Service manual

LUMISCAN ACR-2000 SERVICE MANUAL - SECTION 4.0 THEORY OF OPERATION
4-9
4.5.2 DATA ACQUISITION BOARD (DACQ)
The DACQ board performs all of the signal conditioning and data acquisition functions, including
calibration and output table lookup. In addition it generates the Galvo sweep control waveform and
provides the Film Motor control and drive.
The DACQ is optimized for digitizing image plates up to 14" wide with a resolution of 2048 pixels/line
(146 pixels/inch) and can accommodate increased resolutions with smaller plates (e.g. 256 pixels/inch across
a 8" image plate
Most of the digital logic on the DACQ board is contained in two large Programmable Logic Devices and a
Digital Signal Processor. These are ICs which are SRAM-based and must be downloaded with code before
they become functional. The DSP downloads or boots at power-on and each time a DACQ function is
initiated. However at power-on and after the DSP boots, it then also downloads the two PLD's with their
code. If for some reason the download fails to complete, the board will be nonfunctional.
Signal Conditioning
The analog signal path consists of a logarithmic amplifier. The log amp is of a special type that does not
depend on a semiconductor junction but rather a resistor ladder network. It has inherently high stability,
dynamic range and bandwidth and depends only on resistor values for accuracy. In addition there is a
proprietary self-adjustment circuit which is controlled by the DSP and acts to maintain the low-level signal
accuracy.
Start/end of Film Detection
The Start/End of Film (ISFILM) detector is a photodetector which is located at the cylinder slot such that it
is in the path of the laser sweep when there is no image plate present, which results in a pulse signal. This
signal is blocked while the image plate is being scanned; ISFILM is defined as the absence of this pulse.
Note that if the laser is not on or the sweep is misaligned such that it does not strike the detector then this
will be interpreted as ISFILM being true.
Acquisition Control
The acquisition process begins with the initiation of an A/D sample output of the log amplifier. Control is
primarily by the DSP device (DQDSP.DSP), in conjunction with PLDs (DQCTRL.TDF and
DQDSPIF.TDF). A pixel acquisition sequence is initiated by each PIXTRIG signal from the Data Control
Board. The function performed for each pixel depends on the Mode which is selected in the DACQ CSR.
During acquisition, the functions may include from one to three lookup table steps and a check of the data
against upper and lower limits.