Long Cable Runs and Their Three Constraints: Voltage Drop, Signal Attenuation and Repeater Segmentation

When the sensor is a long way from the control cabinet, the limiting factors are not the sensor itself but three items: supply voltage drop, signal attenuation and interference coupling. The remedies are completely different: voltage drop is handled by increasing conductor size or supplying power locally; attenuation is handled by choosing the right signal form; interference is handled by shielded routing and, where needed, reducing speed. This article gives the order of judgement and the engineering practice for long-run applications. For the output-selection framework, see analog vs. fieldbus selection; for PLC-side fieldbus matching, see which fieldbus does my PLC support.

Constraint 1: supply voltage drop — the power supply is usually the first to fail

Supply current produces a voltage drop across the cable resistance. The thinner the conductor, the longer the distance and the larger the current, the greater the drop. The resistivity of copper is about 0.0175 Ω·mm²/m. Loop resistance is calculated on the out-and-return path: R = 0.0175 × 2L / S (L is the one-way length in m; S is the conductor cross-section in mm²). The voltage drop is ΔU = I × R.

This calculation must be done at the design stage, not after the site reports that "the sensor works sometimes and not others". A typical field symptom is: power-up is normal with no load; as soon as the machine moves and other loads come on, the supply falls below the rated lower limit and the sensor output goes abnormal or even resets. The key troubleshooting action is to measure the voltage at the sensor connector, not at the power-supply outlet; see no-output troubleshooting.

Three practical countermeasures: increase the supply conductor size; move the power module into a field box close to the sensor (the most effective); or use a model with a higher supply voltage to reduce the relative drop. When several sensors share one supply, also check that the total current is within the power-supply capacity.

Constraint 2: the signal form determines how far you can run

The conclusion is clear: for long runs prefer a current loop or a fieldbus; do not choose voltage output. On a long run, 0-10 V suffers both a systematic error from resistive divider action and a lack of common-mode rejection. On the product side, analog options include the current-output variants of the Series 191 analog; fieldbus options include the Series 194 CANopen (up to 1 Mbps; reduce speed deliberately on long runs) or the Series 197 EtherCAT.

Constraint 3: interference coupling scales linearly with length

The longer the cable, the larger the area that couples in parallel with interference sources, and the more common-mode noise is picked up. On long runs you must: use twisted pair with an overall shield; run separately from inverter output cables and solenoid-valve cables; avoid long parallel runs, and cross at right angles where crossing is unavoidable; and earth the shield according to the signal type (single-ended on the cabinet side for analog; both ends when equipotential bonding is reliable on a fieldbus). Practice is covered in shield grounding at one end vs. both. For a remediation approach, see on-site EMC remediation reviewed.

Repeaters and segmentation: when they become mandatory

Once a single segment exceeds the reasonable range of the signal form, "a slightly thicker cable" will no longer solve it. The topology has to change:

• CAN repeater / bridge: split the bus into two segments, each independently meeting the length constraint and each fitted with 120 Ω terminating resistors. Note that every extra repeater stage introduces delay, which must be budgeted when real-time performance is demanding;
• Industrial Ethernet switch / slave daisy-chain: EtherCAT natively supports linear daisy-chaining, extending segment by segment; PROFINET can be segmented with switches;
• Local acquisition plus digital uplink: place a remote I/O or gateway in the field, convert analog to digital locally, then transmit over the long distance — the most robust architecture for long-distance applications;
• Protocol-conversion gateway: gather locally on a short-distance fieldbus, and return on the trunk via industrial Ethernet.

Terminating resistors must be re-planned after segmentation — one 120 Ω at each end of every physical segment. The original arrangement must not be retained, or the result is "more data loss after the modification". See fieldbus data loss and terminating resistors and bus length.

A four-item design-stage calculation checklist

1. Calculate the supply voltage drop to the farthest point and confirm that the voltage at the sensor is still within the permitted range;
2. Confirm that the signal form matches the distance; for a current loop, calculate the total loop load resistance;
3. For a fieldbus, confirm the baud-rate versus bus-length relationship and reduce speed deliberately on long runs;
4. In explosion-protected duty, also check that the cable distributed inductance and capacitance meet the intrinsically safe requirements; see hazardous-area installation.