Chapter 9 479
Controller Section
Display Problems (8560E)
amplitude. LCHAR is low when characters are drawn.
INTEGRATE: Q202 collector
SAMPLE: Q201 collector
LCHAR: U207 pin 9
VECTOR: U213 Pin 9
Figure 9-5 Switch Driver Waveform LCHAR
7. All of the DAC inputs should change state two or more times within
a 5 ms window. If one or more DAC bits are not working correctly,
this will effect the entire display, especially the diagonal lines that
go from lower left to upper right. When these lines are drawn, both
the X and Y DACs are stepped one count at a time. A "stuck" bit will
distort the diagonal in a repetitive manner. The quicker the
repetition, the less significant the "stuck" bit. Horizontal distortions
apply to the X LINE GENERATOR DAC, while vertical distortions
apply to the Y LINE GENERATOR DAC. The DACS have current
outputs so they are not readily observable with an oscilloscope.
Continue with step 8 to observe the DAC outputs.
8. To break the effect of feedback in the line generators and to observe
the output of the DACs, short J201 pin 13 (J201 pin 1) to TP3 (GND)
to observe U201 pin 1 and TP2 (U203 pin 1 and TP1.) Continue with
step 9.
9. Set an oscilloscope to the following settings:
Amplitude scale ............................................ 5V/div
Sweep time .................................................. 1ms/div
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