Some Technical Information

Hardware


The diffractometer is a Philips APD3520 equipped with a copper tube, theta-compensating slit, graphite monochromator, and scintillation detector with pulse height discrimination.The goniometer is driven with a stepping motor having a minimum step size of 0.005 degrees. Shutters, tube current and tube voltage are set manually on the front panel. Other functions are under microprocessor control (6 MHz, 8-bit, 8085A CPU, 2K RAM, 20K ROM).

Communications is through an asynchronous serial interface at 9600 Baud. There is a 16 character buffer.

The diifractometer microprocessor is controlled from a PC, currently a 486-33 with 16 M RAM running OS2 Warp connect. This PC runs the diffractometer while simultaneously processing the data, converting the diffractogram into a GIF format and running the Web Server.

Software


The main control program (in Basic) interprets the input parameters from the user and converts them to suitable commands recognizable by the 8085A-based diffractometer control system. These include parameters such as step size and count time. The control program also does some error checking to prevent possibly damaging commands being sent to the diffractometer. The control program retrieves the 2-theta and counts data at each step. These data are saved to a user-named file, and are plotted to the screen as a diffractogram. A binary capture of the screen is made every 60 seconds and saved on the hard drive.

A screen conversion program (in Rexx) looks at the archive bit of the screen capture file. 5 seconds after a screen update is completed the conversion program reads the file and changes it into a portable pixel map format. A second program (in "C" , freeware from the Web) converts this into a GIF format which is saved and available for transfer via any properly configured web browser. (Care was needed to avoid read/write collisions with the basic program.) Compression takes about 30 seconds to complete and explains the occasional slow-down you might observe in updating screen images. User parameters are submitted through standard HTML forms. These parameters are manipulated by Rexx programs that do password checking and prevent inappropriate commands from reaching the diffractometer. (For example, submission of a starting angle that isn't a multiple of 0.005 degrees would hang the system.)

The server is running GOSERVE 2.3, available from the IBM web site.

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Send technical Questions and Comments to the WebMaster
URL: HTTP://www.geosci.ipfw.edu/

Questions and comments about the diffractometer project to Anne Argast
ARGAST@IPFW.EDU
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