Encoder Interface Protocols
The output of an incremental encoder is a stream of pulses
on one or more channels, while the output of an absolute encoder is a multi-bit
word. To interpret discrete position, speed and other information from this
multi-bit word, both encoder and control system must speak a common language.
architecture choice will typically dictate communication protocol. Dynapar
offers several absolute encoder interface options to meet the needs of system
builders, listed below.
Parallel Encoder Output
Parallel encoder output makes all output bits available simultaneously. It may be provided as straight binary or transformed into gray code. Gray code produces only a single-bit change at each step, which can reduce errors. Learn more about gray code encoders here
Serial Output / Point to Point Encoder Protocols
The alternative to parallel encoder communication is serial. There are several dedicated serial buses available, as well as standard industrial buses. Tradeoffs among these include bandwidth, update rate, hardware requirements, proprietary vs. nonproprietary nature, and availability.
SSI Encoder Protocol: SSI encoders offer an all-digital point-to-point encoder interface with speeds up to 1.5 MHz, depending on cable length. Output options include SSI binary, SSI gray, SSI extended and SSI with Sin-Cos 1Vpp for real time control. Learn more about SSI encoders here
BiSS Encoder Protocol: BiSS is an open source point-to-point encoder interface. BiSS encoders can send full absolute position data whenever the controller polls the encoder, not just at startup allowing for real time control without additional Sin/Cos 1Vpp output. Learn more about BiSS encoders here
Industrial Bus Encoder Interfaces
While point-to-point communication protocols require one slave per master, industrial busses allow multiple salves to be connected to one master. For feedback applications, there are several general-purpose industrial buses that are most commonly used with absolute encoders including DeviceNet, Profibus, Interbus and CAN.
Profibus Encoder Interface: Profibus boasts baud rates to 12Mbps with a maximum segment length of 327 ft and features programmable resolution, preset, and direction. Profibus encoders can communicate speed, direction, and on-time diagnostics.
DeviceNet Encoder Interface: Encoders with DeviceNet have programmable scale, direction, and preset with position data. DeviceNet’s basic trunkline-dropline topology provides separate twisted-pair wires for both signal and power distribution, enabling 24 VDC encoders to be powered directly from the bus. The 0 to 8 byte data packet is ideal for low-end devices with small amounts of I/O that must be exchanged frequently.
Interbus Encoder Interface: Designed by Phoenix Contact in the mid 1980s, Interbus is the longest-standing open industrial network. Interbus is divided into two buses. The remote bus is an RS-485 transmission medium with length capabilities up to 13km. The local or peripheral bus enables connection of up to eight Interbus encoders within a 10m range.
CANOpen Encoder Interface: The CAN network (Controller Area Network) was first used in the automotive industry in the 1980s. The current CAN L2 and CANOpen protocols feature baud rates up to 1Mbps. CANOpen has programmable preset and resolution with L2 having programmable direction and limit values. Both protocols offer speed, acceleration, position, and limit data.
For many of industrial bus protocols, the encoder transmits absolute position only when interrogated by the controller. In others, the encoder sends incremental data as it turns, and provides absolute data either on startup or when interrogated. These methods cut down on bus traffic, but also mean that the control system must know enough to demand an absolute position update whenever there’s a momentary glitch. The update may take only a few milliseconds, but it’s a factor to keep in mind when designing a control system.
Ethernet Based Encoder Protocols
Ethernet based protocols represent the latest in industrial communication networks and overcome the speed limitations of older protocols while offering greater device support for the Internet of Things. For example, an EtherCAT network offers support for up to 65,535 devices.
EtherCAT Encoder Protocol: The EtherCAT protocol is optimized for cyclic process data with the integration of a chip in each slave device, instead of a TCP/IP stack as with Ethernet. EtherCAT encoders have fast cycle times and are ideal for high end speed and position control applications that require real time data with deterministic response times.
Profinet Encoder Protocol: The next evolution of Profibus, Profinet has become one of the leading one of the leading Ethernet based protocols on the market. Profinet allows devices or modules to be added and swapped on the fly without stopping the entire machine and is ideal for building highly flexible, modular machines.