Can you recommend a gateway to convert Siemens AS511 protocol to any newer one (preferable Modbus/TCP)

I need to connect to an HMI that does not have AS511 support.

 

You could use an AS511/TCP-IP port adapter like those from IBH - see http://www.ibhsoftec-sps.de/english/IBH_Link_S5.htm

We use this for remote diagnostics and programming of a group of S5 PLCs from a central PC.

Besides the programming capabilities the vendor includes application examples of programming interface for HMI applications.

If you don't want to develop your own interface code, you could use their OPC server for your HMI to interface with the converters.

There are other vendors that do the same thing - e.g. Softing, INAT, etc