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PS reprint
John Moran, Steven Keleti, Paul F. Hsieh
Smithsonian Astrophysical Observatory, Cambridge, MA 02138
The High Resolution Mirror Assembly (HRMA) X-ray Detection System (HXDS) consists of motors, apertures, and detectors to be used in the calibration of flight mirrors for the Advanced X-ray Astrophysics Facility (AXAF). One of the central design problems has been to meet a 10 millisecond time-base synchronization requirement for detector control. In this paper, we discuss our solution.
The X-ray Calibration Facility (XRCF) at Marshall Space Flight Center (MSFC) is a 500-meter x-ray beamline. The HXDS has nine detectors in three groups: Normalization detectors at both ends of the beamline and calibration detectors in the HRMA focal plane. There are also three types of detectors: flow proportional counters (FPC), solid state detectors (SSD), and a custom-built microchannel plate detector called the High Speed Imager (HSI). The FPC's and the SSD's are controlled by EG&G Ortec electronics chains while the HSI has custom-built electronics which communicate via a combination of RS-232 serial and DR11-W parallel interface standards.
Figure 1: Schematic of the XRCF. The focal plane detectors consist of
1 flow proportional counter (FPC), 1 solid state detector (SSD),
and 1 microchannel plate detector (the HSI). The x-ray source
normalization detectors consist of 1 FPC and 1 SSD. The mirror
normalization detectors consist of 4 FPC's.
Figure 1: 6 Kb
In our design of the the HXDS detector control, we have chosen to partition the design into a high-level coordination layer and a low-level real-time control layer.
The high-level coordination software runs on Sun workstations with Solaris 2.4 for the operating system. The UNIX platform was chosen for the following reasons:
The low-level real-time coordination software runs on PC compatible computers with MS-DOS 6.22 and Sun PC/NFS as the operating system. The PC environment was chosen for the following reasons:
For our time base, we use an IRIG-B signal that is provided by the XRCF. IRIB-B provides a clock that is time-synchronous to within 1 millisecond across the facility.
Figure 2: Data acquisiton control algorithm.
Figure 2: PS 6 Kb
Figure 2 shows the algorithm used to control data acquisition. This algorithm was initially prototyped using ksh and finally implemented in C using standard TCP/IP socket calls. We chose to use C because of its superior signal handling and process control capabilities.
The start time is initially chosen to be two seconds in the future. If any of the detectors fail to start at the designated time, all acquisition processes are terminated and more time is allocated and a new start time selected. After repeated failures, the data acquisition will report a failure to the operator.
Figure 3: Histogram of and fit to relative synchronization between
two detectors connected to different PC's as measured with
an oscilloscope.
Figure 3: PS 14 Kb
Measurements with an oscilloscope show that both the spectral detector
electronics and the HSI electronics have a latency of
milliseconds. The low-level control layer therefore
issues a start command 7 milliseconds before the designated time.
Further measurements with different configurations of detectors and PC's were made. The result from one such measurement, where one PC was controlling a spectral detector and one PC was controlling a spectral detector and the HSI, is shown in Figure 3. This particular set of measurements has the most extreme outliers. From these measurements we see that our 10 ms requirement has been met.
This work was supported by NASA Contract NAS8-40224