In 1973, researcher Bob Metcalfe, Xerox Corp., developed a way to link computers to printers, thus creating a physical method of cabling, known today as Ethernet. The SERCOS interface — also known as Serial Real-Time Communications System — is an open, digital interface for communication between a digital controller and peripherals, such as I/Os, digital servo drives, and actuators. Together, they provide a powerful new motion bus for numerically controlled machines and systems.

Two German organizations, ZVEI and VDW, created SERCOS in the 1980s as a digital drive interface for advanced machinetool applications to replace the +/-10 V analog interface. Since 1995, it has been the only internationally standardized (IEC 61491) motion control and I/O interface. Its applications have expanded to all types of automated machines, including machine tools, printing presses, packaging machines, and robots.

There are many other digital drive interfaces on the market. However, many are proprietary, and therefore, only work with the manufacturer's drives; others are "opened," but are developed and controlled by a single manufacturer, with just a few firms selling products using that bus. In contrast, more than 50 control and 30 drive manufacturers worldwide offer products based on the SERCOS interface standard.

Improving the interface

Industrial Ethernet is the de facto standard for manufacturing information networking and is now reaching the servo drive level. However, synchronizing multiple drives requires a high degree of determinism, which is available in SERCOS, but not industrial Ethernet. While Ethernet is not deterministic, it does offer high bandwidth and low hardware costs. The best-of-both worlds solution is to combine SERCOS with industrial Ethernet as the physical layer. SERCOS III fulfills this need, running at up to 100 Mbit/sec. (By comparison SERCOS I operates at 2 and 4 Mbit/sec and SERCOS II at 2/4/8/16 Mbit/sec.)

SERCOS III maintains the hardware determinism of earlier SERCOS versions operating with submicrosecond jitter to provide precise real-time control. Additionally, an optional IP channel for TCP/IP ( transmission control protocol/Internet protocol) is placed under control of the motion bus.

Further, SERCOS III maintains the protocol structure and the many profile definitions ( parameters and functions) of the original SERCOS, which more than 300,000 applications successfully employ. This provides 500 standardized parameters, lets users link to existing communication infrastructures, makes new features possible, and decreases hardware costs.

One special feature in SERCOS III is direct communication between slaves, allowing them to exchange data in one communication cycle without involving the master. This is possible because of Ethernet physics' full, duplex characteristics.

Defining traits

In SERCOS, data is transmitted via a group of telegrams. The first step involves the controller (master) broadcasting a Master Synchronization Telegram (MST) at the start of a communications cycle to set up and synchronize the timing sequence. Then, the master transmits a Master Data Telegram (MDT) once each cycle, sending data such as command values to the slaves (drives, I/O). Next, the slaves return an Amplifier Telegram (AT) to the master, reporting on speed, torque, and position. Each slave repeats the AT and inserts its data in the appropriate slot.

While operating data is communicated in real time, a service channel is available that transmits nonreal-time data without-disturbing the synchronous data transfer. Each telegram provides space for sending two bytes of service channel data (four bytes in SERCOS III). An additional, optional IP channel can be added to transfer nonreal-time data (such as TCP/IP and UDP/IP) via standard Ethernet frames. The IP channel's job is to directly transmit telegrams from superior networks (office, wide area) "up/down to" the drive or I/O. Transmission occurs in a separate channel and therefore, does not affect real-time data exchange between devices. End users can adapt cycle times and separate real time and nonrealtime channels for particular applications.