Valera,
you could try to use this converter for S7-200, but I have strong doubts about it's useability for S7-300.
The PC/PPI converter from Siemens, used for programming the S7-200, is indeed a fairly simple, data-driven RS232 to RS485 converter. The highest speed used on this converter is, if I'm not mistaken, 19200 or 38400 baud, while the default speed for PPI is 9600 baud. The speed is always the same on both sides of the converter.
The PC/MPI converter used to program the S7-300 is something completely different. It's not only a RS232/RS485 converter, but it also handles some protocol conversion and speed buffering. This is because the speed on the MPI network is defaulted to 187.5 kbaud, which is an unusual setting for RS232. The older converters (such as the ones I still use) are limited to 19.2 kbaud at the RS232 side, while the newer ones go up to 38.4 kbaud (or even over 1Mbaud if you use the USB to MPI converter).
Should you find a way to use your converter to program an S7-300, I'll be glad to hear from you, since I get this kind of question from trainees from time to time.
Kind regards,