The last time I used Ethernet it was specified. The PLCs were GE-Fanuc 90-30. There were 4 PLCs controlling 3 diesel generators for emergency power and co-generation. The Ethernet reads only had to take place every 2 seconds but even at that meager rate, the PLCs were reporting "full mail boxes". The only other device on the network was a computer with FIX-Dynamics on it. Unfortunately, this was not good enough as the generation was at 11kV, the generation system had a neautral earth switch connected to keep the system grounded. If the neutral earth switch opened, the whole system had to be taken offline very quickly to stop the 11kV "floating" to dangerous levels above ground. If that did not happen very quickly, there would have been the potential for cuasing major problems at the sub-stations in the reticulation system. Finished up hard wiring all critical signals from PLC to PLC. The local GE-Fanuc distributor was not surprised at our problem and considered what we saw as normal for that version of their Ethernet.
I have found it non-deterministic and fine for collection of data into a SCADA system or something similar but until I see evidence of a project where the "global bit" idea works effectively, I will not use Ethernet for a critical system, such as generators, where you really do not have any room for "network collisions" to slow down PLC to PLC exchanges.
My preference for these systems is definately a token ring based PLC to PLC network. The network does not clog up and there are no collisions. Data transfer is reliable, seamless and fast.
I have developed many systems with Omron's Controller Link networks. The network is Manchester encoded token ring and, even though the network's maximum speed is 2mB/sec over a twisted pair, I have one system controlling a base load power station with a total of 9 Omron C200HS PLCs, (these are older slower PLCs these days), all with remote I/O, and a 4500 tag Citect data base drawing information from the network, the Citect SCADA still reports in excess of 12,000 digital reads per second and the PLC to PLC I/O update is taking place at the same rate. The Controller Link network also has a certain amount of redundancy built in. One of the PLC cards takes charge of token and communications control. If that PLC is taken off line, another unit takes over.
The setup is seamless. A table is developed that is loaded into each PLC. If one sets up each PLC on the network with, say, 30x16 bit channels of digital data and 50 channels of data memory (register) data for exchange, and loads the same table into each PLC, the same address numbers from one PLC to another are used. For example, PLC1 is setup with digital channels 400-429 for network exchange, then PLC2 reads exactly those channels and can use the bits for information and control.
Quite frankly, it is the easiest and best PLC to PLC network I have used, although I have not experienced GE-Fanuc's global I/O yet. I am lead to believe that it behaves in much the same way. Seamless and reliable with no send or receive commands required.
beerchug