additional 64 Katakana characters are produced by the Japanese version.
• Provides storage for as many as 2048 (2K) eight-bit character codes originated by the
computer for display on the CRT screen (1024 for 960-character version).
• Transmits contents of refresh memory to CRT screen to refresh display 50 or 60 times
per second, depending upon ac input.
• Loops back computer-supplied character code and cursor address, video data, horizontal
sync, vertical sync, and software alarm enable (audio enable) to output multiplexers
for examination by diagnostic software.
The interface between the computer and the VDT controller is a standard 990 CRU interface
consisting of a serial output data line and associated output data strobe (STORECLOCK-), four
CRU address lines that specify to the VDT controller which of its 32 internal latches and/or
registers should accept the serial data or control bit, a module select signal that indicates that the
VDT controller has been selected by the computer as the destination device for the CRU transfer,
a serial input data line, and a device interrupt line.
The computer provides the module select signal, four-bit CRU address, and strobe signal for each
bit of information on the data line (CRUBITOUT) to the VDT controller. The controller responds
to the module select signal by decoding each four-bit CRU address and either latching the accom-
panying bit of information in the appropriate latch or register, or responding to strobe commands
(Increment. Cursor, Reset Keyboard Interrupt, etc.).
During entry of data from the keyboard into the computer, the VDT controller accepts the 11-bit
serial keyboard code from the power/logic pwb. The VDT controller removes the start, parity
and stop bits from the data code and gates the resulting eight-bit data-only code to an output multi-
plexer. The controller then issues an interrupt to notify the computer that keyboard data is ready
to be transferred
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the computer. When ihe computer has accepted the data from the keyboard, it
issues Keyboard Acknowledge to reset keyboard data ready.
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controller with eight bits of character data (generated in computer memory) over the serial data
line (CRUBITOUT). This information determines what is to be displayed on the CRT screen, and
whether the displayed information is to be displayed normally or at high intensity (low-intensity
symbols are displayed with circular dots, and high-intensity symbols with elongated dots). The
computer also provides display and cursor enable signals, and an 11-bit ·cursor address, all over the
same serial output data line. The cursor address specifies the position on the screen where the
information is to be displayed and the location in the VDT controller's refresh memory where
the information is to be stored. The VDT controller continually routes the contents of refresh
memory to the CRT screen to maintain the screen images.
1-8
Digital Systems Division