Wiring 4-20mA Transmitters to PLC Analog Inputs Correctly

Wiring 4-20mA Transmitters to PLC Analog Inputs Correctly

Connecting a 4-20 mA current-loop transmitter to a PLC analog input looks trivial on a wiring diagram, but field failures consistently trace back to a single issue: the return (negative) conductor is not connected, or the analog-to-digital converter (ADC) negative input is left floating relative to the loop power supply common. This reference documents the loop topology, the role of galvanic isolation, the documented and undocumented behavior of common analog modules (including the Siemens LOGO! 8 AM2), and a deterministic procedure to verify any 4-20 mA wiring before commissioning.

1. 4-20 mA Current Loop Topology

A 4-20 mA current loop is a series circuit. The four functional elements are:

1. Loop power supply — typically 24 V DC, capable of sourcing 20 mA plus the voltage overhead of the transmitter and the IR drop across the wiring.
2. Transmitter — regulates loop current between 4 mA (zero-scale) and 20 mA (full-scale) in proportion to the measured variable.
3. Loop wiring — a single pair (or occasionally a shielded pair) that carries the current; the loop current is identical at every point in the series path.
4. Receiver — converts loop current back to a voltage (typically across a 250 Ω precision resistor producing 1-5 V, or directly into an ADC front-end).

Because the loop is a series circuit, current is conserved at every node. Adding a second receiver in series does not load the loop as a parallel resistor divider would; provided the loop supply has voltage headroom, multiple instruments can be inserted into the same two-wire loop.

Reference: Introduction to 4-20-mA current loop transmitters (TI training video); What Is a 4-20 mA Current Loop? (Fluke calibration blog); Acromag Introduction to Two-Wire Transmitters and the 4-20 mA Current Loop (PDF white paper).

2. Transmitter Powering Categories

PLC analog input modules accept all three topologies, but the internal wiring of the module determines whether the negative terminal of the ADC is bonded to chassis, to field ground, or to the PLC backplane 0 V reference. Always check the module's wiring diagram against the loop-supply common before energizing.

3. Return-Path Theory and the "Floating Minus" Fault

A 4-20 mA loop is closed only when both conductors and the supply return make a complete circuit. The most frequent commissioning fault is leaving the ADC negative terminal unconnected to the loop supply common because the installer assumed the ADC "shares ground" with the PLC backplane. That assumption is correct only if three conditions hold:

1. The analog module is not galvanically isolated from the PLC backplane.
2. The PLC backplane 0 V is bonded to the same protective earth (PE) as the loop supply 0 V.
3. No other return path (PE, shield, or parallel ground rod) is carrying loop current.

If any condition fails — for example, the module is isolated, or the loop PSU is ungrounded — the ADC sits on a high-impedance node. The ADC then measures common-mode voltage rather than differential current, producing a saturated, noisy, or pegged reading that drifts with cable motion.

4. Galvanic Isolation: Mandatory vs. Optional

Galvanic isolation between the field loop and the PLC backplane breaks every unintentional ground path. It is mandatory in any of these situations:

• Loop supply is grounded at the panel and the PLC is grounded at a different panel — multiple ground rods create a ground loop that injects 50/60 Hz common-mode noise.
• Loop supply and PLC are on different feeder transformers — different neutrals are not at the same potential.
• Field cabling runs near VFD power cables — high dV/dt capacitive coupling forces common-mode current through the ADC.
• The transmitter is mounted on a pipe or tank that may carry fault current.

When isolation is present, the analog module provides separate field-side and logic-side terminals. You must still complete the loop on the field side. Isolation does not eliminate the need to terminate the negative conductor at the ADC input; it only guarantees that the field side has no DC path to the PLC backplane.

5. Siemens LOGO! 8 AM2 Module Behavior (Documented and Field-Observed)

The Siemens LOGO! 8 AM2 analog input module (6ED1055-1MA00-0BA2 family) and AM2 RTD variant accept two 0-10 V or 0/4-20 mA inputs on four terminals: I1, M1, I2, M2. In current mode, the internal shunt resistor of the AM2 internally bridges the input to the module's internal reference, so the M terminal is not a "no-connect" pin: it is the negative terminal of the differential ADC pair.

Field observation (and the pattern Siemens service engineers confirm in their LOGO! 8 system manual wiring examples) is that when an external 24 V loop supply is used and M is left open, the input still registers a signal because the AM2 internally biases the channel to its own reference potential. The reading appears valid at the bench but is undefined at the panel: any common-mode voltage that develops on the field cabling — sensor heater cycling, nearby relay switching, VFD-induced ground potential — appears at the ADC and shifts the reading.

Always connect the M terminal to the loop supply negative for predictable operation. Refer to the official LOGO! 8 system manual for the AM2 wiring diagram, available from Siemens Industry Online Support.

6. Determining the Polarity and Connection Order for a 2-Wire Loop

For a loop-powered (2-wire) transmitter, the canonical order from supply positive (+) to supply negative (−) is:

1. PSU 24 V + terminal
2. Transmitter + terminal (powers the transmitter electronics)
3. Through the transmitter's internal regulator (current controlled by sensor input)
4. Transmitter − terminal (loop-current return)
5. Receiver (PLC analog input) + terminal (current enters)
6. Receiver (PLC analog input) − terminal (loop-current exits back to PSU)
7. PSU 24 V − terminal

The series path is unbroken. If the receiver is inserted in the return leg instead of the high side, the polarity at the ADC terminals swaps; the engineering count is the same, but verify that the PLC module labels its terminals + / − consistently with whichever leg you choose. Most modules — including the LOGO! AM2 — expect the loop current to enter the I terminal and return through M.

7. Wiring Procedure for a Loop-Powered Sensor to a Non-Isolated PLC Input

7.1 Prerequisites

• 24 V DC regulated power supply rated for at least 0.5 A × (number of loops) with the negative terminal bonded to the panel PE.
• Loop-powered transmitter with documented terminal markings; sensor datasheet showing minimum loop voltage = supply voltage − maximum loop IR drop.
• PLC analog input module datasheet. Verify channel configuration: 0-20 mA vs 4-20 mA, single-ended vs differential, isolated vs non-isolated.
• Calibrated multimeter with 4-20 mA loop current measurement mode or a series-connected milliammeter.
• Shielded twisted-pair cable, shield bonded at one end only (panel end).

7.2 Termination diagram

+24 V ──┬── [Loop-powered TX: +] ── [TX internals] ── [TX: −]
        │
        │      ┌──────────────────────┐
        │      │ PLC analog input      │
        │      │   I+ ◀── return ────┘
        │      │   M- (or -) ────────┐
        │      └──────────────────────┘
        │
       0V ──────────────────────────┘ (back to PSU negative)

7.3 Step-by-step

1. Power down the PLC and the loop supply.
2. Land the PSU 24 V positive to the transmitter + terminal.
3. Land the transmitter − terminal to the PLC analog input's + (or I) terminal.
4. Land the PLC analog input's − (or M) terminal to the PSU 24 V negative.
5. Connect the shield drain wire to PE at the panel end only; do not ground at the field end.
6. Verify with a continuity check that DC current can flow from PSU + to PSU − through exactly the three series elements: transmitter, ADC positive terminal, ADC negative terminal.
7. Apply loop power without the PLC in RUN; measure 4 mA with the transmitter at zero-scale stimulus.
8. Apply the full-scale stimulus; verify 20.000 mA ± 0.016 mA (NAMUR NE43 standard error band) with the multimeter.
9. Power the PLC; read the engineering value at 4 mA (zero) and at 20 mA (full-scale). Confirm linearity at 25 %, 50 %, and 75 %.

8. Wiring Procedure for a 3-Wire or 4-Wire Sensor

3-wire devices require an additional 24 V supply terminal at the sensor; only the signal out pair enters the PLC. The loop current still flows from the sensor's output driver into the PLC input positive terminal and returns on the analog negative terminal — the same topology as a 2-wire receiver side. Connect both signal leads to the ADC; do not rely on chassis bonding.

4-wire (self-powered) sensors isolate their analog section internally; the output pair is normally ground-referenced to chassis through the sensor's PE. Verify the sensor datasheet for "isolated analog output" — if it is not isolated, run an isolated PLC module to avoid a ground loop.

9. Verification: The Five Point Loop Test

Use this procedure for any new 4-20 mA installation or after a wiring change.

1. Continuity — With supply de-energized, meter the loop resistance from PSU + to PSU − through the transmitter and ADC input. Expected: between 50 Ω and 1 kΩ depending on transmitter type and ADC shunt value.
2. Polarity — Reapply supply, meter voltage from PSU + to transmitter + terminal. Expected: 18-24 V (transmitter overhead is typically 12-14 V at 20 mA).
3. Zero — Apply zero-scale stimulus. Meter reads 4.000 mA ± 0.016 mA.
4. Span — Apply full-scale stimulus. Meter reads 20.000 mA ± 0.016 mA.
5. PLC track — Engineering value at the HMI matches the expected zero and span within one count of the ADC resolution (12-bit module: ~4 µA per count; 16-bit: ~0.3 µA per count).

10. Troubleshooting Matrix

11. Edge Cases and Field Commissioning Caveats

• HART multiplexed loops. HART communication rides on the loop as a 1 mA pk-pk 1200/2200 Hz FSK signal. The PLC analog input shunt resistor and input filter must pass 2.2 kHz. A bare 250 Ω shunt with no filtering is fine; an aggressive input filter that rolls off at 1 kHz will mute HART.
• Intrinsic safety barriers. When the transmitter is in a hazardous area through a zener barrier or galvanic isolator, the barrier replaces a portion of the loop. The PLC must still close the loop on the safe side; barriers are not return paths.
• Multi-drop HART. Multi-drop HART fixes the loop current at 4 mA for digital communication. A PLC that reads that loop as "4 mA = zero" will interpret multi-drop devices as zero scale. Verify the analog channel configuration before commissioning a multi-drop network.
• Test-mode buttons on smart transmitters. Many smart transmitters let you push-test the loop to 4, 12, or 20 mA. If the test signal is gated by a remote acknowledgement, the PLC must not require a live sensor variable to read the test point.
• Loop supply failure modes. If the loop PSU shares the panel with the PLC and a brownout trips the PLC while the PSU holds, the ADC pin may be biased up by the input network. Add a resistor divider clamp if your analog channel allows it.

12. Loop Voltage Margin Calculation

The transmitter must receive a minimum voltage between its + and − terminals to regulate loop current accurately. The regulator budget is:

V_TX(min) = V_PSU − V_wire − V_ADC

Where:

• V_PSU = nominal loop supply voltage (e.g., 24.0 V DC)
• V_wire = loop-current × (single-conductor resistance × 2). For 20 mA through 250 m of 18 AWG copper (≈ 21 Ω/km × 0.25 km × 2 = 10.5 Ω), V_wire = 0.21 V.
• V_ADC = drop across the receiver shunt or ADC front-end. For a 250 Ω resistor: V_ADC = 5.000 V at 20 mA. For a non-isolated module with integrated shunt, see the module datasheet — typical is 1.0-5.0 V.

Typical required transmitter compliance: 12-14 V at 20 mA. Required minimum supply: 17-19 V. If V_PSU − V_wire − V_ADC falls below the transmitter's minimum, the loop will not reach 20 mA and will saturate at some lower current — typically interpreted by the PLC as a stuck-mid reading.

13. Safety Notes for Energized Work

Bonding and grounding must follow the local electrical code (typically NEC Article 250 in the United States, IEC 60364 internationally). PE conductors are not normally intended to carry loop current; if a clamp meter shows loop current on the PE, the installation has a parallel ground path that must be eliminated.

14. Putting It Together: Quick Diagnostic Decision Tree

When a 4-20 mA input misbehaves, walk the tree in this order:

1. Is the PLC reading saturated high or low? If low, ADC negative is open. If high, ADC positive is open or reversed.
2. Does the reading drift when nothing is changing? Common-mode issue. The ADC reference is floating.
3. Does the reading correct when you bond the ADC negative to the PSU negative with a jumper? Missing return conductor.
4. Does the reading jitter only when a VFD or large contactor operates? High dV/dt coupling. Use shielded twisted pair and isolated module.
5. Does the reading match the multimeter at 4 mA and 20 mA but mismatch in between? Linearization issue; check engineering units mapping.
6. Does the multimeter show 4.00 mA at zero scale but the PLC reads -32768? Wrong range configuration; switch channel to 4-20 mA mode.

15. FAQ