The following is from the S7-200 Programmable Controller System Manual (S7-200_e.pdf); you can download it from the Siemens support website: http://support.automation.siemens.com/WW/llisapi.dll?func=cslib.csinfo&changelang=true&aktprim=0&siteid=cseus&lang=en
While you are there, look for other S7-200 FAQs, especially the S7-200 Tips and Tricks, they are very helpful for some applications.
In the S7-200 manual, chapter 9 has sample programs for the EM253 positioning module you can use in your program.