Hi Garry,
I have tried with mainly PL2303 based USB to serial converters.
Tried with this as I was using it with my headless PI but when I tried to connect to the PLC, there is no reply. I then connected this to a desktop PC and I ended up getting data only going from the desktop to the notebook but not the other way around.
http://www.dx.com/p/waveshare-pl2303-usb-uart-board-type-a-for-raspberry-pi-blue-282359#.Wbk9RtOg-EI
Rather than troubleshooting the convertor, I bought this other convertor yesterday night and did the desktop test first and data went both ways. Thinking that I will finally succeed, I went ahead to connect to the PLC but then encountered the echo back when connecting with only the serial cable. When connecting both the usb and serial cable, there is no response from the serial side.
https://biskandar.wordpress.com/2014/11/24/accessing-intel-galileo-board-via-3-5mm-serial-cable/
Using both convertors, I am not able to not able to connect to the PLC using direct online using COM port
Now I am at a loss as to whether it is still a cable issue or something else.
Best Regards,
Linus