HI ,

i was surprised with my colleague solution for TCP IP communication between S7-1200 and S7-300 over GPRS, so i had an need to comment it with you. Both PLCs are behind two routers (not Siemens) with static IPs, so we made port forwarding on routers of port 2000 to the addresses of PLCs.

The program was first tested in a way that we connected directly with ethernet cable both PLCs and it worked. The S7-1200 was configured as the active partner.

After connecting PLCs using 2 routers the situation changed, we needed to specify on both PLCs that the remote partner is unspecified with the IP address the static address of the router. But it didn't work before we set that both PLCs are passive partners and that the remote unspecified partner is the active one..

So can somebody please explain why does it work like that ? 

TNX