The interface between controllers and traditional servo drives has traditionally been analog, displaying torque or speed. For years, this seemed like the obvious choice, but analog interfaces have limitations: multi-wire connections (at least 12 lines per drive for command, position feedback, enable, and error handling); susceptibility to electromagnetic interference; temperature drift; limited command resolution (typically 10-16 bits); lack of detailed diagnostic information; and time delays in data conversion (D/A and A/D). Currently, most controllers and drives operate digitally internally. While analog drive interfaces enable PC-based soft motion control, they don't fully utilize today's low-cost, high-performance digital technologies. A better approach is to achieve soft motion control, as shown in the diagram below: Digital technology allows the servo drive to retain all control loops and motor control laws. An AC servo system is shown in the diagram, but the architecture is not limited to any specific servo technology. The position command interface minimizes the controller's computational and network burden, ensuring the servo technology remains unaffected by the controller. The technology that eliminates the analog interface for soft motion and provides a standard interface is the Serial Real-time Communication System (SERCOS). This digital drive interface emerged in the mid-1980s and became the international standard IEC 61491 in 1995. SERCOS is the only motion bus standard, supported by over 120 controller and drive manufacturers worldwide. The SERCOS interface consists of a pair of fiber optic cables, connected digitally (drift-free, with 32-bit commands and feedback), fast (up to 4 Mbit/sec, soon to reach 16 Mbit/sec), and noise-free due to the use of optical media. Fiber optic connections replace hundreds of wires in multi-axis control systems. Low-cost plastic fiber supports point-to-point distances up to 130 ft and a maximum loop distance of 32,800 ft. A typical system feature is a position feedback cable running directly from the motor to the drive (see SERCOS topology diagram). SERCOS-based systems are used in high-performance applications, connecting up to 32 drives and one controller. (The SERCOS specification supports 254 drives and I/O stations.) Its communication protocol provides all drive parameters for synchronous transmission, enabling automatic setup, deterministic control, and intelligent diagnostics. SERCOS networks have relatively low bandwidth requirements because the controller-to-drive interface consists of simple position commands, typically refreshed every 1 or 2 milliseconds. However, to increase the precision of motion control, position commands are micro-inserted internally within the digital drive at a considerably high frequency. SERCOS also helps improve performance because it allows all loops to be closed within the digital drive. This minimizes time delays and maximizes the resolution of all drive parameters. Customers have found that SERCOS makes their machinery run faster and more accurately compared to traditional analog techniques. Furthermore, Windows NT is an acceptable standard for factory-level operating systems. Windows NT enables soft real-time (SRT) control, which operates in a timely manner. For some mechanical control tasks, such as operator interfaces and soft logic control, SRT control performs well.