The Crystal Experiment
Our CAMAC controller consisted of a board which, when inserted on the ISA bus of a computer, could connect to as many as seven different CAMAC crates, each containing up to 22 different specialized devices connected to measuring instruments.
This board was mapped on a known set of ISA bus memory addresses through which a user could send commands to each individual instrument and retrieve the responses. UNIX permits access to physical memory addresses only at the kernel level: it was clear that we needed a set of software routines dedicated to the interaction with the DAQ board, usually called a device driver, to insert into the Linux kernel.
Though aware of the existence of kernel-level device drivers, none of us knew exactly how to write one. (All this happened a few months before the appearance of the good article by Alessandro Rubini. See Resources .) We then decided to ask for help on the comp.os.linux.hardware newsgroup, and less than 24 hours later we were contacted by Ole Streicher, a German researcher who sent us the source code of a device driver he wrote for a different CAMAC controller (see Resources ). Adapting it to our board was a matter of a couple of days, and then experimental data were happily flowing in and out of our DAQ system: the Linux option was finally open.
The ability to dynamically load and unload modules in the kernel, a feature which had been introduced in Linux only a few months before, was of great help in the driver development phase.
From the user's point of view, the CAMAC system was now visible via simple files, one for each different crate, which could be opened, closed, written to and read from. We also provided an interface library in order to hide the low-level details of the CAMAC operations and facilitate code writing. The presence of both the gcc C compiler and the f2c FORTRAN-to-C converter allowed us to provide both a C and a FORTRAN version of this interface library, in order to allow our colleagues to write their own DAQ programs.
Using this library we wrote the main DAQ program which was able to automatically set the run conditions, control the movement of the radioactive source via a serial link to a step motor, send light pulses to calibrate the light sensors, and collect the data coming from the DAQ system and analyze them on the fly. To write the user interface, we used the Tcl/Tk package (see Resources ): all the program controls appeared on a graphical window which could be opened on any X display (Figure 4).
Figure 4. This is a snapshot of our PC screen during an actual DAQ run. In the center you can see the Tcl/Tk-based control interface while the window on the left shows the data collected during the run. The histogram, updated in real-time during the run, is created using the HBOOK and HPLOT packages of the CERN libraries.
Parallel to the DAQ program, we wrote a program to monitor the status of the data acquisition and of some important parameters such as the number of events collected, event rate and average values. With a scientific libraries package called CERNLIB (see Resources ) developed at CERN, freely available along with its source code and widely used in the high-energy physics community, we interfaced the monitor program to a simple analysis facility. This allowed us to access interesting information and execute some preliminary analysis even while the DAQ was being done (Figure 4).
An important factor for a DAQ system is the time performance. If the controlling software is too slow, data may be lost and the time required to collect a useful amount of data can grow to an unacceptable level.
We found the only time-limiting factor in our system was the conversion time of the ADC board; the operating system could easily keep pace with the DAQ task, even while running several other user tasks. This is very important, as this year our bench will move from a prototype level with a single active DAQ chain to an industrial-strength production facility where multiple measurements will proceed in parallel in order to quickly handle all of the many thousands of crystals needed for the CMS experiment.
In practice, we measured the time to execute a single CAMAC operation to be on the order of 10 microseconds, large with respect to the 1.5 microseconds minimum CAMAC operation time, but very good for an inexpensive board such as the CAEN A151 and much lower than the ADC response time of 110 microseconds.
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