Introduction
Ethernet is widely used for information (i.e., office environment) communications today. The technology has availability, familiarity, and cost benefits. Until recently, however, creating an industrial control system using Ethernet was not feasible due to a number of factors, including Ethernet’s lack of determinism, the need for interoperability among devices, and security concerns.
Determinism is the ability to predict when information will be delivered. To guarantee this, an industrial control network must provide scheduled bandwidth (or time slots) that are reserved for time-critical data transfer. Communication over Ethernet, however, is based upon collision detection. If a device attempts to send a message, and that message collides with another message on the Ethernet media, the device backs off and waits to transmit. Thus Ethernet cannot guarantee determinism.
However, recent advances in switch technology have now enabled Ethernet to approach determinism. Switches, unlike traditional bridges and hubs, reduce traffic between the devices attached to their ports. Moreover, the IEEE 802.3 Standard provides for standardized full-duplex operation, which gives a single node - in a point-to-point connection to the switch - full wire concentration. As a result, full-duplex switched Ethernet networks are theoretically able to avoid collisions.
The requirement for device interoperability, the ability of products from different control vendors to communicate with each other, has been answered by the EtherNet/IP (industrial protocol) networking standard. This standard provides interoperability among products from a wide community of automation vendors. It is supported by three major networking organizations: ControlNet International (CI), the Industrial Ethernet Association (IEA) and the Open DeviceNet Vendor Association (ODVA)
