Omron 8025G Maintenance Manual page 54

THEORY OF OPERATION
SECTION 4
housekeeping tasks, will send out another POLL command.
Ouring display time, the address counter on the Refresh Control Card sequen-
tially scans the refresh memory over the MA Data Bus. When the address for the "A"
is reached, the stored ASCII code is read out of memory through the output gate to
the Refresh Memory Card. Here the "A" goes through the RM and half-line gates to the
shift register for transmission over the CG Data Channel.
Full addressing of the character generator ROM on the Video Control Card
includes CG data plus vertical timing signals (V). The former defines the "A"; the
latter specifies the scanning line in the character row to be displayed. The charac-
ter generator ROM decodes the line and character information for the "A" and puts
out the data in parallel form.
After conversion in the parallel-to-serial converter, a video signal con-
taining horizontal line segments of an "A" is gated to the vidlo mixer/driver. This
circuit combines the video with the composite blanking signal (C B ) for application
to the CRT display.
Let's assume now, that.instead of an "A" the keyboard character entered is
a video modification character specifying reverse video (black characters on a white
backgro1:md) .
For a video modification character the operating sequence is the same as
that for the "A" with these exceptions: (1) the character leaving the refresh shift
register contains information identifying it as a video modification character; (2)
the character contains reverse video commands that go to the video modification reg-
ister on the video control card; (3) outputs from the register are decoded by the
video modification logic; and (4) the logic output alters the video signal to dis-
play black on' white until
~ither
the next video modification character arrives or
the end of the line is reached.
Finally, let's assume the NEW LINE key is struck. After determining that
the character is nondisplayable, the CPU addresses the ROM program via the MA Data
Bus anJ receives program instructions over the A Data Bus. In this case the CPU is
instructed to move the cursor position down to· the next character row and to the
left side of the display. The CPU then updates, via the B Data Bus, the cursor
location in the refresh memory's fixed-position address block. The new cursor posi-
tion will subsequently update the cursor position register on the Cursor Control
Card so as to change the cursor position on the CRT screen.
RS-232 I~terface Transfers. Data transfer from the RS-232 Interface Card
i~
basically the same as a keyboard entry. In this case, however, data can be trans-
ferred in either direction.
Assume the RS-232 Interface Card wants to transfer an "A" from a modern to
the CPU for display on the CRT. The "A" received in serial form from the modern on
the BB line, is fed to the receiver-transmitter. On the RS-232 Card, the receiver
converts the serial data into parallel form and puts the "A" in a local register.
When it intercepts a POLL command from the CPU, the interface stops further propa-
gation of the POLL, places its address on the A Data Bus, and selects itself for the
next data transfer. An 100 signal from the CPU in response .causes the interface to
deselect itself and gates the "A" to the A
Dat~
Bus for transfer to the CPU. The re
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