Danaher Controls First To Use New BiSS Communications Bus for Absolute Encoders

Chicago, December 11, 2002 -- In partnership with chipmaker iC-Haus, Danaher Controls has released the first encoder to use a new high speed communications bus called BiSS (Bidirectional Synchronous Serial Interface).  A major supplier of encoders, Danaher Controls adopted the new bus for its ACURO line of absolute encoders, to solve the problems of proprietary protocols, slow speeds, and to reduce machine design costs.

The BiSS bus is nonproprietary, and its VHDL description is freely available under GPL agreement. Running at 10 MHz, it is many times faster than current alternatives. Unlike some encoder buses, it is all-digital, and includes a cyclic redundancy check (CRC) that is more secure than the check used in the SSI bus.  Moreover, BiSS allows for sensor status and diagnostics information.

In addition, the new bus is hardware-compatible with SSI, so existing SSI users can convert to the new system with just a software change.

Background
Absolute encoders have many advantages over incremental encoders, but they have been handicapped over the years by difficulty in communications. An incremental encoder produces very little information—a stream of pulses—so it is simple to connect. An absolute encoder, by definition, produces a multi-bit data word. At first, this was communicated via a parallel interface, but parallel interfaces are bulky and expensive. In response to this, several serial communications methods were developed, but they, too, have drawbacks. Two of them, EnDat® and Hiperface® are proprietary—using them locks a machine builder into just one brand of encoder, and does not allow second-sourcing. Another communications protocol—SSI® (Synchronous Serial Interface) — is freely available, but its performance is limited in several ways, including its comparatively slow data rate of 1.5 MHz and its poor error-checking.

What has been needed is a high-performance nonproprietary communication method for absolute encoders—one that will allow for future growth and yet maintain compatibility with installed equipment.

An Improvement Over Current Options
The BiSS communications bus allows no ambiguity about absolute position. One of the proprietary protocols transmits absolute data only at power-up, and sends only incremental data after that. This solves the problem of not knowing position at startup, but it does not self-correct for momentary data dropouts during operation. With only incremental data coming into the controller, a glitch can cause the controller to lose track of position. BiSS sends full absolute position data whenever the controller polls the encoders. BiSS is purely digital, eliminating the analog outputs used by some other protocols. While using analog signals appears to save on communications bandwidth, it requires that the control equipment contain costly A/D converters. The BiSS bus enables machine builders to avoid this expense because all the signals are digital from the beginning. In addition, BiSS includes a cyclic redundancy check (CRC) with every data transmission, which SSI does not.

BiSS can also carry data other than position. An internal register in the encoder (which can be read and written to by the master) can contain not only relevant data about the encoder itself (identification, device data, resolution, etc.), but can receive other digital data (temperature, proximity, etc.) and transmit it to the master on demand, without interfering with real-time operation.

BiSS can be configured for point-to-point use, with one controller (master) and one encoder (slave), or with one master and multiple slaves. This can significantly reduce wiring expense. The system measures transmission delay for multiple inputs and automatically compensates for it, so system dynamics are not adversely affected by cable length. Using ordinary twisted-pair cable (three pairs), a BiSS system can be 50 m long; with Cat 5 cable, it can be up to about 150 m.

The table below compares BiSS to other available protocols.

For more information
Full technical information on BiSS is available from Danaher Controls and iC-Haus GmbH.  Danaher Controls may be reached at 800-873-8731,www.feedbackdevices.com, Fax: 847-662-6633.; iC-Haus may be reached at  +49 6135-9292-0, Fax +49-6135-9292-192,  www.ichaus.com.

About Dynapar
Dynapar™ - is a respected leader in commercial-light duty rotary encoders and industrial-duty absolute and incremental encoders, including motor mount encoders, for over 50 years. Dynapar has been a division and member company of the Danaher Industrial Controls Group since 1984.

About Danaher Industrial Controls
Danaher Industrial Controls Group designs manufacturers and markets a wide variety of encoders, counters, motion and process controls and timing products used throughout Factory and Process Automation applications worldwide, including Dynapar™, NorthStar™, and Hengstler™ brand encoders, Harowe™ brand resolvers, Veeder-Root™ brand counters, Eagle-Signal™ brand timers, Partlow™ brand controllers and recorders, West™ brand controllers, PMA™ brand controllers and Anderson™ brand and Negele™ brand sanitary process instrumentation. For more information on NorthStar’s new line of harsh-duty optical encoders, contact your nearest NorthStar brand Factory Automation distributor or DICG at 1675 Delany Road, Gurnee, IL 60031. Phone: 1-800-873-8731 or 847-662-2666; FAX: 847-662-6633. Website: www.danaherindustrialcontrols.com.

.EnDat® is a registered trademark of Johannes Heidenhain GmbH. Hiperface® is a registered trademark of Max Stegmann GmbH. All other Products and company names listed are trademarks or trade names of their respective companies.