The Software Concept

SPECTRA is mostly run from menus, the command line or command files. But it is also available as a program library. User code may invoke operations in a way which is very similar to the command line interface. The function gra_command() receives a string which is build according to the command line syntax. It executes the command and returns the completion status. In addition there are query functions which return information from SPECTRA. On the other hand SPECTRA may invoke user code which has been written in a common language like C. This is accomplished by a standard interface to the user-functions.

If the on-line control program runs on a separate host, SPECTRA may be started in a server mode. In this mode SPECTRA accepts commands, executes them and replies with 'DONE', 'ERROR' or a value which has been queried. Any other output is suppressed.

These are the important features of SPECTRA:

• Monochromator control: The adjustment of the setup, including mirrors, is supported by full screen menus. The items that can be accessed through these menus depend on the specific beamline. There is a clear separation between the beamline specific part of the software and those features which are the same for all supported beamlines, like energy scans.
• Interface to the device libraries HCLIB[2]/HVLIB: A device, like a counter, is introduced to SPECTRA by supplying the module type and other information which is needed to access it in a CAMAC or VME crate. A counter has to be cleared, gated and read-out no matter what module type it is or whether it is a CAMAC or VME module. The details are known to the drivers in the device library but they are hidden from the user. All functions of the device libraries are available from the command line of SPECTRA.
• Stepper motor control: Stepping motors can be controlled with the help of a full screen menu. The user may change the motors parameters or perform movements.
• Device menus: There are beamline components which consist of several motors, like absorbers or monochromators. SPECTRA provides menus that allow the user to access a set of motors on one screen.
• Interface to the GPIB bus: SPECTRA has functions that allow the user to exchange data with devices that are connected though the GPIB bus.
• Scan menus: Any scan could be performed by executing a SPECTRA command file. However it is more convenient to use menus to supply the scan parameters, like the scan range, the step width, sample time, etc. These parameters can be entered manually or by qualifiers from the command line.

  Figure 1 gives an example. It shows the on-line display of an EXAFS scan. Four counters have been enabled. Their values are displayed in the upper half of the screen. The absorption of the sample and a reference signal is displayed in the lower part.

• Diffractometer software: Software has been produced that controls four-circle diffractometers.

  The components are:

  • Adjustment of the diffractometer: A full screen menu displays the relevant parameters and allows the user to move single motors. After a successful adjustment it is possible to move the instrument by changing the energy or wavelength and the four diffractometer angles.
  • Crystal orientation: There is a menu that allows the centering of a reflection.
  • Movements in Q-space: When the adjustment and orientation phase is finished, the instrument can be moved in terms of Qx, Qy and Qz. Q-scans are also supported.

  
Figure 1: The on-line display of EXAFS scans