Open Modbus TCP with FB100 problem connect S7 with Foxboro (Invensys) system

UnimogMan

I may be wrong, but last I remember, the length of the total TCP telegram is always contained in the header, whether it's a complete response (to whatever original message) or a packet of a response that needs to be reassembled at the receiving end.

The length is contained in IP-Header (Total length = IP + TCP + UserDataLength) along with IP-Length (20 bytes). The difference is TCP-Length and UserDataLength.
TCP-Header have it's own TCP-Length (20 bytes)that meansCPx43-1"Native-TCP" driverknows when a complete TCP-IP telegram frame arrived and what is the UserDataLength. That is proofed by the ACK & Window update response.
But when it comes to transfer data from the CP to the PLC,Siemens made this complicate/abstract so it force you to figure out how much data you expect to receive in the bufferinstead of giving you each UserData packet at every AG_RECV and the length. This implementation made RFC1006 looksbetter then the Native-TCP indeed

Dec

TryOut

In operating mode server the functionality multitasking according to the MODBUS reference is not implemented.