Here I have a LVP-cable, very similar to yours ...

>>> http://home.vrweb.de/~lotharstolz/st...vpc/index.html

It does NOT work for me, and my RS-232 provides 10 volts.

Is this a problem ?
I thought LVP got "L" from "LOW" voltage, only 5 volts all around.
Am I wrong ?

The resistors here are 10 K, whilst yours are 4K7. Is this important ?
I see 2 mA shall flow in your circuit, and only 1 mA in the LOTHAR desing ...