profinet logo Serial real-time communication system

Sercos is short for " serial real-time communication system" and is an industrial network which is particularly strong in the servo drives and motion control segment. The network originates from Bosch Rexroth and is today maintained by  Sercos International e.V. (SI).

Since the launch of Sercos in the early 1990s, the network has become a globally accepted real-time networking standard for demanding motion control applications. The Sercos technology is known for its real-time capabilities, performance and noise immunity.

Sercos I
was released in 1991 and uses optical fiber. The data rates supported are 2 and 4 Mbit/s, and cyclic update rates as low as 62.5 microseconds. A ring topology is used. Sercos I also supports a "Service Channel" which allows asynchronous communication with slaves for less time-critical data.

Sercos II was introduced in 1999. It expanded the data rates supported to 2, 4, 8 and 16 Mbit/s.

The third generation of Sercos (Sercos III) is an evolution of the original Sercos standard), based on standard Ethernet. The Sercos mechanisms like for example the motion control profiles, the telegram structure, and the hardware synchronization, have been mapped to the Ethernet standard.

To ensure that the real-time requirements are fulfilled in spite of the Ethernet use, Sercos III uses an additional, collision free real-time channel parallel to a non-real-time channel. In the collision free real-time channel, the Sercos defined frames are transmitted using the Ethertype 0x88CD.

A non-real-time channel can be configured parallel to this real time channel, in which any Ethernet telegrams or telegrams using IP based protocols like TCP/IP or UDP/IP can be transmitted.

Sercos III supports the ring structure like the first and second Sercos generation. However, because of the full-duplex characteristics of the Ethernet physics, it is not a single, but a double ring structure. This double ring structure offers the possibility of transferring data redundantly. In case of a cable break at any point in the ring the communication will continue.  The machine continues to function while the integrated diagnosis tool signals the break, which can be fixed without interfering with the machine’s availability.

A line topology is also possible.  This does not offer the redundancy advantage, but it saves a wire connection which can be an advantage in extensive systems.

Sercos III does not use the star topology of the standard Ethernet and therefore hubs and switches are not required.

Sercos fibre optic ring

Network Type:    Scalable Ethernet based advanced communication system
Topology:    Ring or Line
Installation: Ethernet transmission with shielded twisted pair cables and RJ45 or M12 connectors. Alternatively Fiber Optic Media.
Data Rate: 100Mbit full duplex.

max. Stations: 511 for Sercos III, 254 for Sercos II

Data :    Each node: up to 1500 bytes per telegram frame

Network Features :   Real time capabilities, performance, and noise immunity.
User Organization:   Sercos International e.V.

Industrial Communication with Fieldbus and Ethernet (Chapter 18)