Global Data Transfer Between Two PLCs

Hello engineer01;

First, a note for Hati and all the "young" guys here: Global data communications was a simple data exchange structure between S7-300 (and S7-400) CPUs, from the early versions of thid family of PLC. It used MPI as an exchange base, and was "programmed" as a table in Step 7. It was very limited ( I think you could only setup up to 76 bytes of data transmission, and of course it was limited to the nerwork speed of MPI (187,5 kB). 

The problem for the OP is that the GD (global data) table was saved as part of the Step 7 program and downloaded to the CPU as a SDB (system data block), which are not accessible to the user. So, unless you have the original program on your PC, you cannot open and access the GD table.

https://support.industry.siemens.com/cs/ca/en/view/45531107

See this extract from the Step 7 programming manual, as an attachment, showing the elements of a S7-300/400 PLC program that cannot be uploaded. There is also an explanation on Global Data Communication in the following document, look it up if there is archeological interest in the subject.

https://support.industry.siemens.com/cs/ca/en/view/430498

You will have to reprogram your data exchange between the S7-300 CPUs, the easiest way to do this would be to use PUT/GET functions FB 14 and FB 15, the same manual should be able to help you.

Hope this helps,

Daniel Chartier