DH-485: One of the oldest ALLEN BRADLEY networks, it is typcally slow, and I believe supports up to 31 nodes (though I could be wrong). Typically, it runs at 19,200 baud. Useful for programming, and communications between PLC's and HMI's or other PLC's.
DH+: A newer, faster ALLEN BRADLEY designed network, though most of the structure is available in the public domain. It typically runs at 57,600 baud, but also can run at 115 and 230kbaud. Useful the same as DH-485, but supported by more devices. I believe it can support up to 77 nodes. This is a two-wire (blue-hose) network.
RIO: Another legacy AB network, designed primarily for controlling remote-I/O bases. The same speeds, and cabling as DH+, except for some odd reason, the blue and white wires are reversed.
DeviceNet: An AB Network based on the CANBus protocol, which was initially developed for point to point communications in automobiles (go figure). Very good for PLC to Device communications (including drives, limit switches, flow-meters, encoders), but not good for programming or HMI interfacing. Runs on a 4-Wire network that provides both power and communications to slave devices.
ControlNet: One of the newest AB protocols, widely supported, and deterministic. ControlNet runs over coax (basically, RG-6), at a speed of 5MBps. ControlNet can be completely redundant if desired. It is used mainly to replace RIO for remote I/O points, but can also be used for HMI's and programming.
EtherNet/IP: Is another of the newer AB Networks which is becoming a standard. It supports the same CIP protocol as ControlNet, but modified to run over standard Ethernet IP systems. Speeds are generally 10/100MBps on the hardware end, but can be switched to a higer speed backbone (it is, Ethernet after all). This is commonly used for I/O, HMI's, Data Collection, and programming.
Personally, for all new systems, I specify primarily ControlNet for I/O and drives, and Ethernet/IP for HMI and Enterprise communications.
Why are there so many? Technology evolves. Initially, the 'Proprietary' networks gave an edge to one 'brand', as they could sell more items on the premise that all of their devices could communicate well together.
Later on, the networks became more open source, or at least more freely licensed, mainly to increase the potential compatible target markets.
Today, things are moving more and more to universal networks, but there are still the 'biggies' out there that are not interoperable, Profibus/Profinet, Modbus/Modbus TCP, ControlNet, EthernetIP... who knows where the future will take us?
