You will need a 9 - 25 pin serial adapter form any supplier (like Radio Shack) and a Siemens part 6es5-734-1bd20 seial (25pin female) to current loop cable. It comes with the 15 pin male, needed for their PLC's.
You can use a B&B 232PCLR and make you own cables, it is a lot more work and requires an external power supply. I do have a drawing of that.