The logic level conversion RS232 too UART TTL is a complete knightmare..
RS232 SEND ... logic 0 =
High voltage level + and becomes
Low voltage level ( 0.0 - 0.8v ) = TTL logic 0
and the reverse is true on the RECEIVE ... So logic levels are reversed.
I'm now of the opinion that the comms is just RS232 serial logic based....both the puter and controller talking RS232 level logic.
It would seem that the USB comms order should be RS232
Serial logic level .....which is converted by PL2303 to TTL logic level ... this then needs inverting and combining into one wire comms ...
Serial port of Controller.
This inversion is borne out by the CURTIS Controller KPG lead diagram that I found ....
In which the KPG lead colours and signal inversion chip connections were correct ..but the LT1081( RS232 Dual Driver/Receiver) chip weren't.... (TX and RX were in fact erroneusly marked)...I had assumed they were CORRECT and this has caused me no end of grief!
I suspect that I got lucky with my KPG lead and some programming / extra components took care of the MISS MATCH IN LOGIC LEVELS.
Once I get your cable I can play with the above premise that I need to invert the TTL logic and combine the resulting RX/TX.
http://www.best-microcontroller-project ... works.html