Pictures of different adapters, trying to understand how they work (PLC to PC communication)
3 possible issues to always be aware of when using RS232 to USB converters. (in order of most common to least.)


Converter driver comm port number chosen matches comm number in PLC
communication software. 
Power from USB port enough to drive all
devices connect. 
Communication watchdog timer when converting from
multiple protocols.


1st example: see http://bin95.com/Industrial-Training-Videos/AB-PLC-DH485-RS232-USB.htm
2nd and 3rd example: if you connect USB to computer, RS232 to RS232 to RJ485 converter box (pic box), the computer's USB port will not have enough power to power circuits in RS232 to RJ485 converter box. And/or communication during multiple conversion gets slowed down too much giving comm error on PLC side.

Solution: Get a converter cable specifically designed to convert USB to RJ485 instead of connecting two different converters together.