Sensors - November 2003 - BiSS, an Open Digital Interface Standard for Smart Sensors

An open bidirectional synchronous-serial sensor interface—BiSS—is designed to network smart sensors without breaking the bank.

Various bus systems currently exist for networking intelligent sensors for measurement and automation systems, most of them capable of bidirectional controller communications (see Figure 1).

Figure 1.
BiSS Interface in Brief
Data length	Three different data sections: Sensor data: 0–64 bit* (measurement data, alarms, warnings) Multicycle data: 0–64 bit* (additional measurement data) Register data: up to 128 bytes per block (parameters)
Transmission and data rate	Synchronous-serial transmission via unidirectional RS-422 lines: 10 Mbps for sensor data 1-bit multicycle data per sensor data cycle 250 kbaud, transmitted bidirectionally, for register data
Data integrity	Secured by: CRC polynomials of up to 8 bits for sensor and multicycle data, 4-bit CRC for register addressing and data, all write accesses with bit return for verifying
Data capture	Triggered synchronously for all bus subscribers
Bus configuration	Up to eight bus subscribers, automatic addressing sequence
Cable length	Up to 100 m; an adaptive master compensates for delay times
Action calls	Activiation of preset functions, e.g., specific wake-up call for sections of the sensor which have been switched off
Compatibility	Interface can be switched over to SSI format
Initial operation	Supported by manufacturer ID and device ID in conjuction with a Direct Link Library (DLL; device description file in XML, for example)
*Introduced with current applications, but extendable.

Proprietary interfaces have the major market share in motion control and use a mix of analog and digital wires to enable high-resolution position capture. The non-proprietary all-digital BiSS interface opens up the market to other vendors, with reduced link costs for cables, connectors, and electronics. The BiSS interface concept embraces both the usual functions of existing sensor interfaces and the new market demands made by a diverse range of sensor applications, such as time-synchronous data capture for all sensors connected to the BiSS line, speedy data transmission with integrity verification, support for fast control cycles, and plug & play capability.

Market Demands and Costs
To network sensors within a single machine, car, or control unit, you need interfaces with low connection costs, since outlay for hardware and connectors can often make a large hole in the overall budget. You also want to minimize the number of wires to keep cabling costs low.

These requirements also apply to systems such as the CAN bus, Interbus S, or AS-i; at this level connection costs are usually more than $3.00 per node. In simple sensor applications the expense for networking often exceeds that of the sensors themselves. The costs involved in networking sensors divert attention from the real task—the temporally precise acquisition and fast transmission of measurement data.

The following market requirements, among others, have been taken into account in the protocol definition for the new BiSS interface:

The synchronous-serial BiSS interface is already providing many applications with a real alternative to proprietary interfaces and supports “embedded” solutions through simple integration. With regard to transmission performance and security, BiSS can cope with digital drive controls with cycle times of <10 µs.

What Is BiSS?
The bidirectional, synchronous-serial sensor interface (BiSS) is basically a serial communication protocol developed by iC-Haus in Germany and made available to manufacturers of machines and control systems free of charge as common intellectual property. Those interested can download protocol and VHDL descriptions from www.biss-interface.com to implement in field-programmable gate arrays (FPGAs) or microcontrollers for their own applications. As with CANopen, semiconductor manufacturers can license the interface; technical support is included. Alternatively, iC-Haus offers ICs containing BiSS interfaces, e.g., the ADC iC-NQ for sine-cosine interpolation that provides an absolute angle value and 24-bit multiturn position via BiSS. Opto-sensors for absolute angular encoder devices are also found in products now on the market (see “Danaher Chooses BiSS”). Communication via BiSS is entirely digital and based on a synchronous master-slave structure, keeping the protocol simple (see Figure 2).

Figure 2.
BiSS Interface in Comparison
	SSI	EnDat 2.1	Hiperface	BiSS Interface
Connection	point-to-point	point-to-point	bus¹ or point-to-point	bus or point-to-point
Transmission mode (digital)	unidirectional, synchronous	bidirectional, synchronous	bidirectional, asynchronous	bidirectional, synchronous
Sensor data transmission	up to 1.5 MHz²	up to 2 MHz², plus analog	38.4 kbaud, plus analog	up to 10 MHz²
Protocol length adjustable	yes	yes	no	yes
Multicycle data protocol available	no	no	no	yes
Number of directional lines	4 unidirectional	2 bidirectional, 2 unidirectional	2 bidirectional	4 unidirectional
Number of analog lines	none	4³	4	none
Multi-slave synchroni zation	no	no	yes	yes
Alarm/ warning bits	definable	yes	no	definable
Plug & play (autocon- figuration)	no	yes	yes	yes
¹ Digital lines of parameter channel only; ² Limits depending on line driver and length of line; ³ Optional; Hiperface is a trademark of Max Stegmann GmbH.

Hardware. The minimum configuration (see Figure 3) consists of a microcontroller, BiSS master, and sensor. Figure 3 shows a point-to-point connection requiring only six wires—two power supply lines, one of which links the remote power supply to the sensor, and two lines each for sensor and clock data. Differential transmission makes the interface fairly immune to environmental interferences; two unidirectional RS-422 line drivers are sufficient here, although 24 V line drivers are also feasible.

Figure 3. Two unidirectional lines are sufficient for the BiSS interface. The twisted pair lines are each served by a single RS-422 driver and receiver device per side.

Depending on the microcontroller’s duties, it can also simulate the protocol via SPI ports for applications that aren’t time critical.

BiSS Interface Master. The BiSS interface master can be programmed in an FPGA using the VHDL code supplied or delivered as a ready-to-use IC. Using an evaluation board, you can assemble a target system for testing with a microcontroller.

The logic has a minimum of 5000 gate functions for a BiSS interface master in its simplest form. With multimasters (a special master with multiple data return lines to feed back sensor data in parallel) the possible number of slaves can be higher; the parallel sensor data enable more rapid evaluation. A larger port width for 32-bit microcontrollers is also possible here. Figure 4 shows an example of networking using BiSS.

Figure 4. This is a networking example using the BiSS interface master iC-MB3. The two BiSS devices shown here make up a serial BiSS link via separate input and output connectors, each operated by RS-422 receivers and drivers. A device can also contain a number of different sensors as well as allocate several sensor IDs (SIDs) if, for example, further physical properties are to be recorded (pressure, temperature, etc.).

A Simple Protocol. For one to eight subscribers the BiSS master supplies a clock signal for simultaneous capture of all measurement data and its synchronous-serial data transmission. As shown in Figure 5 when the master sends a clock pulse on line MA, the

Figure 5. The BiSS interface line signals on clock line MA and data return line SL. The upper traces represent a high speed transmission of sensor data, the lower traces feature the register mode used during device programming and sensor calibration.

slave answers directly on return line SL with the recorded data (sensor mode). Commands and parameters can be swapped in PWM pulse form (register mode), but this is not necessary to start the protocol.

With each data cycle the master learns and compensates for the line delay, permitting clock rates of up to 10 Mbps even for cable lengths of up to 100 m. Changes in line conditions that occur during cable drag, for example, are corrected. For several position encoders operating along various axes, data capture within the encoders is synchronized to <1 µs.

The BiSS protocol classifies each subscriber in one of the following data sections: sensor data, multicycle data, or register data. These data sections have various setups with regard to access and transmission performance to cater to a number of different sensor applications.

Rapidly changing angle data are assigned to the sensor data area. Framed by one start and one stop bit, sensor data are transmitted at the best-possible core data rate; a single multicycle data bit is optional. Data that alter gradually, such as revolution counts or drive temperatures, are allocated to the multicycle data area. The multicycle data bits make up a second in-band protocol which helps increase the efficiency of the sensor data; permanent monitoring of the position and operation of the drive is possible without interfering with the control cycle. Bidirectional parameter communications for device configuration—also applicable to OEM parameters—is usually consigned to the register data section. As a result, control cycle times of <10 µs are not a problem, even for data words up to 64 bits. There is enough room in the protocol for redundancy and this space is normally used to implement a CRC (cyclic redundancy check).

Register Configurations. The bus protocol enables a second form of communication: register mode. If, following the first falling clock edge, the master signal remains low beyond the set timeout_sens (the bus is idle), the slaves switch to register mode and trigger the master to send PWM-coded signals.

In register mode the registers in each slave can be written and/or read out. BiSS has a special addressing sequence in which 3 bits are used to address the slave and 7 bits to address the register. A 4-bit CRC code is used for address and data verification.

Once the required slave has been triggered with a start address the register data—also CRC monitored—are supplied by the master for write access as a PWM-coded signal. For verification, the slave returns the data it has received (uncoded this time) to the master, bit by bit. An auto-increment function within the slave enables several consecutive registers to be written and/or read without a new addressing sequence being necessary. Read access follows the same pattern as above, omitting the PWM-coded master data after the start address.

If a sensor has more than 128 registers, the sensor can allocate to itself several blocks of 128 registers by accessing several slave IDs. By the protocol definition, only 128 bytes can be addressed per slave ID; if a sensor contains more ROM or RAM, it simply acknowledges more slave IDs.

BiSS Applications
BiSS allows you to send data from smart sensors (which have their own intelligence to condition and process measurement data) in digital form to a master, for example. The advantages for the user lie in the considerable reduction in development and production costs and in improved reliability in tough, noisy industrial environments. BiSS also removes the need for the complex filtering of transmitted analog sensor signals.

Because the receiver device configures itself almost automatically (e.g., on the basis of an XML device description), it removes a time-consuming manual setup step when the device is first installed. The master can also recognize and, if required, initialize a

BiSS interface Support
support@biss-interface.com
www.biss-interface.com

number of different sensors contained in any one product range. Electronic systems that involve large numbers of sensors to be connected over a short distance lend themselves to digital networking with BiSS. Examples of these include all types of machine and drive control units, measurement and automation systems, robotics, monitoring systems, printers, and printing machines, including photocopying apparatus.

Analog (or digital) smart sensors can measure pressure, acceleration, position and angle, temperature, inclination, color, flow, distance, shock, and weight; BiSS lets you transmit these data as digital signals.

In Conclusion
BiSS is particularly well suited for networking all types of smart sensors to the microcontroller. The high data rates possible, coupled with synchronous data capture in several sensors, also enables BiSS to be used in positioning systems and fast control systems. The low cost of the hardware and the availability of a VHDL code enables the interface to be quickly and easily integrated into all intelligent sensors.

Danaher Chooses BiSS

Martin Haller, Danaher Industrial Controls Group

When Danaher Controls started work on its new ACURO line of absolute encoders, it listened to its customers. ACURO would be made in two versions: one for machinery applications, providing mostly position information, and one for motor feedback applications providing both absolute position and speed data. For communications, the decision had already been made to use a serial interface, but how to choose among the available options?

There was a strong customer demand for digital feedback of absolute position in motor feedback applications, but all the absolute multiturn encoders on the market used analog signals (sin/cos) for speed information. Analog signals are susceptible to noise and are subject to distortion when transmitted long distances. Changing encoders from one manufacturer to another can cause problems because of differences in signal amplitude and offset.

The company worked with integrated circuit maker iC-Haus, which developed the new BiSS interface and appreciated that the new interface should be open and nonproprietary, since a proprietary interface would restrict customers’ ability to choose among manufacturers.

ACURO was the first product on the market that used the BiSS interface, but it won’t be alone for long. Another encoder manufacturer showed an absolute encoder with the BiSS at the SPS/IPC/Drives show in Nuremberg last November; there are about eight other companies signed up to use BiSS, and new ones appear on a regular basis.

Danaher didn’t restrict ACURO to just BiSS, of course, and made it compatible with Profibus, DeviceNet, CANopen, CAN Layer2 Interbus, and SSI. The encoder also provides a 2048-step sine/cosine signal (1 Vpp) for use with legacy systems.

BiSS provides other advantages for the ACURO. Since BiSS is bidirectional, it allows you to set the zero point of the encoder electronically, speeding up installation. In addition, BiSS’s ability to carry additional information tied in nicely with ACURO’s built-in diagnostics and data reporting capability. n

Martin Haller is Manager, Product Marketing for Danaher Industrial Controls Group, Gurnee, IL; +49 7424-89317, martin.haller@hengstler.de.