TCP is connection oriented, if you need to do "multipoint" ie analogous to RS485 then UDP is the answer for you, this is stateless, and can be imagined something like junk mail, you post it off and assume that they get to the far end, you never really know if the readers are going to open them up.

Although this sounds flakey, it is no worse than RS485 as your protocol will have (or should have) an acknowledge of some sort to say that the message has been received, which is also sent back as a datagram. (UDP = User Datagram Protocol)

In theory, if you are using TCP (Transmission Control Protocol), you could forget about an error check (you could with UDP as well) and also an acknowledge function in your protocol as the TCP would imply this. You would never do that in real life though, (although you may drop the error checking)

I have used the Lantronix devices in my applications, and have been reasonably happy with them, there is opposition to Lantronix, and Digi springs to mind as having a nice selection of embeddable modules, of course having the unit embedded makes it hard to change if you see a shinier (or cheaper) product from someone else.

Regards

Marshall