In the first part of this blog series, MarTIn Staebler provides an overview of the various motor position encoders and their interfaces. In the second part, I will explain the interface of the bidirectional/serial/synchronous (BiSS) position encoder.
BiSS is an open source agreement from iC-Haus. It defines a digital bidirectional serial interface for actuators and sensors such as rotary encoders or position encoders. (See more.) BiSS allows for serial synchronous data communication in one-way or two-way mode (called BiSS-C continuous mode). The BiSS interface is compatible with Serial Synchronous Interface (SSI) hardware.
The BiSS protocol defines each user/slave that enters the data portion access: the sensor data portion, the actuator data portion, the register data portion, and (if specified) the multi-cycle data portion. Depending on the access and transmission performance, each data section can have different settings - depending on the sensor application. In order to connect to a user/slave, the "BiSS Master" protocol should be predetermined, it can send data to the position encoder and can also receive data from the position encoder. The BiSS master is software and is implemented on a host processor such as a SitaraTM processor or a Field Programmable Gate Array (FPGA).
The BiSS interface has two physical layer (PHY) options, one based on the TIA/EIA-422 standard and the other using the LVDS TIA/EIA-644 standard. Typical interfaces are based on the TIA/EIA-422 standard.
BiSS has two different structural options: a point-to-point structure and a bus structure. In this article, the author will focus on the point-to-point structure. For more information on bus structure hardware, please refer to the design guide for the 5V BiSS Position Encoder Interface TI Designs Reference Design.
Today's encoders typically use a point-to-point architecture. When connecting a BiSS digital encoder with an RS422 or RS485 physical layer to a servo drive, I recommend using a shielded cable with a twisted pair as the core. Encoder cables typically have six or eight cores that can be used as signal and power lines, as shown in Figure 1. Cable lengths of 100 meters or more are not uncommon.
Figure 1: BiSS-C point-to-point structure
Figure 1 shows a typical BiSS configuration for a position or rotary encoder. In a peer-to-peer configuration, only one device (with one or more sensors) is connected to the primary station.
The MA clock frequency is variable. The recommended MA clock frequency depends on the cable length, as shown roughly in Figure 1. I use this diagram generated in Table 1 of the application manual "BiSS Interface AN15: BiSS C Master Station Operation Details".
Figure 2: Recommended BiSS MA Clock Frequency and Cable Length
When designed for the recommended frequency of the BiSS interface, the 10MHz MA clock frequency is converted to an RS422/485 transceiver capable of supporting 20MBaud. These are the minimum requirements. Tests using the 5V BiSS position encoder interface TI Designs reference design show that a faster transceiver will allow you to increase the cable length while still using the highest frequency of the protocol because the transceiver is sensitive to cable distortion noise Sex is not strong.
Power supplies for BiSS encoders usually need to support the parameters shown in Table 1, but you should confirm this data with your encoder supplier's product data sheet.
Table 1: General specifications for encoder power supplies
For power supplies, you will need to consider the range of voltages the encoder can support and the voltage drop of your cable. One option is to use a programmable power supply that can vary the voltage based on the length of the cable. View the TI Designs reference design for power supplies with programmable output voltage and protection for position encoder interfaces.
In the next article in this series (discussing the encoder interface), the author and his colleagues will provide detailed information on how to implement the industrial interface of the Endat 2.2 position encoder in accordance with EMC standards.
Other resources
Learn more about the TI Designs reference design for the 5V BiSS position encoder interface for hardware interfaces.
For an example of a BiSS master software for Sitara devices, see the ARM Microprocessor (MPU) TI Designs reference design with integrated BiSS C master interface.
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