When developing a low cost computed tomography (CT) imaging system using a PC compatible as a computer platform, it
was necessary to create a data acquisition board for the PC with high enough precision (at least 12 bits per pixel) for CT
imaging. Since the CT system uses in its standard configuration an image intensifier/camera detector with a rasterized video
source (such as a TV camera), the data acquisition system (DAS) must be able to capture any part or all of a video frame. We
created a DAS that is capable of 12-bit digitization at a speed of 10 million samples per second. Built on a single PC AT
board, this DAS has all the functions necessary for video-speed single slice or volume CT data collection.
The analog-to-digital conversion device (ADC) used is the Comlinea.r CLC925AI, which has 12-bit output. Since the DAS
can also accept 16-bit digital input from an external ADC, the 12-bit samples are stored as 16-bit values. The DAS has two
1024 x 1024 x 16-bit image buffers and two direct memory access (DMA) paths, one to the AT bus and one to BIR's
proprietary 64-bit data bus. The 64-bit bus connects the DAS, an image processing board, and a 32 MB memory board to
provide the data transfer speed necessary to keep up with data acquisition and processing. It operates at a DMA transfer speed
of 1 1.1 to 13.3 MHz. For transfers between the DAS buffer memory and the data bus, the DAS memory is a read-only data
source; data cannot be written into the DAS buffer from the data bus.