Encoder Cable Length: Limits, Precautions & Solutions
The speed and position signals of rotary encoders often need to be transmitted over long distances that are questionable for reliability.
The distance from the point of “pick up” to the controller (PLC, VSD, or display) needs to be considered and cannot be an indefinite length. Certain measures can be taken to avoid signal problems.
Problems caused by cable runs that are too long can present intermittently or may only present after some time, as interference conditions change, or electronic components such as capacitors age.
Steps should be taken at the design stage of a system to avoid such problems.
Need help with an Encoder installation? Contact the PCA team here or call directly on 1300 236 467.
Incremental Encoders Cable Distances
It is possible to transmit and successfully read incremental encoder pulses over distances of 100m or longer, if ideal conditions are met.
Important factors to consider include:
- Using high-quality, individually shielded twisted pair cables with a thick gauge copper core for low resistance, and designed for low capacitance and inductance if possible. Higher resistance leads to greater voltage drop, resulting in lower encoder pulse voltage at the receiving end.
- The path that the cable follows avoids high voltage cables or any electrically noisy systems.
- Always use a quality shielded cable (steel braid preferred), and make sure that the shield is grounded.
- Should you experience electrical noise induced from Variable Speed drives cable runs, make sure these cables are well screened.
- It is strongly advised that all 6 encoder channels are used, A, B, O, /A, /B, /O. With long cable runs, the inverted, complement, or NOT channels should be physically wired and enabled in the controller. These channels serve as a reference to boost the receiver’s immunity against signal degradation and noise interference.
- When wiring for differential signal (A,/A,B,/B,Z,/Z), A and /A should be wired using one twisted shielded pair; B and /B should be in a second pair, etc. Failure to use complementary pairs (say, using A and B in a twisted pair) will reduce noise immunity significantly.
- An HTL signal is preferable. The voltage of these signals can typically be between 4 ~ 30V. If possible, try to use a high voltage such as 24VDC. 5V is not suitable for longer encoder cable distances.
- It may help to use an external switch mode power supply, as the controller’s onboard power source may not have enough power to cover the long cable distance.
- If the application allows it, an encoder with a lower PPR would be beneficial, especially if the application RPM is high. When you’re specifying an encoder, it’s easy to focus on resolution and forget about the cable. But higher frequencies and higher PPR (Pulses Per Revolution) directly shorten the maximum reliable cable length, because of the electrical properties of the cable itself.
Here’s how:
1. Cable capacitance
Every metre of copper cable acts like a small capacitor that stores and releases electrical charge. At low speeds this barely matters. At high frequencies, the cable can’t charge and discharge fast enough to keep up with the pulses, and the longer the cable, the more capacitance there is to fight against.
2. Signal rounding and distortion
An encoder outputs clean square waves. As frequency and PPR increase, those square waves get compressed in time. Cable capacitance then rounds off their sharp corners, and over long distances the signal degrades into triangle-like shapes the receiver can’t read reliably.
3. Overlapping pulses (inter-symbol interference)
High PPR at high rotational speeds packs pulses extremely close together. Over a long cable, the rounded, distorted edges of these rapid pulses start to bleed into one another. The result: the receiver miscounts pulses or loses track of position entirely.
4. The frequency–length trade-off
Encoder transmission limits are defined by a frequency-to-length relationship (refer to the table below).
5. Voltage drop from impedance
High-frequency signals see higher total impedance in a cable than low-frequency signals. This attenuates the signal voltage faster over distance, so it falls below the receiver’s detection threshold sooner.
The takeaway
Don’t choose more PPR than your application needs. Every increase in resolution raises the output frequency at a given speed, and that eats directly into how far you can run your cable. If you need both high resolution and long distances, consider a lower-capacitance cable, a differential line driver such as RS-422, or a signal repeater partway along the run.
Typical Encoder Cable Distances
| Encoder Output Type | Signal Voltage | Typical Maximum Distance | Characteristics and Limitations |
| Open Collector | Variable (often 5V–24V) | 10~15 metres | Signal rise time is slow due to high output impedance and makes it highly susceptible to noise and the signal degenerates over long distances. |
| TTL (Transistor Transfer Logic) | 5V DC (Single Ended) | 10~12 metres | Voltage drops are frequent as the signal uses a low voltage level. Vulnerable to electromagnetic interference (EMI) |
| HTL (High Threshold Logic) | 10V–30V DC (typically 24V) | 100 ~ 300 metres | Due to the higher voltage threshold, the signal has excellent noise immunity. The distance it can travel varies on its frequency. The higher the frequency, the shorter the distance it can travel. |
| RS-422 (Differential Line Driver) | 5V DC (Differential | 100 ~ 1000 metres | With the use of twisted pair complementary signals, it eliminates common-mode noise. Can reach up to 1000 metres with low frequency. |
HTL vs TTL: Which Travels Further?
When an encoder sits a long way from its controller, the choice between HTL (High Threshold Logic) and TTL (Transistor-Transistor Logic) output can decide whether your signal arrives intact. For long cable runs, HTL generally comes out ahead, with one notable exception.
Higher voltage absorbs voltage drop
HTL operates on a supply of 10 to 30 V DC (most commonly 24 V), compared with TTL’s fixed 5 V DC. Every metre of copper adds resistance, and over a long run that resistance pulls the signal voltage down. Because HTL starts with so much more headroom, it can lose a meaningful amount of voltage along the way and still be read as a clean “high” at the controller. A 5 V TTL signal has far less margin to give before it drops into an ambiguous range.
Greater resistance to electrical noise
Long cables act like antennas, picking up interference from motors, drives and other equipment in industrial environments. HTL’s high switching threshold means a stray voltage spike usually isn’t large enough to flip the signal state, as a change in the order of 10 V or more is typically needed. TTL switches at much lower voltages, so the same noise is far more likely to cause false counts or missed pulses.
The exception: differential TTL (RS-422)
Standard single-ended TTL is only reliable over roughly 10 to 15 metres. Convert it to differential TTL (RS-422), however, and the picture changes completely. RS-422 sends each signal alongside its complement on a twisted pair. Noise picked up along the cable affects both wires equally, so the receiver cancels it out by reading the difference between them. This allows differential TTL to reach distances of up to 1,000 metres.
Selecting the right cable for your encoder application
ADM recommends the following Encoder Cables:
- FLEX-06CORE
- FLEX-08CORE
What makes these cables suitable for encoder use?
- Twisted pair cables
- Low resistance, low capacitance
- High-quality screening, tinned copper braid is far superior to a foil jacket layer.
- PUR outer insulation is oil and heat & cold-resistant, and more resistant to mechanical abrasion than PVC.
- Energy Chain rated for use on moving applications where the cable will be continuously flexed.
If the optimal conditions are not met, or even longer cable runs are required, a few electronics solutions can be considered.
Encoder Signal Repeater for Long Cable Runs
Encoder Signal Booster:
With a signal repeater, such as the Motrona GV210, the maximum distance can be repeated for each signal type. For example, an HTL signal that can travel up to 100m can be extended to 200m whilst maintaining the same frequency.
If the GV210 is not used as a “pass through” device, it can be used to convert HTL voltage levels to TTL and vice versa.
Fibre Optic Encoder Signal Transmission (up to 3,000 m)
With fibre optic cable, the Motrona LW215 and LW216 transmitter receiver pair can be used to transmit signals over a distance of up to 3000m. This depends on the wavelength:
- 850 nm wave length up to 2000 meters wide
- 1300 nm wave length up to 3000 meters wide.
Motrona LW215
Motrona LW216
Absolute Encoder Signal Distances using FieldBus or Ethernet Protocols
Industrial Ethernet protocols such as EtherNet/IP, Modbus TCP/IP, ProfiNet and EtherCAT have a distance limit of around 100m.
Again, consideration in cable quality and route can help maximise the permissible distance to the encoder and minimise reliability issues.
Encoders using more traditional fieldbus protocols such as serial RS485, Modbus RTU, CANbus can run longer distances of between 500m and 1km.
However, these distances are typically achieved by reducing the signal speed or Baud rate to a low value, which is often not suitable in a motion system that requires any level of tight control or precision.
Absolute encoders that have a 4-20mA loop signal can be used for cable runs of up to 500m, or possibly longer, and offer fast response times, whilst being relatively noise immune.
Frequently Asked Questions
Point number 4: FAQ Block
Cable length questions:
How long can an encoder cable be?
Answer: It depends on the type of output you are working with, the frequency of the signal, the type of cable being used, etc. Kindly review the above table to see which signal travels further compared to others.
What happens if my encoder cable is too long?
Answer: When the encoder cable is too long, you might experience signal loss or missed pulses, excessive noise levels if the cable does not have proper screening, the controller can receive intermittent ghost pulses, cause timing errors in the system, etc.
Signal type questions
Does an HTL or TTL encoder signal travel further?
Answer: HTL signals, due to its high voltage level and switching tolerance, runs further than TTL signals.
Why should I use differential (A, /A, B, /B) encoder signals on long cable runs?
Answer: When using differential encoder signals on long cable runs, the signal is protected from noise and interference. This is because differential signaling uses two complementary wires for every channel (e.g., A and /A). This allows the signal to cancel electromagnetic interference, prevents ground loop issues, and maintains high speed integrity.
Does encoder resolution (PPR) affect maximum cable length?
Answer: yes, a higher PPR at high RPM means a higher frequency and a shorter usable run. See point 3 for more details.
Cable selection questions
What type of cable should I use for an encoder?
Answer: individually shielded twisted pairs, low capacitance and a braided screen. Link to the FLEX cables. Answer: To ensure reliable signal transmission and prevent data corruption, you should use Shielded Twisted-Pair (STP) cable with low capacitance. Using standard electrical wire or unshielded cables will cause missed pulses and position errors over almost any distance. Some characteristics to look out for:
- Twisted Pairs
- Overall and individual shielding
- Low capacitance
- Correct wire gauge
- Flex rating
Solutions questions:
How can I extend an encoder signal over a long distance?
Answer: By using a signal repeater like the GV210. For longer distances, the LW range will be suitable as well. Link to GV210 and LW products.
How far can encoder signals be sent over fibre optic cable?
Answer: up to 2,000 m with the LW215/LW216, or up to 3,000 m with the LW213/LW214.
The products mentioned above are available from stock in PCA Encoders’ Australian warehouse.
To consult with our expert engineers who are readily available to advise on any of your encoder feedback concerns please contact PCA Encoders.
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