Research in structural biology conducted at synchrotron sources using large area electronic detectors represents a significant challenge for beamline control and data acquisition systems. An intimidating quantity of data and rapid data rates must be successfully accomodated in order to achieve data acquisition in near-realtime. These control and data acquisition systems are also interfaced with data analysis applications, which further complicates the task.
An EPICS-based (Experimental Physics and Industrial Control) data acquisition system developed for the Structural Biology Center Collaborative Access Team (CAT), Advanced Photon Source (APS) Sector 19, will be presented. The SBC-CAT data acquisition system has been designed for speed, using both specialized hardware and the EPICS distributed architecture to conduct data acquisition tasks in parallel. Each 18 MB image can be acquired and written to disk in less than 3 seconds. This data acquisition system has also been interfaced to d*TREK, a single-crystal macromolecular x-ray diffraction analysis package, written by Jim Pflugrath, Molecular Structure Corporation. This represents an integrated approach to data acquisition and analysis. Currently, data analysis is the rate-limit for the SBC-CAT.
Another factor influencing the need for a high performance data acquisition system is the use of the APS 1 Detector at the APS. Synchrotron experimentation generally result in short exposure times. For example, at the APS beamline 19-ID, the total flux onto a sample at 100mA and 12KeV when the beam is fully-focused (38 microns vertical and 83 microns horizontal) is 10õº Xph/s.
Finally, the cost of operating APS and each sector is high. DOE and other funding agencies have made big investments, experimentation should be productive as possible.
Other issues that will be discussed are interfacing of the APS 1 Detector data acquisition system to d*TREK and the use of data compression.
ECT developers have also implemented custom EPICS device support for the APS 1 detector, see the EPICS MEDM (Motif Editor and Display Manager) Screens shown in Figure 4. Software modules have been developed to set up and monitor individual CCD gains, offsets, and temperatures via an RS-422 Interface. Software modules have also been provided to set up and monitor detector enclosure temperature and humidity and individual CCD Thermo-electric Cooler Controller currents and detector power supply voltage analog signals via a XYCOM Analog-to-Digital VME Module. Software modules used to monitor and control approximately 200 APS 1 detector parameters (such as readout mode and detector state via an RS-422 Interface) have been implemented. This software serves to aid detector engineers with routine detector setup, monitoring, and control tasks. This software has been implemented via EPICS device support and EPICS databases, which are downloaded to the APS 1 Detector Setup & Monitor IOC.
When the APS 1 detector is read out, the data are passed serially in a fixed order from 18 pixels at a time (each of the nine CCDs has two readout amplifiers) and transferred to a VME Multiplexer module in the Data Acquisition IOC. The 18 pixels are scattered across the detector face. The Multiplexer module rearranges the data into 16-bit parallel words, which represent the signal measured by each pixel where the signal is proportional to the number of x-rays detected, and passes the 16-bit words to a VME Descrambler module. The VME Descrambler reorders the pixels assembling a sequential image, sequential in x- and y-positions, in VME memory.
While the data is transferred from VME memory to SGI Challenge memory and written to disk, the next image is taken and transferred to a second descrambler and memory module. By "ping-pong"ing between memory modules, the data may be transferred to disk without additional overhead. We'll look at the parallel nature of the data acquisition system later in the talk.
The data acquisition system design centers around a beamline file and compute server, the SGI Challenge. SGI Challenge has the following high-bandwidth features (see Figure 5):
A 32-bit file system limits file size to 2 GB and file system size to 8 GB. With a 32-bit file system, the largest APS 1 Detector dataset that could be contained in a single file system, subsection of the file tree, would be 444 images. Based upon protein crystal symmetry, the experiment can require up to 1000 images/dataset.
Disk I/O is handled by striping the data across 3 SCSI-2 FWD (20 MB/s/controller ) connected RAID Arrays seen as 1 large logical volume using XLV Logical Volume Manager. With this I/O configuration, an 18 MB image can be written to disk in < 1.0 s. Other features to note, RAID arrays have hot swappable disks and power supplies.
The IRIX operating system comes with a set of realtime programming features which are also an integral part of the APS 1 Detector data acquisition system. The data acquisition system makes use of the following IRIX REACT Extensions, a realtime programming library, capabilities:
In this data acquisition system, a HIPPI network is used as a dedicated data acquisition pathway. Image data is transferred from VME memory to SGI Challenge memory, via the HIPPI protocol. On the Challenge, the HIPPI module and driver are provided by SGI. On the data acquisition IOC (input-output controller), a commercial HIPPI-VME module is used. A custom HIPPI vxWorks driver was implemented by ECT control system developers to support this interface. A raw-character-mode protocol, rather than a TCP/IP protocol, was used to eliminate the software overhead needed to implement the TCP/IP protocol. A HIPPI Server process running on the SGI Challenge, accepts image data and header information and writes the image with header to disk asynchronously(see Section 2.3.5).
To achieve this, the Delta Tau Systems PMAC Programmable Multi-Axis Controller, PMAC-VME, motor controller is used to control the position and speed of the crystal orientation axis, also called the goniometer Omega axis. The PMAC-VME programmable feature is also used to calculate and maintain a constant speed of rotation during image acquisition.
ECT developers use the PMAC-VME programmable feature to define the start and end positions for an image using real-time angular readback data from a shaft encoder attached to the Omega axis motor and to output control pulses at each position. These pulses are used to precisely control an x-ray shutter which defines the sample x-ray exposure interval time, the gating of the APS 1 Detector, and any detector used to measure the actual dose of x-rays to the sample (i.e., a beam intensity monitor).
To assure that the gating of all devices is done accurately and with minimal latencies, which rules out software control, a hardware solution was devised. A Data Acquisition Sync Module was designed with an on-board general-purpose VME interface. The DAQ Sync Module has the ability to delay the gating of any device by a predefined time; in this way the delay in opening of the x-ray shutter can be accommodated.
Testing of the fast shutter electronics and software subsytem was conducted in a laboratory setup using the shutter filter module, timing scaler, goniometer sync module, and a sample shutter attenuator unit. Additional tests also involved a motor controller and a goniostat rotary table. A laser diode was directed through the shutter's opening at a photodiode, and a scope was used to monitor the timing signals driving the shutter blades and the output of the photodiode. A 40 millisecond mechanical shutter blade delay opening and closing and a mechanical shutter jitter of approximately 1 millisecond.
The sequence of events during data acquisition for a single image is as follows:
First some background, data acquisition involves a variety of programming techniques (C++ and C programs, EPICS sequence programs, PMAC Motion Control Programs) running on multiple CPUs. The CPUs involved are:
We have discussed the SGI Challenge in Section 2.3.2, the HIPPI Server in Section 2.3.3 and the PMAC-VME in Section 2.3.4.
The Experiment IOC is the data acquisition "master" controlling the sequencing of data acquisition using a sequence program. EPICS provides a state notation compiler that is a C preprocessor which converts state notation language source code into C code. ECT developers wrote the program using state notation language as a sequence of related states (typically, hardware states) within a database(s). State notation language also provides easy access to EPICS database information.
Interprocess communication occurs as both hardware and software signals:
|Parallel Path #1 Tasks||Measured Time (s)|
|Parallel Path #2 Task||Measured Time (s)|
In this overlapping of data acquisition tasks, the header formatting becomes the rate limit. We now use C++ classes from d*TREK which use a lot of dynamic string manipulation. We feel that we can significantly reduce this time in order to bring the time intervals closer.
|Parallel Path #3 Task||Measured Time (s)|
|Parallel Path #4 Tasks||Measured Time (s)|
|Header Information Current||0.3|
|Header Information Next||1.2|
|Parallel Path #5 Task||Measured Time (s)|
The motor speed is currently the rate limiting factor, requiring (>2.5 s) to perform these motions. A higher-speed torque motor is planned. This motor will perform to allow the motions to be completed in under 1.8 s.
For short exposure "still" images, where the motor is not the rate limit, the APS 1 Detector data acquisition system presently collects and writes to disk in 3+ seconds. If header manipulations can be tuned, this performance will be directly affected. For images with motor motion and short exposure times, the image to disk time is 5+ seconds. The current goniometer Omega motor cannot perform within the 3+ second time. Rewind, backlash correction, and ramp of omega motor to speed for next image, which involves 3 accelerations and 3 decelerations. Because of this hardware limitation, actual rotation image to disk time is 5+ s.
While data acquisition proceeds, they may monitor data acquisition status and a variety of sensors. In this case, the PIN diode detector (ua) and APS ring current (ma). The dtcollect GUI allows the user to collect a single data scan, series of images collected in sequential angular position.
Multiple scans may be requested by the user with the dtcollect Scan screen (see Figure 9), brought up from the dtcollect Collect menu. Here the user has the same input with the addition of specifying a series of scans. Also, interesting to note, the scans may be read in from an image header file, reducing the user input required, and thereby automating the task. The user uses the "Scan" button to proceed and may "Abort" or "Pause"/"Resume" the scan at any time.
The advantage of the MIFF format is that the data format can be easily determined and therefore, an image read routine used to process these images with other data analysis programs may be easily implemented. Also, the UNIX "more" command may be used to view image headers and avoid viewing binary data.
Once the crystallography community adopts a standard image format, like the Crystallographic Binary File (CBF) format for image data, APS 1 Detector images will be formatted in that standard format.
The image header format consists of an ASCII string that has a length that is a multiple of 512 characters. The string consists of human readable i text of the form KEYWORD=VALUE; where, KEYWORD is a case-sensitive string of up to 32 characters. VALUE is the value of the KEYWORD, which may be a number (integer or decimal), a string, or an array of numbers.
APS 1 Detector image header information serves the following functions:
Jim Pflugrath will present the d*TREK data analysis GUI, dtprocess. dtprocess obtains initial parameter values for the image headers. d*TREK was written in modular form and data is analyzed as a series of processes. Basically, all of the d*TREK data analysis interprocess communication is handled by the reading and writing of image header files.
They serve as an electronic log of experiment (ie, an electronic notebook). The experiment may be reconstructed from the 200+ static and real-time parameters stored in the image header.
This API consists of 6 c++ classes (coded by ECT control system developers) which interface to the EPICS-based control system.
Together these device classes form the d*TREK DTDEV Device Library. The DTDEV Device Library contains the site-specific, hardware- dependent code that interfaces d*TREK dtcollect to the beamline and experiment, allowing d*TREK to remain device-independent.
Methods were provided to control and monitor beamline and end-station hardware used in an experiment, such as APS 1 detector, goniometer, ion chamber, PIN diode detector, double crystal monochromator, and fast shutter. These methods use the EPICS Common DEVice (cdev) C++ class library. Cdev provides a standard interface between the class methods and the EPICS-based Control System.
SBC-CAT acquisition of processible x-ray diffraction images demonstrates the successful integration of d*TREK and the APS 1 Detector data acquisition system. This API has also been successfully applied to a Windows NT version of the software by MSC for use with a commercial CCD detector system.
|Image Data||lys023s1a (raw)||lys011s3a (raw)|
|dclz Algorithm Compression Ratio||1.38||1.32|
|Image Data||lys023s1a (raw)||lys011s3a (raw)|
|Image size (MB)||18.88||18.88|
|Unix "compress" Time (s)||22||22|
|Unix "compress" Compression Ratio||1.51||1.42|
|GNU "gzip" Time (s)||83||72|
|GNU "gzip" Compression Ratio||1.63||1.58|
|Unix "pack" Time (s)||14||14|
|Unix "pack" Compression Ratio||1.51||1.43|
The factors contributing to the decision not to use data compression in the initial implementation are:
|Image Data||lys011s3a (raw)|
|Image size (MB)||18.88|
|Byte-Offset Compress Time (s)||2.5|
|Byte-Offset Compression Ratio||1.94|
For our system, the byte-offset compression algorithm done in software and conducted in parallel with other data acquisition tasks could be used without degrading image acquisition. We could conduct byte-offset software compression of the current image in parallel with the writing of the previous image and reading of the next image (see Table 6).
|Readout Next Image Tasks||Measured Time (s)|
|Byte-Offset Compress Curent Image Task||Measured Time (s)|
|Data Compression||<2.0 (without disk I/O)|
|Disk Write Previous Image Task||Measured Time (s)|
Futhermore, the data acquisition system design does not prohibit installation and use of data compression hardware. If the byte-offset compression algorithm could be implemented in hardware on a VME module, it could be used as an integral part of the VME data handling pipeline (see Figure 3).
In the process of 20 months of validation, the first crystal structure has been solved and published during this past year. ECT was involved in the "Human fhit" experiment, which used the technique of MAD Phasing. fhit is important to our understanding of cancer in biological systems and in the development of biotechnology.