This means that when you upload in GX IEC, there is no symbolic information to read back so it will uploac the program in instruction list. So when someone comes along and modified the program using a different editor (medoc or GX dev), the symbolic information will be destroyed (and there is no going back!). For A&FX this memory area is straight after the program code finishes. There are dangers, the most significant one is that the symbolic information is stored in a seperate area to the program code. Thus both the list and symbolic will be transferred to the PLC, then, you will be able to come along with a blank laptop and upload the symbolic information and you will have your "IEC" graphical code uploladed. When the compiler does its job and converts your symbolic information into List and transfers to the PLC, there is the option to also download the symbolic information (on the download project options). Try writing a small program in IEC and then read it back in GX and you'll see what I mean. The PLC itself does not understand, for example, what structured text is! GXIEC contains a "compiler" which translates all your nice symbolic graphical work (which you understand) into raw instruction list (which the PLC understands). At the end of the day, all that is written to the PLC is melsec instruction list. What is written to the PLC is not what you see on the screen. So say for example your program consists of 5 pou's each written in one of the editors, Lad, FBD, ST etc. In GXIEC you can create your programs in any or all of the 5 IEC language editors. In GX, what you write in ladder or list gets sent to the plc - straight forward. There are significant differences between GX IEC Developer and GX Developer, but I'll try to explain the most significant difference which is the way that it handles the code that you write.
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