An absolute multi-turn draw-wire sensor delivers a true position word on every power-up, eliminating the dedicated homing move that an incremental draw-wire sensor requires after every loss of supply voltage [S1][S7].
The practical split is straightforward: incremental draw-wire units with A/B quadrature and a Z (index) pulse cost less and resolve higher pulse frequencies, but every controller restart triggers a controlled homing routine against a mechanical datum [S5][S8].
How each technology actually reports position
An incremental encoder only reports changes in position via A/B quadrature channels, with an optional once-per-turn Z index used to re-anchor the count [S5]. Absolute encoders report the actual shaft position at any instant, using Gray code on parallel lines or a serial protocol such as SSI, BiSS-C, Modbus RTU, CANopen, or PROFIBUS PA [S2][S6].
On a draw-wire body the rotary encoder is driven by a spring-loaded drum, so the wire pull length maps to a shaft angle through a fixed pitch; an absolute multi-turn device adds a revolution counter that survives across the full measurement stroke without software bookkeeping [S3][S4]. For reference on the mechanical form factor itself, see the draw-wire sensor primer and the underlying rotary encoder basics.
Power-loss behaviour and the homing tax
The defining drawback of an incremental multi-turn setup is the loss of all position information at power-down, which forces a homing sequence against a limit switch, hard stop, or index pulse on every restart [S1]. Homing is only required when the motion controller runs in absolute positioning mode, and is not needed in pure relative (velocity or endless-rotate) mode [S8].
Absolute multi-turn encoders keep the turn count through power cycles either by gearing to a magnetic Wiegand counter, by an electronic counter with a backup cell, or by an energy-harvesting design that counts revolutions from the rotation itself [S3]. That is why a draw-wire sensor built on an absolute multi-turn encoder reports stroke position in millimetres the moment the controller issues its first read, with no movement of the actuator and no risk of a homing crash on a machine that is already mid-stroke.
Protocol, resolution, and update-rate trade-offs
Incremental outputs are simple: HTL (8 to 30 VDC) for industrial noise immunity, TTL/RS422 (4.75 to 5.5 VDC, often up to 30 V) for long cable runs with a differential receiver, plus legacy NPN/PNP open-collector variants [S4]. The trade-off is protocol simplicity versus the bandwidth ceiling: incremental channels struggle at very high pulse frequencies, while absolute channels trade edge rate for serial cycle time [S2].
Common absolute draw-wire protocols span RS485 Modbus RTU, RS422, TTL Modbus RTU, CANbus, CANopen, SSI, BiSS-C, SPI, plus analogue 4-20 mA, 0-5 V, and 0-10 V retransmissions; SSI and BiSS-C are point-to-point clocked links, Modbus RTU and CANopen are multi-drop, and 4-20 mA is the slow but EMC-rugged fallback for long cable runs to a PLC analogue input [S2].
Decision matrix: incremental with homing vs absolute multi-turn
Cost per channel: incremental HTL/TTL draw-wire bodies are the lowest-cost option because the encoder module is just two or three digital lines and a counter; absolute multi-turn units add a serial transceiver and (for geared types) a Wiegand counter, which pushes unit cost up, often 1.5 to 3 times the incremental equivalent at the same stroke length. [S4]
Startup behaviour: incremental loses all counts on power-down and must run a homing routine; absolute multi-turn holds position across power cycles with no homing move [S1][S3]. Maximum pulse rate: incremental can resolve higher edge rates because the count is done in the controller FPGA or counter; absolute is gated by the serial cycle time, typically a few kHz of position read rate [S2].
Cable and EMC: HTL and RS422 incremental outputs are the most tolerant of long cable runs in noisy plant floors; 4-20 mA absolute is the most tolerant of all for kilometre-scale analogue runs; SSI/BiSS-C absolute is best kept short (a few metres) because the clock and data edges are sensitive to skew [S4][S2]. Failure mode: incremental reports a stuck or wrong count silently until the next homing; absolute can flag a position-code parity or CRC error on the serial frame [S6].
Where each option fits, and where it does not
Pick an incremental draw-wire sensor with homing when the machine has a safe homing direction, when the actuator can be moved at low speed against a hard stop without process interruption, and when the budget for both sensor and PLC high-speed counter is tight; this is the common choice on shop-floor length measurement, crane hoist position, and simple cut-to-length stations where a one-second homing move at start-up is acceptable [S4][S8].
Pick an absolute multi-turn draw-wire sensor when the machine cannot tolerate a homing move on restart, when the draw-wire is mounted on a moving axis that may already be mid-stroke when power returns, or when the controller runs a safety-rated stop that must be reported as a true position rather than a relative offset; typical fits include presses, elevators, mobile machinery booms, and any retrofit where adding a homing switch is mechanically inconvenient [S3][S1].
For deeper reference on how absolute position is transmitted over a single twisted pair versus multi-drop buses, see the absolute pressure transmitter protocol discussion, which covers the same 4-20 mA plus HART and digital serial choices. For comparison with a non-contact linear measurement family that avoids the wire-and-drum wear item, see linear encoder basics.
Field-reality caveats and sourcing
Incremental draw-wire bodies still ship as NPN, PNP, push-pull HTL, RS422 TTL, voltage, and line-driver variants, and HTL is widely treated as the modern replacement for NPN/PNP open collectors in 24 V industrial control panels [S4]. TTL/RS422 incremental parts typically draw under 60 mA per channel, which is comfortable for any PLC or microcontroller high-speed counter input [S4].
Battery-backed and Wiegand-pulse absolute multi-turn designs degrade at end of life, so the maintenance schedule for the backup cell and the duty cycle on the harvester both belong in the spec sheet, not in a footnote [S3]. For a closely related cable-and-wire selection question on long-run analogue or digital signal cables, see cable and wire guidance.
One trackable signal for buyers is the steady migration of new draw-wire catalogues from pure incremental A/B/Z bodies to absolute variants with SSI, BiSS-C, or Modbus RTU, since that single change removes the homing routine from the PLC programme. A second signal is the rise of 4-20 mA absolute draw-wire bodies for retrofit projects where the existing PLC has spare analogue inputs but no high-speed counter, which keeps the absolute multi-turn data flowing on a single two-wire loop [S2].
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