Encoder Output Types: Quadrature, TTL, HTL, Differential and Serial Interfaces

An encoder output defines how a control system receives motion or position information. Labels such as A/B/Z, TTL, push-pull and EnDat describe different layers: channel arrangement, electrical levels, output circuitry and communication protocol. One encoder can legitimately combine several of them. ^[1,2,9]

For example, “incremental A/B/Z with 5 V TTL/RS-422 outputs” is a coherent specification. Whether it uses optical or magnetic sensing is a separate question. Neither the sensing technology nor a familiar connector proves electrical compatibility. ^[2,5]

Start with the information: A, B and index

A quadrature incremental encoder provides A and B channels separated ideally by 90 electrical degrees, or one quarter of a waveform cycle. Their transition order identifies direction. This is not necessarily 90 degrees of shaft rotation. A single pulse channel can report movement, but cannot on its own distinguish both directions. ^[3]

Z, index or reference commonly supplies one pulse per revolution on rotary encoders equipped with that channel. Pulse width and alignment with A/B depend on the model; index is not universal. Linear encoders can have different reference-mark arrangements. The controller decides whether an index event resets, captures or otherwise affects its count. ^[1,3,5]

Explore three different signal views

The example uses four A-channel cycles per shaft revolution. The differential view exposes the complementary lines carrying the same information. Resolver mode shows angle-dependent carrier amplitude. The speed, index width and carrier window are illustrative, not a product specification or captured trace. “Restart” returns a stopped demonstration to its starting point.

Output circuitry: open collector, voltage and push-pull

NPN open collector: The transistor pulls the output low when conducting. An appropriate pull-up establishes the high state when it is off; that resistor may be in the receiving equipment or external. Select resistance, voltage and loading within the device ratings. A separate pull-up supply is not automatically permissible on every encoder. ^[1,2]

Voltage output: Some encoder catalogues use this term for a circuit with an internal pull-up. It does not automatically mean a 0–10 V analog measurement. Read the circuit, logic levels and load specification. There is no universal 10 mA output limit covering all encoders. ^[1]

A push-pull or totem-pole stage actively drives both logic levels. “Push-pull” does not mean bidirectional data communication. Its current capability and output levels must still match the receiving input. ^[2]

Differential pairs are not additional position channels

A line-driver output can provide A+/A−, B+/B− and, where fitted, Z+/Z−. The differential receiver evaluates the voltage difference within each pair. In the ideal logic view, the minus line complements the plus line. A−, B− and Z− are not ground terminals. ^[2,5]

RS-422 specifies an electrical interface, not a connector or a position-message format. Use compatible receivers, twisted pairs, cable construction and termination. Complementary HTL signals are not automatically safe for an RS-422 input. The same channel information can therefore be incremental and differentially transmitted at the same time. ^[1,2,5]

TTL and HTL: distinguish supply from signal level

TTL and HTL identify signal-level/interface classes in encoder specifications. TTL outputs are commonly in the 5 V class; HTL versions use higher levels, often related to their supply. However, an encoder powered from 24 V can provide 5 V TTL outputs. Read supply requirements and output ratings separately. ^[1,5]

A 24 V output can damage a 5 V input. Conversely, a 5 V signal may not reach the high-state threshold of a 24 V input. A compatible-looking PLC connector does not establish a compatible circuit. Check input thresholds, interface type and frequency limits. ^[1,5]

Two different meanings of analog encoder output

1 Vpp Sin/Cos channels are periodic position-related analog signals. Suitable interpolation electronics can resolve smaller steps within each cycle. That remains a useful analog encoder interface, rather than an inherently obsolete technology. 1 Vpp means one volt peak-to-peak, not ±1 V. Consult the actual electrical specification and reference conditions. ^[6]

Other position devices provide scaled 0–10 V or 4–20 mA outputs. The level represents an assigned angle or measuring span; it is not counted like A/B pulses. Establish zero, full scale, direction and behavior at the end of the range. A periodic Sin/Cos channel and a scaled analog position value are different interfaces. ^[8]

Why a resolver needs its own evaluation electronics

A resolver is an electromagnetic angular transducer requiring excitation. For an ideal single-pole-pair example, Vsin = K × sin(θ) × sin(ωt) and Vcos = K × cos(θ) × sin(ωt). Here θ is rotor angle, ω is excitation angular frequency, and K incorporates excitation amplitude and transformation ratio. ^[7]

The raw terminal signals are therefore carriers whose amplitudes depend on angle, not simply two waveforms running at shaft speed. A resolver reader extracts the angle. A conventional resolver has no native A/B/Z channel set, although subsequent converters can generate such outputs. A resolver input is not interchangeable with a 1 Vpp Sin/Cos encoder input. ^[7]

Absolute position, serial interfaces and networks

An absolute encoder encodes position within its resolution. A parallel interface exposes separate data bits; a serial interface transfers them sequentially. Check binary/Gray format, word length, singleturn/multiturn behavior and reader support. Counting transitions at an ordinary high-speed input does not decode a serial position message. ^[10,13]

SSI and BiSS-C are synchronous serial interfaces with different framing and diagnostic capabilities. EnDat is bidirectional and can exchange position plus supported parameter and diagnostic information. Serial EnDat 2.2 uses differential CLOCK and DATA pairs, with power supplied separately. Equal wire counts do not establish protocol compatibility. ^[9,10]

HIPERFACE and HIPERFACE DSL must also be distinguished: the conventional interface combines analog and digital channels, whereas DSL is fully digital. DRIVE-CLiQ is another communication system requiring compatible drive and evaluation electronics. ^[11,12]

Networked encoders are available with CANopen, PROFIBUS/PROFINET, DeviceNet and EtherCAT, among other interfaces. The controller needs the matching network implementation and relevant device configuration/profile support. These protocol choices are a different classification layer from “differential output.” ^[13]

Certain servo-motor encoders also provide U/V/W commutation channels. They report electrical-position information for commutation, not three interchangeable speed outputs. Match them to the motor pole arrangement and the drive’s feedback system. ^[15]

Calculate channel frequency separately from decoded counts

Check the manufacturer’s definitions of PPR and CPR. These abbreviations may refer to waveform cycles or to positions resolved after four-edge decoding. In the calculation below, N explicitly means complete A-channel cycles per mechanical revolution. A high-speed counter specification needs the same care. ^[3,4]

fA = N × rpm / 60. With 1,000 cycles/revolution at 3,000 rpm, channel A runs at 50 kHz. Decoding all rising and falling edges of A and B produces 4,000 counts/revolution and 200,000 counts/second. The physical A-channel frequency remains 50 kHz. A once-per-turn index repeats at 50 Hz.

Establish whether the input limit specifies channel frequency or aggregate counting rate. Minimum pulse width and filtering also matter. The example’s numerical angle step is 360° / 4,000 = 0.09°; that is not a guarantee of mechanical accuracy. ^[1,16]

Wiring and fault checks that prevent expensive mistakes

Do not combine independent encoders’ A outputs into one counter input. Even where open-collector circuits permit a wired logic function, overlapping pulses do not preserve independent motion counts. Directly connecting push-pull or line-driver outputs together can also create electrical contention.

One encoder feeding two receivers is a different problem. Assess driver loading, input circuits, termination and branches together; use a suitable signal splitter where required. A passive cable fork is not automatically appropriate, particularly at high frequency or over long runs. ^[14]

No counts: start with supply, pin assignments and interface compatibility. Missed counts at speed: inspect frequency, pulse shape and filtering. Reversed direction: check A/B order and the controller’s direction setting. Do not ground a complementary line or alter drive feedback parameters at random as a troubleshooting shortcut. ^[1,2,3]

Make the machine safe before changing servo feedback wiring. Remove the risk of unexpected movement and follow the product’s connection and safety instructions. The name of an interface does not itself establish a safety capability.

Technical references

[1] OMRON — Rotary Encoder Terms · [2] Dynapar — Encoder Output Drivers · [3] Dynapar — Quadrature Encoders · [4] US Digital — Resolution Terminology

[5] Baumer — TTL/RS-422 and Supply Specifications · [6] HEIDENHAIN — 1 Vpp Signal Conversion · [7] Analog Devices — Resolver Signals · [8] ATEK — Analog Absolute Encoder Interfaces

[9] HEIDENHAIN — EnDat 2.2 · [10] Novanta — BiSS and SSI · [11] SICK — HIPERFACE and HIPERFACE DSL · [12] Siemens — DRIVE-CLiQ

[13] Hohner — Absolute Encoder Interfaces · [14] Dynapar — Encoder Signal Splitter · [15] US Digital — Commutation Outputs · [16] US Digital — Resolution, Accuracy and Precision