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Introduction to Sercos interface
Applications Types
Sercos III
Sercos I and II
Fiber Optics
Timing & Interface
Sercos IDNs
I/O Functions
Cycle Times
Cyclic Operation
System Safety
Packaging Profile
Sercos Standardization
With generation I and II Sercos interfaces, one fiber optic ring is used to communicate 32-bit data between controllers, drives, I/O and sensors. This includes commands, status, parameters and diagnostics. In Sercos-III, standard Industrial Ethernet is normally used for this purpose. Note that Ethernet fiber optics can be used for communication in Sercos-III. 

Fiber optics provide noise immunity and eliminate the immense requirements for conduit, wiring and terminations normally required with an analog interface. The ring architecture reduces the number of components required on a motion control. Adding an additional motion axis often requires nothing more than opening the ring and placing the new drive in the ring. 

A Sercos ring is composed of a number of fiber optic segments having no braches master-to-slave, slave-to-slave and last-slave-back-to-master as illustrated in topology. Each device in a system receives signals via a fiber optic receiver with an F-SMA connector (Fiber Optic Subminiature Version A), and transmits the signals to the next device via a fiber optic transmitter, again with a standard F-SMA connector.

Fiber Optic Cable

The fiber optic cable consists of 1 mm Plastic Optical Fiber (POF) or 200 m Hard Clad Silica (HCS) (glass) multi-mode optical fiber with a step index profile or graded index profile. Fiber optic cables and cores can be used. Only fiber optic cables with a wavelength of 650 nm are specified for Sercos. For fiber optic materials currently in use, the attenuation is approximately 220 dB/km for plastic and 6 dB/km for glass. 

The following drawing illustrates the structure of a typical fiber optic cable. Note that the Sercos interface standard does not specify the outer diameter of the cables. Typically, 2.2 mm diameter cables are used inside equipment cabinets. These cables do not have the strength members shown in the drawing. Typically, 3.6, 5 and 6 mm diameter cables are used for cabinet-to-cabinet connections.

Structure of a single-core fiber optic cable

Cable Specifications

Plastic Fiber - POF Glass Fiber - HCS
Core diameter 980 m 200 m
Cladding diameter 1000 m 230 m
Numeric aperture 0.47 0.37
Bandwidth >= 5 MHz * 1 km >= 10 MHz * 1 km

Fiber Optic Connectors

Sercos fiber optic cable connectors are specified as: 
-- Shall correspond to F-SMA standard (see IEC 60874-2). 
-- Shall have a quality level of at least 5. 
-- Shall have a metallic connector ring. 

In addition, it is recommended that fiber optic cables have a strain relief.

Sources For Fiber Optic Cables

Assembled and tested Sercos fiber optic cables are available from all Sercos members who supply controls and drives. See our member lists and contact the sales department of the appropriate company or check their website for your local distributor.

Sercos N.A. member Industrial Fiber Optics is a specialist in fiber optics, supplying fiber optic components, test equipment, connectors, optical cable and completely assembled and tested Sercos plastic and glass fiber optic cables in various cable types and lengths. See the Sercos page on the Industrial Fiber Optics website. Contact Randy Dahl, president, with any questions about optical fiber problems or solutions. 480/804-1227.

Contact Ronald Larsen for a list of additional fiber optic cable suppliers.

Fiber Optic Ring Lengths

A Sercos ring is composed of a number of fiber-optic segments. Maximum cable lengths are:

Cable Type Maximum Cable Length
Plastic fiber POF (1 mm diameter)  
Node-to-node 40 meters
Maximum ring length (254 nodes per ring)

10,000+ meters
Glass fiber HCS (200 m diameter)  
Node-to-node 200 meters
Maximum ring length (254 nodes per ring) 50,000+ meters

If a coupling is used to pass a cable through a cabinet wall, the maximum length of that segment is reduced by 6 meters. 

The maximum number of drops per fiber-optic ring is 254. However, the number of drives that can be serviced per ring depends on three application requirements: 
-- The communication cycle time. 
-- The volume of operational data. 
-- The communications speed required. 

The number of rings that can be synchronized together is limited only by the controller, which will have a limit to the number of axes it can process. Sercos interface-compliant controllers can have a much higher axis limit than their predecessors, since the Sercos interface supports distributed processing, which relieves the controller of many time-intensive tasks, which are now handled by the drives.

Performance of Sercos interface Rings -- Generation I and II

Per Drive

(MDT + AT)
Data Rate
2 ms 32 Bytes 2 Mbit/sec 8 8 kbit/s
(2 bytes)
1 ms 32 Bytes 4 Mbit/sec 8 16 kbit/s
(2 bytes)
1 ms 36 Bytes 8 Mbit/sec 15 32 kbit/s
(4 bytes)
0.5 ms 36 Bytes 16 Mbit/sec 14 128 kbit/s
(8 bytes)
2 ms Standard
16 Mbit/sec 112 8 kbit/s
(2 bytes)

Fiber Optic Transmitters/Receivers

Fiber optic transmitters and receivers from Avago Technologies and Honeywell have been tested and approved by the Sercos Technical Working Group. The Literature section of this website provides links to downloadable data sheets for these products. Signal levels are defined in the IEC standard

Avago Technologies (formerly Agilent Technologies):

Avago offers transmitters and receivers specified for operation at 2, 10 and 16 Mbit/s. They are available in both plastic and metal housings.

Avago HFBR1602/1604 transmitters and HFBR2602 receiver -- Optical components specified up to 2 Mbit/s. These models are not recommended for new designs.

Avago HFBR1602Z/1604Z transmitters and HFBR2602Z receiver -- Optical components specified up to 2 Mbit/s.

Avago HFBR1505A/2505A -- Optical components specified up to 10 Mbit/s only, although Avago says they typically will operate up to 16 Mbit/s. These models are not recommended for new designs. 

Avago HFBR1505AZ/2505AZ -- Non-conductive plastic housing optical components specified up to 10 Mbit/s only, although Avago says they typically will operate up to 16 Mbit/s.

Avago HFBR1505AFZ/2505AFZ -- Metal housing optical components specified up to 10 Mbit/s only, although Avago says they typically will operate up to 16 Mbit/s.

Avago HFBR 1506AM/2506AM -- Optical components available with metallized plastic thread. Specified for operation up to 16 Mbit/s. These models are not recommended for new designs.

Avago HFBR 1506AMZ/2506AMZ -- Metal-coated plastic optical components available specified for operation up to 16 Mbit/s.

Avago HFBR 1506AFZ/2506AFZ -- Metal housing optical components specified for operation up to 16 Mbit/s. 

Products can be purchased from Avago's distributors

Europe: For additional information, please contact Mr. Schwaiger.
Tel +49(0)7031 436 3201, Fax 49(0)7031 436 3369.


Honeywell offers a transmitter (HFE7000) which is applicable for 16 Mbit/s. Receivers HFD 7000 and HFD 7500 comply with the specification. HFD 7500 is cheaper than the HFD 7000, but it needs a comparator, since the output signal is not interoperable with TTL. 

Honeywell offers its optical units with either plastic or metal thread.

In North America, these products are distributed by Arrow. For additional information, please contact Mr. Joch.
Tel +49-(0)89-35 81 33 17

Fiber optic receivers and transmitters are available from other manufacturers as well, but thus far, they have not been tested for compliance with the Sercos specifications.

Fiber Optic Repeaters

Periodically, we receive inquiries regarding fiber optic repeaters for Sercos; however, there are no repeaters specified. For longer lengths, use glass fiber up to 400 meters in length. If that isn't adequate, increase the optical output power of the transmitter. 

Rotary Fiber Optic Joints

Rotary fiber optic joints for use in Sercos systems are manufactured in North America by Sercos N.A. member Princetel, Inc.

Contact Barry Zhang, President
Princetel, Inc.
1595 Reed Road, Suite 300
Pennington, NJ 08534
Fax: 609-895-9552

Rotary fiber optic joints are manufactured in Europe by Schleifring. 

Schleifring und Apparatebau GmbH
Am Hardtanger 10
D-82256 Fuerstenfeldbruck. Germany 

Schleifring North America, LLC
226 Lowell Street 
Wilmington, MA 01887 
Tel: 978-988-8780 
Fax: 978-988-8783