GX Developer programming software allows the user to create more than one program within a single Qn PLC. This can be useful for structuring an application and/or for controlling program flow as each program can be triggered by any of the following settings:

Wait – Scan – Initial – Low Speed – Fixed Scan

What is less obvious is what effect having multiple programs has on available PLC program memory.

Design Considerations

For every program within a Qn PLC, a space in memory is automatically allocated for online changes (making changes when the PLC is running). This is often called an overhead or online change space. The default allocation for this is 500 steps. This means that if you have 10 programs within a Q02 CPU that has 28K steps of program space, you will have only 23K steps left for the actual program.

28K – (0.5K x 10) = 23K

The default online change space can be changed but only when you are downloading the project to the PLC as follows:

From the Online menu in GX Developer select Write to PLC. Then from the Param+Prog setting in the file selection tab, select all the programs to download as shown below.

You can now go to the Program tab and set the online edit space, in number of steps.

Unfortunately the setting required is just off the edge of the window so you may need to scroll across as shown below.


The selections above are only active when connected to the chosen PLC for programming. Setting the online programming space too small will cause problems and may stop the user from being able to carry out an online change. This multiple program online change issue is not relevant to GX IEC Developer as this only creates 1 PLC program (2 programs if SFC is used).

Further Information

For further information please refer to the appropriate PLC programming manual.

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