I would like to have a communication between the CPU 1518 (TIA) and CPU 551 (TDC) to control the drive. The drive is controlled by a TDC rack involves CPU551 and CP50M0. Currently another TDC rack station is used as a master controller and there is a GDM between two TDC racks. I would like to use a CPU 1518 as a master controller instead of the TDC, remove the GDM and communicate the CPU1518 with the CPU 551 via CP51M1 (Ethernet). 

How could I communicate them each other? 

Thank you in advance.

Hello.

Look at the manual, chapter 3.5.

This should be like connection to S7-400, look at this thread and this.