I have a slightly old InSight 5400-C11, with Firmware revision 4.10.5 (the highest one offered for this camera, AIUI) that I use for various test and experimental work. I recently tried to connect to it using Telnet, instead of using EIP or ProfiNet, and ran into a maze of strange issues.
I did eventually manage to make it work, but I'm still trying to work out the root cause.
I was using two different Telnet clients -- MobaXTerm, and PuTTY. Whenver I tried to make a Telnet connection to the Cognex with either one, I would see the "Welcome" message from the camera, but the connection would then close instantly, before I hit a single key.
I eventually went through a complete factory reset of the camera, and a firmware update (I had previously been running 4.10.02). But still had exactly the same problem.
However, then I tried PuTTY in Raw mode, instead of Telnet mode, and was able to log in and control the camera. I still need to research exactly how these modes are different, but from what I understand, Telnet mode includes some low-level protocol handling that is inherent to the Telnet specification, while Raw mode only opens the port, and starts sending/receiving ASCII strings. I'll note that Raw mode works more like RS232, just over TCP/IP instead of a serial cable. And Cognexes were originally built around RS232 communications, before Ethernet took over the world.
I'm still trying to figure out if there's a way to "tweak" the Telnet mode settings to resolve this problem, and what the ramifications of this issue might be when I start writing PC programs to access the Cognex via "Telnet." Or, maybe this is something specific to my camera or firmware, I'm not sure.