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

Business Editors

CHICAGO--(BUSINESS WIRE)--Dec. 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(R) and Hiperface(R) are proprietary -- using them locks a machine builder into just one brand of encoder, and does not allow second-sourcing. Another communications protocol -- SSI(R) (Synchronous Serial Interface) -- is freely available, but its performance is limited in several ways, including its comparitively 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.

                     SSI          EnDat(R)    Hiperface(R)  BiSS
----------------------------------------------------------------------
Connection         Point-to-   Point-to-Point  Bus or      Bus or
                    Point                     Point-to-   Point-to-
                                               Point       Point
----------------------------------------------------------------------
Transmission mode  Unidirec-   Bidirec-        Bidirec-    Bidirec-
 (digital)          tional,     tional,         tional,     tional,
                  synchronous  synchronous    synchronous synchronous
----------------------------------------------------------------------
Sensor data        1.5 MHz     2 MHz, plus     38.4 kBaud, 10 MHz
 transmission                    analog        plus analog
----------------------------------------------------------------------
Protocol length    Yes         Yes             No          Yes
 adjustable
----------------------------------------------------------------------
Multi-cycle data   No          No              No          Yes
 protocol available
----------------------------------------------------------------------
Number of lines,   4, unidirec- 2 bidirec-    2 bidirec-  4 unidirec-
 direction           tional      tional, 2     tional      tional
                               unidirectional
----------------------------------------------------------------------
Analog lines       None         4              4           None
----------------------------------------------------------------------
Multi-slave        No           No             Yes         Yes
 synchronization
----------------------------------------------------------------------
Alarm/warning bit  Definable    Yes            No          Definable
----------------------------------------------------------------------
Plug & Play (Auto- No           Yes            Yes         Yes
 configuration)
----------------------------------------------------------------------

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.dancon.com, Fax: [phone].; iC-Haus may be reached at [phone], Fax [phone], www.ichaus.com.

About Danaher Controls

Danaher Controls designs, manufacturers and markets a wide variety of encoders, counters, motion controls and timing products worldwide, including Dynapar, Northstar, and Hengstler brand encoders, Veeder-Root brand counters and Eagle Signal brand timers. For more information, contact your nearest Danaher Controls distributor or Danaher Controls at 1675 Delany Road, Gurnee, IL 60031. Phone: [phone] or 847-662-2666; Fax: [phone]. Website: www.dancon.com.

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