Dear Experts,

I'm using Open TCP communication with  S71511-1 PLC (TIA v14Sp1).

I have established a TCP communication with a PC application .Communication works as expected. The client is the PC app, the PLC is the Server providing data.

This PC application (client) at the end of the communication automatically sends a TCP signal [FIN, ACK].

Usually on the other end there is also a PC application which I replaced with my PLC, so I know that in this case the PLC should send the [ACK] + [FIN,ACK] signals. Gracefully closing the communication. But instead what happens that after receiving the [FIN, ACK] signal, the PLC sends an [ACK] + [RST, ACK] Resetting the communication. This generates an error on the Client side.

So to summarize: I was able to monitor many many PC-PC (Server-Client) implementation with WireShark and any of those TCP implementation closes the communication after the [FIN,ACK]. The PLC TCP implementation is however different. It closes it by sending the [RST, ACK] signal.

My problem is that this signal handling is deep in the PLC firmware. I hoped that TIA v15 with a S71516-3 PLC will make any difference but no.

Is there any hope that I can "tell" the PLC to gracefully close the communication without resetting it?

I also tried with TSEND_C and (TCON+TSEND). The behave the same way.

If I can't solve this the client will full with error logs.

Cheers

Hello,

I also find out that there is a different between S7-1500 and S7-300

S7-300: Sends a FIN after TDISCON
S7-1x00: sends a RST after TDISCON

Please have a look in this PC Application
https://support.industry.siemens.com/cs/de/de/view/109475508

Here, you find a little workaround to handle both types of closing. The source code you find in the Download.

Or on this post

https://support.industry.siemens.com/tf/de/en/posts/behaviour-of-tdiscon-how-does-it-terminate-the-connection/198410/?page=0&pageSize=10