One question, what kind and how many lines of RS-232 to TTL converter are you using in your 8051!
I find that some converter (IT, ICL) with buildin condenser supply, haven't work properly.
Specialy in situation when two transmiters works with different levels.

Typical transciver in PC work with internal supply +12 and -12 V and have bigger current then IC with internal condeser supply. Try that your cable is correct and there isn't any short circuit or big capacity.