QUESTION:
How can you link an S7-300 module as DP slave to a DP/PA-Link in the hardware configuration of STEP 7? 

ANSWER:  
The normal way of linking the above-mentioned modules from the PROFIBUS-DP catalog of the hardware configuration of STEP 7 to the DP master segment of the DP/PA-Link is not possible. Linking the S7-300 as slave to a DP/PA-Link is only possible via the GSD files. When handling CPU-I slaves via GSD file you must make sure as user that the I/O areas match each other.   First download the GSD files required for the above-mentioned modules onto your STEP 7 system.  If these GSD files are already incorporated in your hardware configuration, you can skip this step. The GSD files are available in the Internet in the Entry ID: 113652 "PROFIBUS GSD files : SIMATIC".   Via the menu item "Options > Install New GSD…" you can incorporate the GSD files in your hardware configuration. Please close your S7 station before doing this.

Fig. 1: Installation of the new GSD file of the catalog   

Open your hardware configuration again and configure the module IM157 (e.g. the DP/PA-Link with MLFB: 6ES7157-0AA81-0XA0). From the PROFIBUS DP catalog/subdirectory "DP/PA link" you can link this module directly to the segment of the redundant master system using drag-and-drop.

Fig. 2: Selecting the DP/PA link in the HW Config   

Specify a PROFIBUS address for the DP/PA-Link and select "Interface Module for PROFIBUS-DP" (see Fig. 3).

Fig. 3: Selecting the interface module in the HW Config   

You will then find the S7-300 modules in the PROFIBUS DP catalog under "Additional Field Devices" (see Fig. 4). 

Fig. 4: Selecting the S7-300 module   

Now use drag-and-drop to copy these modules to the master system. Make sure here that you select the right bus system (in Fig. 5: PROFIBUS (5)) and a PROFIBUS address.  If you use the GSD files available at the time, then the CPU 315-2 DP is incorporated as DPV0 slave.

Fig. 5: Configuration in the HW Configuration   

Now you still have to create the HW configuration for the CPU being operated as DP slave. In the case of a standard slave, you usually only have to set the DP address and the baud rate.   

Create an S7-300 station according to your HW configuration. 
Connect the DP interface to a bus. (You now have to select a bus other than the one on your DP/DP link, because you have already assigned the address for the DP slave here). 
Set the DP interface of the CPU to slave mode (double-click DP Interface > Operating Mode tab). Please make sure that the PG functions are not activated.

Fig. 6: Settings as DP slave   

Set the bus parameters for the bus. Use the same bus parameters as you have set on the secondary side of your DP/DP link. 
Load the configuration into the DP slave CPU.   

Keywords:  
Interface modules, I/O devices