Mark-

SWAG, Get_Attribute_List

Yeah the "keep-alive" at the TCP level and the "watchdog/keep-alive" at the CIP level are different. I guess the first question is why keep a polling/response type connection active/open unless it is used. That was rhetorical. Leave the TCP level to itself and only work at the CIP level because that is what is timing out. Did you retry the "List identity" command?  

Do the test.

I was going to write a bunch of stuff but decided at least one more test was needed. With no other connection to the PLC, issue the register session and forward open. Then doing nothing. How long before the PLC drops the connection?  

Are you asking someone to commit a crime?

OK, I asked because the specification is where I find solutions to most returned error codes. Word search (and patience) is your friend.

You are welcome. Do you have the CIP specification to read?

I do not know if that is the issue. I see the result "connection in use or duplicate forward open" and know I code so the connection serial number is unique across all connections.

Are you using a different connection serial number?  

You are welcome. You can use other port number yes. Is the slot (controller) part of the connection path?

And to add not all devices support/reply to "ping" request.

You are welcome. From the DF1 manual: For higher addresses, setting this byte to FF expands this field to three bytes total. Use the second and third bytes for the expanded file address (low address byte first).

You are welcome.

You are correct. Two reads, both using the same CIP sequence count value (1). The controller sees a duplicate read and responds with the same reply as the first message.  

> The problem we are facing is the PLC doesn't send out a response of more than 240 bytes, You are only requesting data for 11 reals. Ask for more tag data and I assume the device will reply with more data, up to the requested specified buffer size, in the large forward open request.