Tektronix 2445, Instruction Manual

Page: 55 / 351

Theory of Operation—2445 Service
Like the switch matrix scanning, the Front-Panel pot
scanning routine is performed every 32 ms. When entered,
the routine reads the settings of the last-moved pot and one
unmoved pot. Each succeeding scan continues to read the
last-moved pot, but a new unmoved pot is read. In this way,
each pot is monitored, but most of the scan time is devoted
to the pot that is most likely to still be moving (needing
continuous updating).
As the initial pot settings are determined, a digital
representation o f each value is stored in memory. The pro­
cessor then checks each pot against its last-known value to
determine if a pot has moved. If a pot is detected as moving,
the processor executes a routine that converts the move­
ment (displacement from last-set value) into a corre­
sponding control voltage.
When producing the actual analog control levels, the
processor can manipulate the digital values read fo r the
various pots before sending the output data to the DAC.
This allows many o f the oscilloscope parameters to vary in
an enhanced fashion. The pot data is manipulated by the
processor in a manner that produces such features as
variable resolution, continuous rotation, fine-resolution
backlash, and electrically detented controls.
In the cases o f the TRIGGER LEVEL, Horizontal POSI­
TION, VO LTS/DIV VAR, and SEC/DIV VAR controls, the
processor reads the magnitude and direction o f pot rotation
and produces variable-resolution control voltages. If a pot's
direction o f rotation changes, the magnitude o f the change
from the last-set position remains small, or if it was not the
FROM DATA
LATCHES U2034
AND U2134
*N 0 T E !N 0 T ALL MATRIX PO SITIO NS ARE USED
3831-04
3-13
Figure 3-4. Front-Panel Switch matrix.