A dead encoder on an axis, the label photographed, a quote in your inbox for the same model. Before you order, settle the one thing the label almost never prints: the output interface. A HEIDENHAIN encoder reports position as sinusoidal 1 VPP voltage signals, sinusoidal 11 µAPP current signals, TTL or HTL square waves, or as a serial EnDat data stream. Your control accepts one of those and cannot use the others. The same 12 pin M23 coupling carries more than one of them, so the wrong encoder plugs in cleanly and then behaves oddly. That is why this mistake tends to get made twice.
This guide covers what each interface sends, how to establish which one you have, and the substitutions that look safe and are not. For the number itself, see matching a HEIDENHAIN encoder by Id.Nr., and for faults, HEIDENHAIN encoder problems and error messages.
What each interface actually sends
Five interfaces cover almost everything built in the last forty years. Cable lengths are HEIDENHAIN's own figures.
| Interface | Signal form | Level | Max cable | Where you meet it |
|---|---|---|---|---|
1 Vpp |
Two sinusoids A and B, 90° elec. apart, plus reference mark R | Typically 1 VPP, tighter grades 0.8 to 1.2 VPP | 150 m | CNC axes since the 1990s |
11 µApp |
Two sinusoidal current signals I1 and I2 | Typically 11 µAPP | Short runs | Older TNC controls, ND readouts |
TTL |
Square waves Ua1, Ua2 and inverted pairs, reference pulse Ua0 | RS-422, typically 5 V | 100 m | Counter cards, readouts |
HTL |
Square waves at a higher level | Typically 10 V to 30 V | 300 m, HTLs 100 m | Long runs, noisy plant |
EnDat |
Bidirectional serial, four lines, clocked by the control | Clock 100 kHz to 2 MHz | 150 m | Absolute systems |
The interface lives in the Id.Nr., not in the model name
Inside one current family the digits do carry the interface. In the ROD 400 series, ROD 426 is TTL, ROD 436 is HTL and ROD 486 is 1 VPP. Among motor feedback encoders, ERN 1321 is TTL while ERN 1381 is 1 VPP.
Do not extrapolate that pattern backwards. It holds within a family and a generation. Older variants such as the ROD 420, ROD 446, ROD 450 and ROD 456 predate the current numbering, and the middle digit will sometimes mislead you. The Id.Nr. is the only part of the label that settles it.
Sinusoidal or square wave, and who does the counting
With 1 VPP the encoder hands over raw sine and cosine and the interpolation happens in the control. That is why such a system resolves far finer than its line count suggests. With TTL the encoder has already digitised everything and the resolution is fixed before the signal leaves the housing.
You can cross that boundary, but not with an adapter plug. HEIDENHAIN's IBV, EXE, APE and IDP units turn sinusoidal signals into TTL, usually interpolating on the way through.
There is a useful tell in the cabinet. If the encoder cable lands in an EXE box, the encoder feeding it is 11 µAPP. If it lands in an IBV, it is 1 VPP. Either way the control input is TTL, so order for the converter's input, not the control's. People read the control manual, order a TTL encoder, and find the box in the middle the hard way.
Absolute encoders: EnDat and the drive brand trap
EnDat is serial and bidirectional, needs only four signal lines, and carries diagnostics and an electronic ID label alongside the position. The RCN 226 angle encoder on the shelf is the 16 384 line version: 67 108 864 positions per revolution, ±5 arcseconds, with 1 VPP incremental signals alongside the serial channel.
Here is the trap. HEIDENHAIN builds the same mechanical encoder with EnDat, Fanuc αi, Mitsubishi, Panasonic and Yaskawa serial interfaces, differing only in a suffix. An EnDat unit will not talk to a Fanuc αi drive input, however well it fits the bore. Check the ordering designation held in the encoder: EnDat01, EnDat02 or EnDat22.
The two substitutions that waste a day
The first is the commutation track. The ERN 1387 and the ERN 1381 are the same family, both 1 VPP, and from a metre away they are the same encoder. The 1387 carries a second Z1 track giving one sine and one cosine period per shaft revolution, which a drive uses to find rotor position for sine commutation at power on. Fit a 1381 where a 1387 was and position feedback is perfect, but a permanent magnet synchronous motor has no rotor position at switch on, so it will refuse to enable or run rough and hot. The ERN 1326 does the same job with three UVW block commutation signals.
The second is line count. We hold two ROD 486 units with different line counts, 2048 and 3600, same model name on the label. Drop the wrong one in and the axis moves a wrong distance per revolution until somebody edits machine data, which on a retrofitted machine is often nobody. Linear systems do it differently: scale and scanning head are a matched pair, so a replacement AE LS 107 scanning unit must suit the scale already on the machine.
Quick answers
Can I connect a 1 Vpp encoder to a TTL input?
Not directly. The connector will mate, but a sinusoid of about one volt peak to peak is not an RS-422 square wave and the counter will not read it. You need interface electronics between them: an IBV for a 1 VPP encoder, an EXE for 11 µAPP.
How do I find out which interface my encoder has?
By the Id.Nr. on the label, not the model name. HEIDENHAIN's database resolves an Id.Nr. to the exact interface, line count, cable outlet and connector. Their PWT 101 and PWM 21 test units identify an encoder automatically from that number.
What is the difference between EnDat 2.1 and EnDat 2.2?
EnDat 2.2 is purely serial, runs at a higher clock frequency and supports safety related applications up to SIL 3. It also includes the EnDat 2.1 command set. Whether your control accepts a given encoder still depends on the ordering designation held in that encoder, so check for EnDat01, EnDat02 or EnDat22.
Is 11 µApp still a sensible thing to buy?
It is end of life and nothing new is built with it, so you buy used or you convert. Converting one axis means the scale, the scanning head, the cable and the control input, and on a 1980s machine that usually costs more than the machine is worth. A matched used unit of the correct Id.Nr. is normally cheaper and faster.
My replacement has the same model and line count but a different Id.Nr. Will it fit?
Possibly not, because the Id.Nr. also encodes the interface, the cable outlet direction, the cable length and the mounting variant. Two encoders can share a model name and a line count and still send different signals. Our Id.Nr. matching guide covers which parts of the number you can compromise on.
Before you order: a checklist
- Photograph the rating plate. Model, line count and Id.Nr. all legible.
- Look up the interface by Id.Nr., never from the model name alone.
- Find what consumes the signal: control input, counter card, readout or converter.
- If there is an EXE or IBV in the line, order for its input, not the control's.
- Ask whether the motor needs commutation. On a permanent magnet synchronous motor, assume it does.
- Match the line count exactly, or budget time to change machine data.
- Check the connector and cable outlet direction. Radial where you need axial will not fit.
- Get the replacement's Id.Nr. in writing before it ships.
The same logic governs sealed scales such as the LS 403C, where head and scale housing belong together. Our stock sits in the HEIDENHAIN collection and the encoders collection, with condition stated on each listing. Our parts carry a functional warranty of between 14 and 30 days depending on the item.
Got an Id.Nr. and an axis that is down? Send us the number from the label and what the cable plugs into, and we will say whether what we hold is a straight swap. Contact us directly. EU stock, inspected and bench tested where applicable, dispatched the same day on orders placed before 14:00 CET.
Related reading on Samonde
- Matching a HEIDENHAIN encoder replacement by Id.Nr.
- HEIDENHAIN encoder problems: error messages and fixes
- SINUMERIK 840D alarm codes
- Used industrial parts buying guide
Interfaces are the part of encoder work where being nearly right is worse than being lost, because a nearly right encoder gives you a machine that runs and lies. The full signal specifications sit in HEIDENHAIN's Interfaces brochure. Start a page in the machine file with one line per axis: model, line count, Id.Nr., interface, and what the cable plugs into. It takes an afternoon across a shop and turns every future encoder failure into a phone call rather than an investigation.
