Differences between MODBUS, MODBUS+ and CTnet

If your working with PLCs, then it looks like MODBUS is probably the way to go...

http://en.wikipedia.org/wiki/Modbus

MOdbus is generally considered an "open protocol" and many mfr's devices support it. Modbus+ is bit more closed. I don't know much about +, but regular MOdbus is a somewhat slow master/slave (or multiple slave) serial protocol that lends itself well to HMI to PLC or HMI to Drive comms. I think of it as "human speed" comms. Some mfr's even use a modified form of their own modbus as the program download protocol. I would not use it for machine IO, but I would use it for process IO.

CTNet is a proprietary protocol from Control Techniques. It is peer-to-peer and is fast enough for machine IO. They use CTSync for drive syncronization, though. It also allows connection anywhere on the network that allows access to everyone on the network. CT offers some HMI's that connect to one drive via modbus but have a driver that allows that one HMI to look through the CTNet network to parameters in other drives.

TCP/IP - if you mean MOdbus/TCP, then you can think of it as the Ethernet version of MOdbus, but now you have speed and might consider machine IO on the network if you can control the traffic. It generally requires a "master" (I think they use the "Server/Client" terminology, thought)

What are you trying to accomplish?

PS - I did not sign in nor register, so I may not see your reply.