Case Studies
Deep PCI Capture of Fast CCD Array Data
A customer wants to digitize data from a 256x256 infrared CCD array. The array data is clocked at 4 MHz through two output channels. Exactly two clock cycles occur between 12 bit line signals, and a dozen or more clock cycles occur between successive frames. The customer wants to digitize the data for a minute or more. Since light level signals from the array can vary widely in amplitude, 12 bit resolution is required.
GAGE'S SOLUTION
The customer will use the CompuScope 512/PCI with the External Clock option, operating in Hardware Rearm / PCI Real Time Transfer mode, together with a 2 GB MMD-5400 PCI memory board. The CS512/PCI is a 12 bit, dual-channel card that transfers data through the high-speed PCI bus in the PC. The two output channels of the CCD array are connected to the two input channels of the CS512/PCI, and the CCD clock signal is connected to its external clock input.
Most Gage CompuScope boards may be operated in Multiple Record mode, in which multiple acquisitions are rapidly captured and stored in on-board memory. Between successive data captures, the board is hardware-rearmed for the next trigger within four data points, with no software intervention. The board cannot be rearmed between the two clock cycles that occur between successive CCD line triggers. However, the board can be rearmed during the dozen or more clock signals that occur between successive frame triggers.
In conventional capture and transfer mode, data cannot be captured while it is being transferred to PC RAM. This is unacceptable in this application, because the customer does not want to miss any CCD data. In Real Time transfer mode, however, captured data is placed directly on the PCI bus and concurrently transferred to PC RAM. With 2 channels and 2 bytes per 12 bit sample, the data rate is
2 * 2 Bytes/S * 4 MS/s = 16 MB/s
This is far below the sustained 100 MB/s rate attainable by the PCI bus in the Pentium-based GagePC.
The hardware rearm of Multiple Record and PCI Real Time Transfer are not mutually exclusive and are simultaneously invoked to meet the customer's requirement. The frame start TTL signal (and not the line start signal) is connected to the external trigger input of the CS512/PCI. Between 256x256 = 65536 sample PCI data bursts, the card is hardware-rearmed to await the next frame trigger. Since the customer knows that there are exactly 2 clock cycles between each line signal, the lines may easily be distinguished within a single frame capture. The 256x256 = 65536 sample frames are being captured through two channels at 4 MS/s by the CS512/PCI. If we ignore the small delays between lines and frames, then the frame refresh rate is
2 * 4 MS/s / (65536 S/frame) = 122 frames/s
The target of the PCI Real Time Transfer is the 2 GB MMD-5400 PCI memory board. With a 16 MB/s data rate, over 2 minutes' worth of data may be captured and transferred to the MMD board. The fastest available SCSI-based hard drive should be able to empty the MMD board in under 5 minutes.
PRODUCT RECOMMENDED
CompuScope 512/PCI -12 bit, dual-channel, 5 MS/s A/D and Scope Card for the PCI Bus
MMD-5400 -Memory Board for the PCI Bus with 2 GB of memory
This application note is provided "as is" without any warranties of any kind, either expressed or implied, including but not limited to the implied warranties of merchantability or fitness for a particular purpose. Gage Applied Technologies further does not warrant the accuracy and completeness of the material contained herein. Gage Applied Technologies may make changes to this material, or to the products described in it, at any time without notice.
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