Zenith Z-100 Series, Technical Manual

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Page 4.48
CIRCUIT DESCRIPTION
Video Logic Circuit Board
Video Processing Circuits
Cathode Ray Tube Controller (CRT-C)
The CRT-C, U330, fetches the characters to be displayed and
provides horizontal and vertical timing. It also keeps track of
the affected character if the light-pen circuits are used.
Briefly, here's what each line does. See the 6845 IC data
sheet for more information.
PPPC Power-on clear, from the S-100 bus. Sets all re-
gisters to their initial conditions on power-up or reset.
6845~ Chip - s elects the CRT-C for accessing the in-
ternal registers.
ECLK La t ches the data into or out from the registers
on its trailing edge.
BAO Helps select a specific register inside the CRT — C.
OUT Wh en low, writes data into the selected register.
Otherwise, reads data from it.
DIOO-DIO7 Data bus used by the CPU to access the
CRTC registers.
CLK Pr o vides character-clock timing to the CRT-C.
HSYNC Ho r izontal sync pulse.
VSYNC Ve r tical sync pulse.
CURSOR Pr o vides an indication where the next char-
acter will be printed.
DISEN Di s ables the display during horizontal and verti-
cal retrace.