SERCOS III has been designed to maintain the most desirable features of SERCOS I & II, yet take advantage of new features made possible by use of Ethernet physics.

Topology

SERCOS-III has a ring structure like the first and second generations of the SERCOS interface. It uses a single daisy-chained Industrial Ethernet cable, but the Ethernet physics allow a double ring structure in regard to signal flow in the network.

The double ring signal flow structure offers the possibility of redundant data transfer. With SERCOS III, in case of a break at any point in the ring, the protocol automatically switches over to the line structure, so that the communication and manufacturing can continue while the integrated diagnostics tool signals the break, which can be repaired without interfering with the plants' performance.

In addition to the ring structure, a linear structure is also possible. This doesn't offer the redundancy advantage, but it does save a wire connection, which can save money when the SERCOS bus runs the length of a long machine or system.

SERCOS-III doesn't use the star topology of the standard Ethernet. No hubs or switches are needed. Delay times and jitter are reduced to a minimum, as all nodes process the real-time data "on the fly."

Note that a single daisy-chained cable is used with both structures.
The drawing illustrates signal flow in the Industrial Ethernet cable.
Sercos III Topologies

The installation of a SERCOS network is very simple and does not require any specific network configuration proceedures. All nodes are simply connected by patch cables or cross-over cables. The Ethernet ports of the devices are interchangeable and can even be used to connect standard Ethernet nodes (e.g., laptops) to a SERCOS real-time domain. In this case, full Ethernet- and IP-connectivity is provided, but the real-time behavior is not affected.

Communications Structure

SERCOS III uses a master-slave communication structure, the same as the previous generations. Direct communication between slaves isn't possible with SERCOS I and II, as telegrams are transmitted by the master in one direction only. However, this feature is advantageous in some motion control applications, because data can be directly exchanged between slaves within one communications cycle without involvement of the master. The full-duplex characteristics of the Ethernet physics enable such a slave-to-slave data transfer, so SERCOS-III supports this feature.

Timing/Interface Placement/IDNs/Cyclic Operation/Telegrams

These functions all operate the same as in SERCOS I and II. See:
Timing
Interface Placement
IDNs
Cyclic Operation
Telegrams

Cycle Times

The description of cycle times for SERCOS I and II applies to SERCOS III. The one difference is that the shortest SERCOS III cycle time is 32.25 microseconds, as opposed to 62.5 for SERCOS I and II.