Software Requirements: Ethernet represents only the physical layer of the OSI 7-layer model, although most people also consider that it includes as a minimum the Data Link Layer as well. Therefore, some software will be required. Be sure to check that the equipment you are purchasing supports TCP, IP and UDP as a minimum, since most industrial networks use these three protocols to transmit messages from A to B. Other useful additions include support for DHCP, SNMP and VPNs to avoid having to purchase additional equipment to assign addresses or perform system maintenance remotely.
Security: Design both physical and architectural security concepts into your system. Use the ISA-99 standards as a guide, although you also need to remember that, in a controls environment, the equipment on the ends of the wires is at least as important as the servers and other devices in the middle. It is the devices on the ends that keep the process under control.
Power Supply: Don't forget that industrial environments often experience harmonics from adjacent equipment. Follow good engineering practices, i.e., space cable runs appropriately and isolate critical power supply panels from lighting panels, so that when the electricians change a light they don't shut you down as well. A UPS is a minimum step, although be careful of the type--depending on the other loads in the facility, a UPS can make harmonics worse not better.
Grounding: Ground loops are the bane of all instrument engineers since they often are overlooked as the cause of poor signals. Ground loops can be prevented only by having all the connected equipment referenced to a single point--not an easy task to accomplish. Most facilities, therefore, ground all the equipment at only one end and allow it to "float" in the field.
Redundancy: "A pair and a spare" applies to critical control systems as well. Redundancy is common practice in the control environment, but unfortunately is not a standard in the Ethernet world. To get redundancy today requires that you purchase all your equipment from a single supplier. This is not too bad a thing, because then you only have to train on that equipment and can carry fewer spares on the shelf.
Price vs. Performance: All things electronic get cheaper by the day. Similar to how you think about your home computer, prepare a curve of price versus performance, take note of the "inflection point" when you are purchasing, and base your decision on that. The equipment will not be obsolete tomorrow and your project manager will be happy because you didn't buy the "gold-plated" version.
Growth: Plan for growth and replacement by making your system as modular as possible, with a layout consistent with the process plant. This way, if the plant expands, so can you. Also, if there is an outage or if you need to work just on a certain part of the network, you will only affect one part of the process. Further, if you have a problem with one of the redundant components, repair is possible while running on the single non-redundant node during the repair process.
Connectors: The RJ-45 connector is not always best for use in the field because the contacts are quite small and easily fail, as do the clips that hold the connector in place. A number of manufacturers make connectors for the industrial environment and some industrial Ethernet protocols have standards in place for their network connectors.
Environment: The industrial environment usually is not the same as the office, so be sure to specify the equipment you install for the conditions you expect it to experience. This is particularly important for temperature since, not only do high temperatures shorten equipment life, the electronics compound the problem by themselves giving off heat. Vibration is another environmental issue, leads to loose connections and, because it is intermittent, it is difficult to identify.
To paraphrase a well-known quote: "You're not in the office environment any more, Dorothy," so you had best take that into account with your designs.