Absolute encoders can communicate with the controller via parallel or serial communication, fieldbus or Ethernet-based protocols such as EtherCAT. Of these interface options, serial communication is a simpler solution than parallel communication (which requires a pair of twisted pairs of wires for each output) and is well suited to applications that are not so complex as to require fieldbus or Ethernet-based protocols. Here we will look at the four most common currentAbsolute EncoderDifferences between serial interfaces: SSI, BiSS, Hiperface DSL and EnDat 2.2.
SSI: Synchronous Serial Interface
As the name suggests, SSI is a synchronization protocol, which means that data is transmitted synchronously from the encoder to the controller via clock signals or pulses provided by the controller. The encoder output can be either binary or Gray code, with 1 bit transmitted per clock pulse, and a standard word length of 13 bits for single-turn encoders and 25 bits for multi-turn encoders.
Fig.1 The SSI encoder interface is simple, with only four wires for communication (twisted pair for data and twisted pair for clock signals) and two wires for power.
According to the RS-422 standard, the synchronous serial interface uses two pairs of twisted-pair wires for communication: one pair for differential data signals and one pair for differential clock signals. Two more wires are used to power the encoder. The clock frequency or data rate can be up to 1.5 MHz, the actual exact frequency depends on the length of the cable. To ensure data integrity, some SSI encoders support multiple transmissions (also known as "multipath" or "circular shift" transmissions), where the same data is transmitted multiple times and the controller compares the transmitted data to ensure they match.
Biss: Bidirectional synchronous serial interface
The Bidirectional Synchronous Serial Interface is an open protocol, similar to SSI, where the data transfer is synchronized by a clock signal from the controller, using the Biss protocol with clock rates up to 10 MHz.Biss still uses two pairs of twisted pairs of wires: one pair for the differential data signals and one pair for the differential clock signals. In addition two more wires are added for the power supply.
Unlike SSI, which only supports one-way communication, Biss supports two-way communication, which means that the controller can read and write to non-volatile memory in the encoder, where the registers contain encoder identification information.The Biss encoder can also send data, such as temperature, to the controller as required. Another unique feature of Biss compared to SSI is that the host determines and compensates for any transmission delays during each data cycle, allowing data rates of up to 10 Mbps.
The latest version of BiSS is BiSS-C (C = Continuly), which is often referred to simply as "BiSS".
Fig.2 BiSS allows bi-directional communication, so two wires are used to communicate with the controller (MA+ and MA-), and two wires are used to communicate with the encoder (SL+ and SL-), plus two power wires.
Unlike SSI encoders, Biss encoders can be connected point-to-point or over a bus. When connected via a bus, data from all encoders is clocked (synchronized) to the master in one continuous frame, rather than being transmitted individually.Biss also uses cyclic redundancy check (CRC) for error checking, which is a more reliable method than multiple transmissions. There is also a Biss safety interface for safety applications in accordance with the IEC 61508 SIL 3 standard.
Hiperface DSL
Hiperface DSL, the HIgh PERformance InterFACE Digital Servo Link, was originally a proprietary interface developed by SICK. However, in 2016, SICK "opened up" the interface through a licensing model, allowing other manufacturers to integrate the technology into their products.
Unlike its Hiperface predecessor, Hiperface DSL is an all-digital protocol that uses only two wires for bi-directional communication and encoder power, bundled with the motor power wires (although a transformer is required to improve common mode noise rejection). This has the advantage of eliminating the need for a separate encoder connection between the motor and the controller.Hyperfaces DSL is RS-485 compliant and has a data rate of 9.375 Mbaud.Data can be transmitted either cyclically (as fast as possible) or synchronized with the controller clock.
Fig.3 The Hiperface DSL includes channels for position feedback, parameter exchange, process data, safe position and condition monitoring (SensorHub) data, all of which are transmitted via two wires (which can be integrated into the motor cable).
The Hiperface DSL architecture also includes channels for the transmission of motor parameter data, condition monitoring data and integrated safety motions, with the data being transmitted over two digital communication lines. This redundancy and error checking enables the Hiperface DSL interface to comply with the SIL3 safety standard.
EnDat 2.2
HEIDENHAIN's encoder data or EnDat 2.2 interface is a synchronized bidirectional standard that uses four lines for communication: two lines for differential data and two lines for differential clock signals. In addition to two wires for the power supply and two wires for the battery buffer or parallel power supply, EnDat 2.2 can be supplied with a clock frequency of up to 2 MHz, and on some models, additional compensation for propagation delays makes frequencies of up to 16 MHz possible.
Since Hiperface DSL has become an "open" interface, EnDat is now the only one that remains proprietary.Absolute Encoderserial interface (it should be noted that the original Hiperface protocol also remains proprietary).
FIG.4 With transmitted delay compensation, EnDat 2.2 can provide clock frequencies up to 16 MHz.
EnDat 2.2. can also read, write or update information stored in the encoder and can transmit data (e.g. sensor information or diagnostic information) from the encoder to the controller. The type of data transferred (e.g. absolute position, diagnostic or parameter information) is sent from the controller to the encoder via mode commands. Like the Biss and Hiperface DSL, EnDat 2.2 complies with the SIL3 safety standard.
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