Omron 8025G Maintenance Manual page 52

THEORY OF OPERATION
SECTION 4
The keyboard interface controls data transfer from the keyboard to the
CPU.
To transfer data, the keyboard encoder sends a strobe signal to the keyboard inter-
face, which leads to the following sequence of events. Upon receipt of a POLL I/O
command from the CPU, the interface responds with its address over the A Data Bus.
When the CPU in turn responds with an I/O command, the interface gates KB Data to
the A Data Bus, and signals the encoder that the data was transferred. If a key is
depressed for more 'than one-half second, the interface also signals the encoder to
repeat the last character transferred until the key is released.
Each time the keyboard interface transfers KB Data to the A Data Bus, it
causes the click/beep circuit to produce an output pulse heard on the keyboard speaker
as a click. When the click-beep circuit receives a signal fro~~ the beep decode cir-
cuit on the Refresh Buffer Card, it produces an output heard on the speaker as a
beep.
Circuits on the Video Control Card generate the signals that produce and
control the display on both the terminal CRT Display and an optional external display.
Characters stored in the character generator ROM are addressed for display
by d'ata from the shift register on the Refresh Buffer Card. The line to be displayed
in the character row is specified by the V inputs to the ROM. Parallel data out of
the ROM is converted into serial form and gated, under video modification control,
t6 two mixer drivers. In one of these, the video is mixed with the blanking signal
(CB) for use by the CRT display. In the second, the video is mixed with the comp-
sync signal (CS) for use by an external display.
Video modification. data from the shift register on the Refresh Buffer Card
is stored in the video modification register. The video modification logic decodes
the register outputs, video status data on the CS Data channel, and the other input
signals shown to produce modification commands. These commands are applied to the
video gate to modify the video display as required (e.g., reverse video, underline,
etc.).
The Keyboard Card contains an X-V switch matrix, a shift and control cir-
cuit, and an encoder. An X output from the encoder is connected to a Y input to the
encoder when a key is depressed. The encoder converts the X-V coordinates into an
ASCII code, which it places on the KB Data Channel. At the same time, the encoder
sends a strobe signal to the keyboard interface on the Cursor Control Card to indi-
cate that the keybQard has data for the CPU. After the data is transferred, the
interface sends a signal to the encoder to reset the strobe selector and enable the
encoder for the next key stroke. Shift and control inputs to the encoder initiate
keyboard shift action and control character code generation.
The RS-232 Interface Card provides for compatability between the 8025 Term-
inal and a data communications system. Parallel data supplied by the CPU via the
MA Data Bus is converted into serial form in the receiver-transmitter, and then trans-
mitted, under control of the control logic, to the data communications system (output
BA). Incoming serial data (BB) goes through the control logic to the receiver-trans-
mitter where it is converted into parallel form. The control logic enables the re-
ceived data gate to transfer the data to the A Data Bus for entry into the CPU.
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