Service manual

LUMISCAN ACR-2000 SERVICE MANUAL - SECTION 4.0 THEORY OF OPERATION
4-11
X Only and XY Pixel Averaging Modes
The ACR-2000 READER Data Acquisition board is capable of averaging pixel values together, either in X
only or in both X and Y. The mode bits are selected in the DACQ CSR. In X-only mode one data value is
produced for each two pixel clocks (one DATAVAIL per two PIXTRIGs). The data value is the average
of two samples. Thus the amount of data is reduced by two, and the film speed should double in order to
maintain a 1:1 pixel aspect ratio.
In XY averaging mode two lines are averaged in addition to the pixel averaging described above. Thus
each data value is the average of four adjacent pixels (2x2) and the amount of data is reduced by four. In
this case the film speed is the same as for no averaging.
Galvo Control
Control of the galvo in the ACR-2000 READER is different from control in the LS100/200 series in that
the basic sweep and retrace times are not programmable. These parameters are fixed in firmware; the PLD
must be changed in order to change these parameters. The present settings are 480 kHz up clock (8.53ms
up ramp) and step return to start (no down ramp).
The SCANCLK signal from the Data Control Board no longer controls the sweep speed. The sweep speed
is now fixed within the generation circuit (DQFMGV.GDF) and can be changed only by changing the
firmware. The SCANCLK signal now controls the repetition, or line, rate. This is fixed for each model and
should never be changed.
For the ACR-2000 READER the Scanclk frequency is 10,000,000/25000 = 400Hz. This signal is always
sent by the DCB; active galvo scanning is enabled by the Galvo Enable bit in the DACQ CSR.
This is divided by 8 in DQFMGV.GDF, which generates the SOS signal. Thus the scan synchronization
begins here. The SOS signal initiates a galvo sweep and also is sent back to the DCB to begin the line
acquisition sequence.
The Galvo board is now a more standard servo control amplifier and the interface is simpler. The sweep is
linearized with a lookup table (GLVLUT.DAT) which converts the linear stairstep output from the
generation circuit into an S-shaped waveform. The amplitude and DC offset of the output signal can be
varied by two on-board potentiometers.
Galvo Rest Positions
If the SCANCLK signal is present and the GLVENB bit is set in the DACQ CSR, then the galvo sweep
signal will be generated, causing the galvo to sweep. If the GIVENB bit is cleared the galvo will stop
sweeping and will take a static position according the settings of the "PARK" switched and the "CTR"
jumper. If the CTR jumper is installed the position will be at count = 2048, which is the electrical midpoint
of the sweep. This is irrespective of the setting of the PARK switches.