Canon A-200 series Service Manual page 95

The heart of this circuit is the LS161 Synchronous Counter.
When the all output terminals OA, OB, OC and OD become HIGH, a HIGH level signal is output
from the CY terminal and then a inverted LOW level signal is input to the LOAD terminal. In this
case, values at the A, B, C and D terminals are input and then they are output at a rising edge
of the next clock cycle. Further, these values are counted up at the every following clock cycle.
The fundamental clock signals for this display board is made using these output signals. Detailed
description for every clock signal is as follows:
CRTC-ClK
This signal is emitted by ANDing the OC and OA output signals and is fed to the CRTC.
CPu/CRTC
This clock is made by NANDing the OC and OD outputs and switches the addresses of the
V-RAM between the CPU mode and CRTC mode. In the CRTC mode, this signal is used as the
OE signal, the attribute signal and the timing signal for latching of the character signals, etc.
S/L
The
SIC
signal is generated by inverting the CY signal and used as the load signal for the LS161
and for the character shift register (LS166).
WE
WE signal is generated by shifting a signal input to the D terminal of U5 for one clock cycle.
The signal to be sent to the D terminal is made by NAN Ding the EX-ORed output of OB and
OA and the CRTC-CLK signal. This WE Signal becomes the writing signal to the V-RAM when
the CPU cycle.
• Attribute Generator
Figure 6-33 is an overview of this circuit.
Descriptions for each output signal is as follows:
NON-DISP
This signal is made by decoding the attribute signals. That is, when the ATO, AT1, AT2, AT4, AT5
and AT6 signals are all LOW, the NON-DISP signal also becomes LOW. In this case, no character
is displayed on the screen.
REVERSE
This signal is also made by decoding the attribute signals. When the ATO, AT1 and AT2 are LOW
and the AT4, AT5 and AT6 is HIGH, the REVERSE signal becomes HIGH. In this case, the video
signal becomes to the reverse mode.
UNDERLINE
When the attribute signals ATO is HIGH, AT1, AT2, AT4, AT5 and AT6 are LOW, this signal is output.
In this case, an underline sign previously set in the CRTC is output on the screen.
BLINK
When the ATT' signal is HIGH, the BLINK-EN signal is HIGH and the CURSOR signal is not
active, the BLINK signal make the CRTC to generate a signal which is made by dividing the V­
SYNC signal into one 32th. When this signal is LOW, displayed characters are disappear and
when H IG H, re-appear.
CURSOR*
This signal is made by ANDing the CURSOR signal from the CRTC and a signal which is divided
the V-SYNC signal into one 16th. This CURSOR signal is controlled by the software.
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