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Theory of Operation
3–100
1780R-Series Service Manual
The output from U429B drives U223B, which enables the Digital Delay
Counters. U429B and U223B are both cleared with the /RAMP START signal,
which becomes true at the end of the digital count from the delay counters.
The remaining outputs from U426 are described as follows:
LINE SEL PHASE: Presets flip-flip U429A during the selected line in line
select mode so that one of the timing dots is present each time a line is
displayed. Note that since there are two timing dots, and since only one
timing dot can be displayed during one line, that each dot is displayed only
half of the time on that line, causing the displayed timing dots to flicker at
half the rate of the selected line.
FREQ CAL CLK: During line select, FREQ CAL CLK occurs just after the
selected line is displayed. During non-line select, it occurs during the vertical
interval. This logic selects V SYNC or W LINESEL L as the FREQ CAL
CLK source, depending on line select or not.
/CLK RUN: /CLK RUN is true low, and runs the timing cursor clock
(Oscillator Q214). If either COUNTER ENABLE or FREQ CAL is low,
/CLK RUN becomes low.
CURSOR: This signal is true high. SUBCARRIER DOTS EN, TIME DOTS
EN, LINE SEL, and WFM READOUT EN are input control signals for
selecting T CURSOR IN or SUBCURSE. These control signals determine
whether T CURSOR IN or SUBCURSE is steered into CURSOR. In
non-line select mode, no timing cursors are displayed when the readout is
being displayed. In line select mode, timing cursors are displayed during the
readout time so that they will be visible on the voltage cursors. This also
causes the timing cursors to be displayed on the readout text when in line
select mode. When subcarrier dots are being displayed, SUBCURSE is
selected as the input for CURSOR.
/CURSOR LOCATE: This signal is true low. /CURSOR LOCATE is a 1 ms
pulse which coincides with the timing cursor pulse, CURSOR. The
difference in the two is their destination. Only the leading edge of CURSOR
is used to make the Z-Axis bright-up dot, where the whole of /CURSOR
LOCATE is used to brighten the Z-Axis. /CURSOR LOCATE is low when
/T C LOCATE is low, WFM READOUT EN is high, T CURSOR IN is
high, TIME DOTS EN is high, and LINESEL is low. /CURSOR LOCATE is
also low when LINESEL is high, /T C LOCATE is low, T CURSOR IN is
high, and TIME DOTS EN is high.
Freq Cal Timer. U420A outputs the FREQ CAL pulse. The duration of this pulse
is eight lines, set by U423, which counts H pulses.
Digital Delay and Counters. U233, U239, U236, and U243 are latches that input
delay information from the Microprocessor Data Bus to output counter delay