The common ethernet hub simply broadcasts each message it receives to every one of its ports. Each ethernet device also has to wait for its turn to ‘talk’ to the hub, increasing the likelihood of message collisions; therefore, real-time operation is jeopardized and determinism is difficult. Alternatively, the industrial ethernet switch automatically determines and remembers where an Ethernet device is located and routes messages only through the appropriate port. Internal to as industrial switch is a memory for Ethernet MAC addresses so it remembers the routing for even the largest industrial network. In addition, it contains message buffers to store incoming messages until it is safe to transmit them over the appropriate network segment. This buffering not only avoids collisions, but it can further speed up the system by transmitting message packets over multiple network segments simultaneously.
Industrial ethernet switches further enhance the performance of your network by performing automatic speed changes on the appropriate port. Some user devices communicate at 10 Mbits per second (which is more than adequate for the small lightly loaded messages that some devices need). These devices, when connected to an industrial ethernet switch can be uplinked to a controller or computer at 100 Mbits per second. In effect, the computer can talk to ten times as many stations at the higher speed. The industrial switch routes the messages to multiple devices at the reduced speed in parallel (at the same time).
By avoiding collisions and directing network traffic efficiently, the Industrial Ethernet Switch promotes real-time performance in your deterministic industrial system.
There are two types of switches – managed and unmanaged. The unmanaged switch requires no setup and does not have an IP address. It regulates traffic according to preset parameters in its firmware. It is a Plug ‘n’ Play device.
A managed switch, allows the user to change the parameters of operation. An IP address can be given, as well as a name and location, so that the switch can be identified by an SNMP software package – such as IntraVUE. A managed switch stores port data and traffic data to memory locations that can be viewed with SNMP software. Security features, such as, who can access the switch, turning unused ports off, and Alarm emails can be programmed by the user to indicate fault conditions. VLANs, Redundancy, ring or RSTP, are set up by the user in a managed switch. IGMP snooping is a feature to control PLC multicasting – it is extremely important to use managed switches when I/O over ethernet is implemented.