S7 TCP Connection problems

Hi. I am experiencing a problem while trying to establish an S7 TCP connection between my PLC and various customer PLC's. We are using TCP connections and when monitoring their status they all show 'closed'. Every now and then this message is replaced by one indicating that the connection it trying to be established but then returns back to 'closed'. As far as I and the customer engineer can tell, the connection configurations are correct (ip addresses and port numbers). We have also tried ISO-on-TCP connections which also seem to suffer the same issue.

The plc's are all connected via a single switch and all reply successfully to a ping test. The customer PLC's also successfully 'talk' to each other.

I have exhausted my attempts at getting this working, recompiling harware and downloading, recompiling netpro config and downloading, cycling power etc but to no avail.

At present I have only 3 connections configured to 3 customer PLC's and all have the same issue. As mentioned previously the customer PLC's are either 300's or 400's and none have more than 3-4 connections configured so am confident the issue is not a resource related thing.

Any suggestions???

My hardware is CPU315-2 DP and CPC343-1 (1EX21) ethernet card.

Who is active?

How are your connections setup exactly ?
Unspecified ?
Fully specified ?
Fully specified to dummy partners ?

What are the remote partners exactly ?

For a TCP or an ISn_TCP connection, you must use Send/Receive blocks. If you are doing this already, are the send/receive blocks programmed in the remote partners ?