Open, digital sensor interface (BiSS-C)
Open Standard Sensor/Actuator Communication
The BiSS-C-Interface features bidirectional isochronous communication between sensors, actuators and industrial controls. The purely digital link and its protocol has been designed for maximum performance, transmission reliability and security. Remarkable cost savings are obtained by reducing efforts for hardware, for installation and maintenance tasks.
Without affecting the payload data of measurements or interfering with control cycles the communication protocol incorporates a permanent, bidirectional access to slave registers. That way device parameters and additional measurement data, or an electronic ID plate and OEM data, can be accessed at any time – device monitoring and diagnosis is made easy.
For an unlimited subscriber count the interface master provides the clock signal for simultaneously triggered actions. For an example, a typical RS422 link can support frame repetition rates of 10µs even with data words of up to 64 bits.
Data transmission is fully CRC secured for the bidirectional command and register communication and for each single-cycle data channel separately, with an assignation of a start value ensuring channel identification by safety controls.
With the BiSS-C-Interface the ASSP / ASiC manufacturer iC-Haus has established an open standard interface for individual automation solutions. On the basis of an OEM license available free of charge interface embedding is supported by IP modules such as VHDL code and an API library for software developers.
Sensor-to-PLC communication with two data words and bidirectional control data: master to slave by CDM, slave to master by CDS. The BiSS master adjusts automatically to line and measurement delays each cycle.
Sensor/actuator network controlled by master output data (MO). The slave devices build up a serial shift register and simply pass on clock and data.
BiSS-C introduces a bank-switch to address extended memory in blocks of 64 bytes each, and direct addresses supplying measurement data and device information. Fixed addresses are agreed for pointers to the following information: electronic data sheet (BiSS EDS), standard device profile, serial number, device description file (XML) and device manufacturer ID.
The bidirectional digital sensor interface (BiSS-C) assures the communication between the encoder and the measuring device or drive ontrol and can if required simultaneously transfer the measured values of up to 8 sensors.
For 1 to 8 subscribers the interface master provides the clock signal for the simultaneous capturing of all position data as well as for the subsequent synchronous serial data transmission. Only 4 unidirectional RS422 data lines are required; the minimal slave electronics is located directly in the sensor ICs.
When the master sends the clock pulse on the line MA, the slave will reply on the return line SL with the captured position data. Commands or parameters are exchanged via a PWM clock sequence, although this is not necessary for the startup of the BiSS-C protocol.
With every data cycle the master learns and compensates for the signal transit times, thus enabling high clock rates up to 10Mbit/s even with cable lengths of 100 m. Varying cable conditions, for example due to drag movement, are corrected.
The synchronization accuracy between several encoders on a number of axes is less than 1 microsecond; moreover the master keeps the signal transit times that have been experienced transparent for the controller and thus enables a further optimization.
The BiSS-C protocol classifies each subscriber into various data areas: sensor data, multi-cycle data and register data. These data areas are laid out differently with respect to the possibility to access them and to their transmission performance, so covering a wide variety of sensor applications. A bidirectional communication parameter for configuring the device, and if need be for so-called OEM parameters, is placed as is customary in the register data area. Data that change slowly such as speed of rotation or motor temperature occupy the multi-cycle data area, whereas data that are changing quickly occupy the sensor data area.
This means that there is no problem in achieving control cycle times under 10 - even for data words up to 64 bit. Enough space is available for redundancy and as a rule is used for implementing a CRC (cyclic redundancy check). As they are only framed by a start and a stop bit, the sensor data is transferred at the best possible user data rate; a single multi-cycle data bit is optional. Similarly detected and triggered, the multi-cycle data bits form a second inband protocol and contribute to the redundancy of the sensor data – permanent monitoring of the drive status and operation is possible, without interfering with the controller cycle. Specific device developments by individual users are not restricted or made more expensive by a need to be compatible with other BiSS products. A BiSS subscriber is described with only a few parameters and an XML device description file that comes with the product simplifies the startup of the controller.
Output circuit and recommended input circuit for absolute encoders with a BiSS output.
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