What Is an EnDat Signal? EnDat 2.1, EnDat 2.2 and Encoder Feedback

EnDat is HEIDENHAIN's digital bidirectional interface for position encoders. It is used with rotary, angle and linear encoders where a drive or control system needs precise measurement feedback. The interface primarily transfers position values, but supported devices can also exchange parameters, identification information, diagnostics and additional sensor data. ^[1][2]

That makes EnDat more than a digital replacement for a pulse signal. In a servo system, it forms part of the feedback channel between the encoder and the drive electronics. The controller can calculate velocity from successive position samples. The encoder does not automatically become a torque sensor simply because the feedback link is digital.

What Actually Happens on an EnDat Link?

An EnDat connection has a master on the control side and an encoder on the measurement side. The master may be integrated into a servo drive, CNC controller or dedicated encoder interface. It provides the CLOCK and selects the required transaction. The encoder responds with the requested position or other supported information. This command-and-response capability is why EnDat is described as bidirectional. ^[1][2][5]

It is useful to teach the link as a stream of ones and zeros, but a real transaction is more structured than “one clock pulse equals one position bit.” The master issues a mode command and the reply is delivered as a defined data frame. Depending on the transaction, the frame can contain position information, status information and error checking. CRC protection helps the receiver detect corrupted communication. ^[2][5]

Does EnDat Use Only Two Wires?

No. Two logical signal functions must not be confused with two physical conductors. A serial EnDat 2.2 interface uses four signal conductors: a differential CLOCK+ / CLOCK− pair and a differential DATA+ / DATA− pair. Encoder power is additional. A real shielded feedback cable can therefore contain more conductors than the four communication lines. ^[2][4]

CLOCK is generated on the master side. The DATA path is bidirectional, allowing the master to send commands and the encoder to return data on the serial link. Differential transmission gives the interface strong noise performance when the complete installation is designed correctly. It does not mean the cable is immune to every EMC problem. Shielding, termination, grounding, cable construction and signal timing still matter. ^[2][4]

The binary sequence is illustrative. Real EnDat word length and framing depend on the encoder's position format and the command being executed. The graphic is intended to explain synchronized serial transfer; it is not an oscilloscope trace or an electrical-level specification.

What Can EnDat Carry Besides Position?

Position remains the central measurement. With a suitable absolute encoder, the control receives a position value representing the measured location. EnDat 2.2 can also carry supported diagnostic information, encoder parameters, identification data, temperature information and other additional data. Certain safety-related configurations can transfer a second position value as part of their architecture. ^[2][3]

Resolution is not the same as accuracy. A large position word can provide very fine digital increments, while mechanical and sensing errors still determine how accurately those increments represent the real shaft or axis position. Bit count alone is therefore not an accuracy specification.

On a rotary axis the measured position represents angular position. Velocity is typically derived by the drive or control system from how that position changes with time. This distinction matters when specifying a feedback system: position measurement and speed calculation are closely related, but they are not the same physical measurement.

EnDat 2.1 and EnDat 2.2

EnDat 2.1 established the earlier generation of the interface. EnDat 2.2, introduced in 2003, moved the system toward purely serial operation and expanded the handling of additional information and diagnostics. HEIDENHAIN designed EnDat 2.2 with continuity toward EnDat 2.1 in communication, commands and timing. ^[1]

Do not infer the incremental-signal arrangement from the words “EnDat 2.2” alone. Some encoder variants combine EnDat 2.2 with separate 1 Vpp incremental signals. A HEIDENHAIN EnDat02 product is one example. Other models identified as EnDat22 operate without separate incremental signals. ^[6][7]

This is why EnDat and Sin/Cos are not automatically competing alternatives. A device can provide a serial EnDat interface and a separate sinusoidal incremental channel. The serial DATA signal itself must not be described as a 1 Vpp Sin/Cos signal.

EnDat Compared with Other Encoder Feedback

Incremental A/B/Z: A conventional incremental encoder reports motion through periodic channels. The controller can use pulse information to calculate movement, direction and speed. EnDat instead transfers formatted serial position information.

TTL and HTL: TTL and HTL generally describe electrical levels used by incremental feedback. They do not describe a protocol comparable to EnDat. Likewise, a differential output describes a signaling method rather than the complete application protocol.

Sin/Cos: A high-resolution analog encoder may provide sinusoidal incremental signals for interpolation. Those signals must not be confused with the EnDat serial DATA pair.

SSI: SSI also uses a control-generated clock and serial DATA, but its traditional role is the synchronous transfer of an absolute position word from the encoder. EnDat adds a bidirectional command structure that supports parameter access and richer diagnostics.

Resolver: A resolver is an electromagnetic analog position transducer, not a serial digital protocol. It needs suitable resolver excitation and evaluation electronics. The resolver-versus-encoder guide covers the distinction in more detail.

The broader encoder output guide is useful here because measurement principle, position format, electrical signaling and communication protocol are separate design choices. Calling every non-EnDat encoder “traditional analog” hides those distinctions.

Why EnDat Is Useful in Servo Feedback

A servo motor control loop needs reliable rotor or axis position information. An EnDat encoder provides that measurement feedback to compatible drive electronics. The drive can then use the position stream for position control, velocity estimation and, where the motor design requires it, commutation-related processing.

The wider servo feedback system therefore involves more than encoder resolution. Control-cycle time, interface compatibility, mechanical coupling, encoder accuracy and drive configuration all affect the final motion-control performance.

Can an EnDat Encoder Be Wired Straight into a PLC?

A normal PLC input is not automatically an EnDat master. Even though EnDat is digital, the controller must implement the required serial protocol and electrical interface. A standard high-speed counter is appropriate for certain pulse encoders; it does not decode EnDat by counting DATA transitions.

Where the controller has no native EnDat input, use a compatible drive, EnDat master or encoder interface unit. Verify the supported EnDat version, required commands, position format and additional-data requirements rather than assuming that a physical connector provides protocol compatibility.

Cable Length, EMC and Clock Rate

Differential signaling helps EnDat operate in electrically demanding drive systems, but the communication channel still has physical limits. Cable delay, shielding, termination, cable construction and clock rate need to be considered as one system. EnDat 2.2 implementations can use propagation-delay compensation to support demanding timing requirements. ^[2][4]

Keep encoder-feedback installation practice separate from protocol marketing. A robust serial protocol cannot repair a damaged connector, incorrect shield connection or unsuitable cable route. Digital transmission improves error tolerance; it does not remove the need for sound EMC engineering.

Selecting a Compatible EnDat Encoder

First specify the mechanical measuring task: rotary or linear, singleturn or multiturn, absolute position requirements, shaft or mounting arrangement, operating speed and environment. Then evaluate resolution and actual accuracy separately.

Next verify the interface: EnDat 2.1 or 2.2, serial-only or combined with an incremental channel, connector and supply arrangement, and exact compatibility with the drive. The word EnDat on two data sheets is not enough to prove that two devices can simply be plugged together.

The sensing technology can also matter. The choice between optical and magnetic encoders is a different decision from choosing EnDat as the feedback interface.

Troubleshooting an EnDat Link

No communication: verify encoder supply, connector pinout, differential CLOCK and DATA pairs, supported EnDat version and whether the receiving equipment actually contains an EnDat master.

Intermittent communication: inspect the cable, shield, connector, termination, grounding and clock configuration. A fault that changes with cable movement or clock rate should not be treated as a software problem by default.

Position is received but incorrect: check the encoder's position word length, scaling, datum offset, direction convention, singleturn/multiturn configuration and drive parameterization.

The system works only at a lower clock rate: investigate signal integrity and propagation delay. Raising the clock frequency without checking the complete transmission path can reduce reliability rather than improve the machine.

Functional Safety Requires More Than the Protocol Name

HEIDENHAIN offers EnDat 2.2 encoder solutions intended for safety-related applications, including architectures supporting applications up to SIL 3. That capability belongs to the complete specified safety chain. An EnDat label alone does not make an ordinary encoder-and-controller combination a SIL 3 system. ^[3]

Technical References

[1] HEIDENHAIN — EnDat 2.1 and EnDat 2.2 · [2] HEIDENHAIN — EnDat 2.2 Encoder Characteristics · [3] HEIDENHAIN — EnDat Functional Safety

[4] Texas Instruments — Four-Wire RS-485 Interface for EnDat 2.2 · [5] Texas Instruments — EnDat 2.2 Master Communication

[6] HEIDENHAIN — EnDat02 with 1 Vpp Incremental Signals · [7] HEIDENHAIN — EnDat22 without Incremental Signals