An absolute encoder answers the question of where the shaft is with a single, unique value at any instant, without needing to have watched the shaft move. Every position corresponds to a distinct code, so the moment the encoder powers up it already reports the true position, no counting from a reference required. That is the key contrast with an incremental encoder, which only tracks change and forgets everything on power loss. This page contrasts absolute with incremental encoders, explains single-turn versus multi-turn designs, shows why absolute feedback eliminates homing, and describes the serial protocols, BiSS, EnDat, and SSI, that carry the position code.
Absolute Encoder in one line: An absolute encoder reports a unique digital code for every distinct shaft position, so it always knows the true position even at power-up, with no homing required. Single-turn versions give a unique position within one revolution, while multi-turn versions also count how many revolutions have occurred. Serial protocols such as BiSS, EnDat, and SSI transmit the position code to the controller.
The fundamental difference is what the device actually reports. An incremental encoder outputs pulses as the shaft moves, and the controller counts those pulses to track how far the shaft has traveled from wherever it started. It measures change. An absolute encoder instead reports a distinct code for each physical position, so reading it at any moment directly yields the position itself, with no dependence on having observed the motion that led there.
This changes the behavior across a power cycle in the most important way. When an incremental encoder loses power, the accumulated count is gone, and on restart the controller has no idea where the shaft is until the axis is homed. An absolute encoder retains its knowledge of position because each position simply has its own code; power it back up and it immediately reports the correct, true position exactly as it was, whether or not the shaft moved while power was off.
That retention is the property most applications are buying when they choose absolute feedback. It removes an entire category of startup ritual and an entire category of risk, the risk of operating on a position count that was never properly anchored. For machines that must know their true position the instant they wake up, tools left mid-cut, axes that could be pushed while unpowered, safety-relevant positions, absolute feedback is the natural fit.
A single-turn absolute encoder provides a unique code for each position within one revolution of the shaft. It tells you exactly where the shaft is angularly, but once the shaft passes a full turn the code repeats, so a single-turn device alone cannot distinguish one revolution from the next. That is perfectly sufficient for a rotary joint that never travels more than one turn, or where the control system tracks turns by other means.
A multi-turn absolute encoder adds a count of how many full revolutions the shaft has made, on top of the single-turn position within the current revolution. This gives a unique position across a large range of travel, essential for a lead-screw axis or a geared joint that makes many turns to move the load through its range. Multi-turn revolution counting has historically been maintained through power-off by a backup battery or by a gear mechanism, and some designs use energy-harvesting techniques to count turns without a battery; the important point for a user is confirming that the multi-turn count survives power loss the way the design intends.
The headline operational benefit of absolute feedback, single-turn or multi-turn, is that homing is not required. Because the encoder reports true position immediately at power-up, the machine can begin working without first running an axis to a reference to establish zero. That saves startup time, removes the wear and risk of homing moves, and lets a machine recover its exact state after a power interruption. For equipment that must resume precisely where it stopped, this is a decisive advantage over incremental feedback.
Because an absolute encoder outputs a multi-bit code rather than simple pulse trains, it transmits that code over a serial digital interface. Several protocols are common in industrial servo systems. SSI, the synchronous serial interface, is a long-established, straightforward way to clock the absolute position word out of the encoder. EnDat and BiSS are bidirectional serial protocols that carry not only position but additional information, and they allow the controller to read encoder parameters and diagnostics, which SSI in its basic form does not.
These bidirectional protocols matter because they let the encoder report more than position. Diagnostic and status information, and in many cases the encoder's own identification and calibration data, travel over the same link. That means the control and drive system can verify it is talking to a healthy, correctly configured encoder, and can surface warnings if the device reports a degraded condition, all through the digital interface rather than requiring separate wiring or inspection.
For monitoring, absolute feedback and its serial diagnostics fit naturally into a SCADA picture. Because the position is trustworthy from power-up, a monitoring layer can rely on reported axis positions without worrying whether the axis was homed. A cloud SCADA platform such as Merobix can collect these positions along with any encoder diagnostics the drive exposes, giving operations of remote or distributed machines confidence that a machine reporting its position after a power event is reporting the real one, and flagging encoder health warnings before they become faults.
An incremental encoder outputs pulses as the shaft moves and the controller counts them, so it tracks change and loses position on power loss, requiring homing. An absolute encoder reports a unique code for each position, so it always knows the true position and retains it through power cycles, with no homing needed.
A single-turn absolute encoder gives a unique code for each position within one revolution, but the code repeats each turn. A multi-turn encoder also counts how many full revolutions have occurred, giving a unique position across many turns. Multi-turn is needed for lead-screw or geared axes that rotate many times across their travel.
Absolute encoders send a multi-bit position code over a serial digital interface. Common protocols include SSI, a long-established one-way synchronous interface, and the bidirectional protocols EnDat and BiSS, which carry position plus diagnostics and encoder parameters. The bidirectional protocols let the controller read encoder health and configuration, not just position.
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