SSI

The Synchronous Serial Interface (SSI) is a widely adopted communication protocol used in industrial automation systems for transmitting position data from absolute encoders to controllers, drives, or other host devices. It is particularly prevalent in applications that require reliable and robust data transfer in harsh industrial environments. 

The SSI communication process follows these steps: 

1. The master device (controller or drive) initiates the data transfer by sending a series of clock pulses to the slave device (encoder). 

2. The slave device responds by transmitting one bit of the position data for each clock pulse received. The data is typically transmitted in a predefined format, such as binary or Gray code, with a fixed number of bits (e.g., 24 or 25 bits). 

3. The master device receives and processes the incoming data bits, reconstructing the complete position value based on the predefined data format. 

4. Optionally, the SSI protocol may include additional features like parity bits for error detection or support for multi-turn absolute encoders, which can track multiple revolutions. 

While SSI is a robust and widely used protocol, it is important to note that it has limitations in terms of data transfer rates and distance. However, for many position feedback applications in industrial automation, SSI provides a reliable and cost-effective solution. 

Synchronous Communication

SSI relies on an external clock signal generated by the master device (controller or drive). The clock signal ensures precise timing for data transmission, with each bit of data being sent in perfect synchronization with the clock pulses. This synchronization minimizes the risk of data misinterpretation due to timing errors and enhances data integrity. 

Serial Data Transmission

Information is sent sequentially, one bit at a time, along a single data line. This method contrasts with parallel transmission, which sends multiple bits simultaneously across several channels. The serial approach reduces wiring complexity and cost, facilitates easier installation and maintenance, and is less susceptible to electromagnetic interference (EMI). 

Unidirectional Data Flow

SSI operates in a master-slave configuration where the master device controls the data transfer by generating clock pulses. The slave device (encoder) responds by transmitting one bit of data for each clock pulse received. This unidirectional data flow simplifies the communication protocol and ensures reliable data transfer. 

Absolute Position Data

SSI is primarily used with absolute encoders, which provide the current position value immediately upon power-up without the need for initialization or reference homing procedures. This feature is crucial for applications requiring precise and immediate position feedback. 

Data Transmission Rates

SSI can achieve data transmission rates, depending on the cable length and the data frame to be transmitted, of up to 10 Mbit/s over distances up to 1200 meters, making it suitable for most industrial applications. 

Simple Wiring Requirements

A typical SSI setup requires only four wires for data transmission: two for power (V+ and Gnd) and two twisted pairs for the clock (C+ and C-) and data (D+ and D-) signals. This simplicity in wiring reduces installation costs and the risk of incorrect wiring. 

SSI finds widespread applications in industrial automation and communication due to its simplicity, reliability, and robustness. Here are some detailed explanations of its key applications: 

Position Feedback in CNC Machines and Precision Positioning Systems

One of the primary applications of SSI is in providing precise position feedback for CNC (Computer Numerical Control) machines and other precision positioning systems. These systems require highly accurate and real-time position data to ensure precise control of tool movements, cutting operations, and other critical processes. 

In such applications, SSI absolute encoders are directly coupled to the motor shafts or linear positioning mechanisms. The SSI interface allows the encoders to transmit their absolute position data to the CNC controller or motion control system in a reliable and noise-immune manner. This position feedback is crucial for maintaining tight tolerances, minimizing errors, and achieving high-quality machining or positioning results. 

Robotic Control Systems

Robotic systems, particularly those used in industrial automation, rely heavily on accurate position and motion control. SSI plays a vital role in these applications by enabling robust and precise communication between the robotic arm's encoders and the control system. 

The SSI interface ensures that the robotic controller receives reliable position data from the encoders mounted on each joint or axis of the robotic arm. This real-time position feedback is essential for coordinating the complex movements of the robotic arm, ensuring smooth and accurate operation, and preventing collisions or positioning errors. 

Motor Control and Servo Drives

SSI is widely used in motor control applications, particularly in conjunction with servo drives and motion control systems. Servo drives require precise position feedback from encoders to accurately control the speed, position, and torque of the motor. 

SSI absolute encoders are often directly mounted on the motor shaft, providing the servo drive with real-time position data through the SSI interface. This position feedback is critical for closed-loop control algorithms, enabling the servo drive to make continuous adjustments and maintain precise motor control, even in demanding applications with varying loads or environmental conditions. 

Industrial Automation Systems

In various industrial automation systems, such as those used in manufacturing, packaging, and material handling, SSI plays a crucial role in enabling reliable communication between encoders and programmable logic controllers (PLCs) or other control systems. 

SSI absolute encoders are used to monitor the position of conveyor belts, robotic arms, packaging machines, and other automated equipment. The SSI interface allows the encoders to transmit their position data to the PLC or control system, which can then make decisions and issue commands based on this real-time feedback, ensuring smooth and coordinated operation of the entire automation system. 

Building Automation and Elevator Control

While SSI is primarily associated with industrial applications, it also finds use in building automation systems, particularly in elevator control. Elevator systems require precise position feedback to ensure accurate floor leveling and smooth operation. 

SSI absolute encoders are often used in elevator systems to provide real-time position data to the control system, enabling it to accurately control the elevator's movement, stop at the correct floor level, and ensure safe and comfortable operation for passengers. 

Minimal Wiring Requirements

SSI requires only four wires for data transmission: two for power (V+ and GND) and two twisted pairs for the clock (C+ and C-) and data (D+ and D-) signals. This simplicity in wiring reduces installation costs, complexity, and the risk of incorrect wiring. 

Fewer Components

The SSI protocol uses a minimal number of components, such as encoders and interface circuitry, making it a cost-effective solution compared to more complex communication protocols. 

Synchronous Communication 

SSI operates in a synchronous manner, where the master device generates a clock signal, and the slave device (encoder) responds by transmitting one bit of data for each clock pulse received. This synchronous operation ensures reliable data transfer without the need for complex handshaking mechanisms. 

Differential Signaling

SSI employs differential signaling for the clock and data lines, which enhances immunity to electromagnetic interference (EMI) and noise, common challenges in harsh industrial environments. Differential signaling allows for longer cable lengths without loss of signal quality. 

Efficient Data Transfer

Despite being a serial protocol, SSI can achieve high data transmission rates, depending on the cable length and the data frame to be transmitted. The protocol supports transmission rates of up to 10 Mbit/s over distances up to 1200 meters, making it suitable for most industrial applications. 

Industry Standard

SSI is a widely adopted standard in the industrial automation industry, ensuring compatibility with a wide range of controllers, drives, and other devices from various manufacturers. This compatibility facilitates seamless integration and interoperability within industrial systems. 

Absolute Position Data

SSI is primarily used with absolute encoders, which can provide the current position value immediately upon power-up, without the need for initialization or reference homing procedures. This feature is crucial for applications requiring precise and immediate position feedback. 

Robustness

The synchronous and unidirectional nature of SSI, combined with differential signaling and error detection mechanisms, contributes to its robustness and reliability in harsh industrial environments with potential noise, vibrations, and temperature variations. 

As one of the world's leading manufacturers of hardware and software for industrial communication, Hilscher offers a wide range of industrial communication interfaces. The basis for this is provided by the company's multiprotocol-capable netX chip technology, which serves as the fundamental basis for the portfolio and has integrated SSI interfaces. 

The multiprotocol SoCs can be integrated into automation components as required and their extensive chip peripherals enable powerful, efficient and flexible solutions. A protocol change is achieved by simply reloading Hilscher's own netX firmware. Building on this, the company also offers embedded modules and PC cards in all form factors in order to realise the netX communication interface with less integration effort. 

Hilscher also offers a comprehensive managed industrial IoT range under the netFIELD brand. This ranges from edge gateways as an application-oriented computer platform with integrated container management and the Edge OS Runtime running on it to the central cloud portal, via which the docker containers are deployed to the edge devices, through to turnkey containers for communication applications. 

Gateways and switches, devices for network diagnostics as well as masters and bridges for the wireless connection of IO-Link sensors round off the automation portfolio.