Repair manual

2-55
THEORY OF OPERATION
c. Filter Wheel Position Control
The Filter wheel Position Control circuit (Figure 2-9, sheet 5), in
conjunction with the microprocessor network, positions the
colored filter specified by the host computer Expose Color
command between the monitor display screen and the selected
camera.
Before an exposure takes place, the Expose Color command is sent
by the host computer via the applicable parallel interface to the
Digital Palette buffer memory. Upon receipt of this command, the
microprocessor network begins to execute the exposure sequence
specified by the Expose Color command. This network decodes
the specified filter position indicated by the Expose Color
command and then signals the stepper motor to rotate its filter
wheel assembly to the specified colored filter position.
As the stepper motor rotates its filter wheel assembly to the
specified colored filter position, a notch adjacent to each filter
position on the filter wheel assembly interrupts the photo
interrupter (position sensor). The photo interrupter contains a
LED that emits infrared light and a photo-transistor that turns on
when it sees the light emitted from the LED.
The photo interrupter generates a filter wheel sense (FWSENSE)
output pulse that is applied via connector J8 to a dual 4-line to
1-line data selector/multiplexer network. Upon command, this
network converts the FWSENSE pulses generated by the photo-
interrupter into a 2-bit filter wheel sense signal. It applies this
outputted 2-bit filter wheel sense signal onto the Address/Data
communications bus.
Specified data select signals from the microprocessor network
inputs the 2-bit filter wheel sense signal via the Address/Data
communications bus to the microprocessor network indicating that
the filter wheel assembly reached its selected colored filter
position. The microprocessor network determines when the
specified colored filter is properly positioned by decoding the 2-bit
filter wheel sense signal.
If it is determined that the filter wheel is positioned correctly
(specified colored filter between monitor display screen and the
selected camera), the microprocessor network stops the stepper
motor leaving the specified colored filter in front of the monitor
display screen.