Connecting 4-20 mA Pressure Transmitters to LOGO! 12 VDC Inputs

Connecting 4-20 mA Pressure Transmitters to Siemens LOGO! 12 VDC Inputs

The Siemens LOGO! 12/24 RCE and the older 12/24 VDC base modules ship with four onboard analog inputs (AI1–AI4) that are hard-wired for 0–10 VDC. This is convenient when you want to use a 0–10 V pressure transmitter, but most industrial pressure transmitters output 4–20 mA. Bridging the two domains is one of the most common first-time questions new LOGO! users hit when they try to instrument a pump skid, a compressed air line, or a hydraulic press.

This reference covers three practical methods to bring a 4–20 mA pressure loop into a LOGO! 12 VDC base module, the math behind each conversion, the programming steps in LOGO!Soft Comfort, and the verification you should perform before commissioning. It also flags the field-proven pitfalls (loop voltage drop, resistor tolerance, signal ground) that are not obvious from the marketing brochure.

1. The Interfacing Problem in One Diagram

The disconnect is simple but rigid:

The base unit accepts voltage, the transmitter produces current. You either convert current to voltage with a precision resistor, or you add an extension module that natively reads 4–20 mA. Both are valid; the choice is driven by panel space, supply voltage availability, and accuracy budget.

2. LOGO! 12/24 VDC Hardware Overview

The relevant 12 VDC base module for this application is the LOGO! 12/24 RCE (Siemens part number 6ED1052-1CC08-0BA1, firmware ≥ V8.3) or its predecessor 6ED1052-1CC01-0BA8. Key analog characteristics, taken from the LOGO! 8 System Manual:

Power supply to the LOGO! base must be 12 VDC or 24 VDC. The supply range is wide (10.8–28.8 VDC on the 12/24 RCE), but the sensor side has its own minimum loop voltage requirement that usually dominates.

3. Pressure Sensor Selection Criteria

For a 12 VDC LOGO! system you have three practical sensor classes:

1. 0–10 V pressure transmitter — wire directly to AI1–AI4. Simplest. Power supply must be ≥ 12 VDC and stable.
2. 4–20 mA pressure transmitter, 2-wire (loop-powered) — most common industrial type. Use a 500 Ω sense resistor.
3. 4–20 mA pressure transmitter, 3-wire — separate supply and return. Same conversion method, simpler wiring.

AutomationDirect's digital pressure switch / transmitter lineup is a useful reference for catalog numbers, response times, and display options if you want a part with a local readout. When you are not using the display, treat the output exactly like a standard 4–20 mA loop.

For a 2-wire loop-powered 4–20 mA transmitter, the loop voltage budget is the limiting factor:

V_supply ≥ V_transmitter_min + (I_max × R_loop) + (I_max × R_sense)

With I_max = 20 mA, R_sense = 500 Ω, and a typical V_transmitter_min of 10 VDC, you need at least 10 V + 1.0 V = 11.0 V at the transmitter terminals. A clean 12 VDC regulated supply is therefore the practical minimum. A 24 VDC supply gives you 13 V of headroom and is strongly preferred.

4. Method 1 — 500 Ω Current-to-Voltage Conversion

This is the workhorse method for converting 4–20 mA into the LOGO!'s 0–10 V range. Place a single precision resistor between the analog input terminal and the negative supply reference.

4.1 Conversion Math

By Ohm's law, the voltage developed across the sense resistor is:

V_out = I_loop × R_sense

Using 500 Ω gives the full 0–10 V dynamic range of the LOGO! input, with a 2 V live-zero that lets the program detect a broken wire (the input would read ~0 V, which is below the 2 V live-zero). The trade-off is 10 V × 20 mA = 200 mW of heat dissipated in the resistor and 10 V of compliance voltage consumed in the loop.

A 250 Ω resistor halves the loop drop (5 V at 20 mA) and the heat (50 mW) but uses only half the ADC range (1–5 V), which means a 2× loss of resolution. For 10-bit ADC that is still 1024 counts over 4 V, or ~3.9 mV/count — adequate for most process work. For 4-wire ratiometric measurements on low-pressure transducers (≤ 5 psi), stick with 500 Ω.

4.2 Resistor Selection

Use a metal-film, 0.1% or 0.5% tolerance, 25 ppm/°C or better, 1/4 W or 1/2 W part. A 5% carbon-film resistor will introduce a calibration error of ~0.5% FS at room temperature, and another 0.5% per 30 °C of drift — unacceptable on a calibrated pressure loop.

Recommended types:

• Vishay RN60 series (military-grade metal film, 0.1%, 10 ppm/°C)
• Yageo MFR-25FBF52 series (1% metal film, 50 ppm/°C, generic-industrial grade)
• Any 0.1% resistor in a 2512 SMD or axial leaded package, derated 2:1 for voltage and 4:1 for power

5. Method 2 — LOGO! AM2 Extension Module

When the panel already has 24 VDC, the cleaner solution is the LOGO! AM2 analog input module (Siemens part number 6ED1055-1MA00-0BA0), which accepts 0–10 V or 0/4–20 mA on each of four channels, switch-selectable per channel.

The AM2 requires 24 VDC at the module's power terminals L+ and M; it will not function on a 12 VDC supply even if the base unit does. If your system is 12 VDC only, you must use Method 1.

For 4–20 mA on the AM2, wire the transmitter's positive lead to the AM2 input terminal and the negative lead to M. The AM2 already contains a 250 Ω internal sense resistor in current mode; no external components are required. The 0/4 mA live-zero and broken-wire detection are handled in software by checking for a value below 0 or above 1000.

6. Wiring Topology

6.1 2-Wire Loop-Powered Transmitter with 500 Ω Resistor

The transmitter sinks current through the 500 Ω resistor. The voltage across the resistor (2–10 V) is measured by AI1–AI4 on the LOGO! base. The transmitter and the LOGO! must share the same M / negative reference for the loop to be complete.

6.2 3-Wire Transmitter Wiring

For a 3-wire transmitter with separate supply, signal, and common:

The 3-wire topology isolates the supply current from the signal current, which is helpful when long cable runs cause the supply voltage at the transmitter to sag.

7. LOGO!Soft Comfort Programming

Programming in LOGO!Soft Comfort V8.3 or later follows the same pattern regardless of whether you used the resistor method or the AM2 module. The base block is Analog Input (AI) with the input source set to AI1 (or whichever physical input you wired).

1. Open your project in LOGO!Soft Comfort.
2. From the right toolbar, drag the Analog Input block onto the workspace.
3. Double-click it and set:
   • Input: AI1 (0–10 V) — for resistor method
   • Input: AM2 AI1 (4–20 mA) — for AM2 module
   • Sensitivity: depends on whether you want a switching threshold or a scaled value
4. Insert a downstream block (Threshold Trigger, Analog Comparator, or Math block) to perform the actual logic.

7.1 Analog Input Block Parameters (resistor method, 0–10 V)

7.2 Analog Input Block Parameters (AM2 4–20 mA)

8. Analog Input Scaling Math

The AI block returns a normalized 0–1000 integer (10-bit). To map it back to engineering units, use the LOGO!Soft Comfort Analog Amplifier, Math, or an Arithmetic Instruction.

8.1 Linear Mapping Formula

Engineer_Units = (AI_raw / 1000) × (P_max − P_min) + P_min

Example: 0–150 psi transmitter with a 500 Ω resistor:

• 4 mA → 2.000 V → AI_raw = 200
• 20 mA → 10.000 V → AI_raw = 1000
• Live-zero span (4 mA) maps to 200, not 0. This is by design — it preserves broken-wire detection.

To convert AI_raw to psi on the LOGO! 8 base:

Pressure_psi = ((AI_raw − 200) / 800) × 150

Implement with the Math Instruction block (function: Gain × Value + Offset), where:

• Gain = 150 / 800 = 0.1875
• Offset = −37.5 (this is applied after the gain, so sign matters — verify in simulation)

On LOGO! 8 the gain and offset fields are floats; on older LOGO! 6/7 the gain is an integer (1–999) and a separate PreGain handles fractional scaling. Use the simulation mode in LOGO!Soft Comfort to confirm before downloading to hardware.

8.2 Broken-Wire Detection

A broken loop drops the current to 0 mA, producing 0 V at the AI. The live-zero (4 mA → 2 V) means AI_raw < 200 is unambiguously a fault condition. Wire a Threshold Trigger with threshold = 150, on-time = 0 s, off-time = 0 s to flag Q_fault when this happens, and use it to disable downstream process logic.

On the AM2 in 4–20 mA mode, the live-zero is interpreted natively: the AI block returns negative values below 4 mA, so a value < 0 in the downstream Math block indicates a wire break.

9. Threshold / Alarm Configuration

Use a Threshold Trigger block to convert the analog value into a digital alarm:

This produces a high-pressure alarm output that energizes at 120 psi, de-energizes at 100 psi, with 2 s of debounce on the rising edge to avoid nuisance trips from pump pulsations. For a low-pressure trip, swap the On/Off threshold values.

For proportional control (PID-style), use the PID Controller block in LOGO!Soft Comfort. Feed the AI value into the PV input, set SP = 600 (= 75 psi on the 0–150 psi scale), and route MV to an analog output or a PWM block driving a proportional valve.

10. Verification and Calibration

After the program is downloaded, perform these checks before trusting the reading.

1. Loop integrity. With the transmitter disconnected, the AI raw value should read 0 (or below the live-zero threshold). This confirms there is no stray voltage pickup on the input terminal.
2. Zero check. Apply 0% pressure (vent to atmosphere for a gauge transmitter, or apply vacuum for an absolute). Verify the AI raw value matches the expected live-zero (200 with 500 Ω, 0 with AM2 4–20 mA). If it does not, adjust the AI block offset.
3. Span check. Apply a known pressure from a calibrator (or a dead-weight tester, or a hand pump with a reference gauge). Verify the AI raw value. Adjust the AI block gain to match.
4. Two-point linearity. Apply 25%, 50%, 75% of full scale. The error should be within the combined tolerance of the transmitter (typically 0.5% FS) and the 500 Ω resistor (0.1% for a quality metal film).
5. Noise check. Monitor the AI raw value on the LOGO!'s online status screen with the system running. The variation should be < ±2 counts for a clean installation; ±5–10 counts is typical in an industrial cabinet; > ±20 counts indicates a grounding or shielding problem.

10.1 Total Loop Error Budget

Worst-case error for a 0–150 psi transmitter on the resistor method:

For most process monitoring, this is more than adequate. For closed-loop control or custody transfer, use the AM2 module and a higher-accuracy transmitter (0.1% FS).

11. Troubleshooting Matrix

12. Field-Proven Caveats

• Loop voltage is everything. A 2-wire transmitter at 4 mA is fine, but at 20 mA the transmitter needs at least 10 V across its terminals plus the 10 V drop across the 500 Ω resistor. On a 12 VDC system that leaves only 2 V of headroom — too tight for most industrial transmitters. Use 24 VDC if at all possible.
• Ground the negative. The LOGO! 12/24 RCE base and the pressure transmitter must share the same M reference. If you power the LOGO! from a separate supply, tie the two M terminals together with a dedicated wire — do not rely on panel chassis ground.
• Do not exceed 28.8 V on the AI. The onboard AI is rated for continuous 28.8 V max. A loop-powered transmitter running on 30 V with the 500 Ω resistor will produce up to 10 V — safe. But a miswired system can push full supply into the AI and damage the input.
• Shield one end only. Shielded twisted pair cable from the sensor should have the shield grounded at the LOGO! panel end, not the transmitter end, to avoid ground-loop currents in the shield.
• The AI channel conflict on I7/I8. On a 12/24 RCE base, I7 and I8 are physically shared with AI3 and AI4. If you need both digital input 7 and analog input 3, you cannot have them on the same physical terminal — choose AM1 or AM2 expansion instead.
• LOGO!Soft Comfort online monitor shows normalized values, not engineering units. The AI block displays 0–1000. To see psi or bar, you need a Math block downstream with the gain/offset applied, and you monitor that block, not the AI block directly.
• Filter, but verify. LOGO!Soft Comfort offers an analog low-pass filter on the AI block. A 100 ms filter smooths display noise but adds 100 ms of dead time to your control loop. For alarms, this is fine; for closed-loop pressure control, a 100 ms filter can destabilize a fast loop. Start with 50 ms and adjust.

13. Putting It Together

For a typical air-line pressure monitoring application with a LOGO! 12/24 RCE and a 4–20 mA pressure transmitter:

1. Source a 24 VDC supply (or a 12 VDC supply if your transmitter is rated for 12 V operation and your loop budget allows it).
2. Wire the transmitter in 2-wire mode through a 500 Ω, 0.1%, 10 ppm/°C metal-film resistor to AI1 on the LOGO! base.
3. Program the AI block with sensor type 0–10 V and engineering range 0–150 psi (or whatever your transmitter range is).
4. Add a Math block to convert AI_raw (0–1000) into engineering units, accounting for the 4 mA live-zero.
5. Add a Threshold Trigger to drive an alarm output at your high- and low-pressure setpoints.
6. Calibrate at zero and span using a pressure calibrator or hand pump with a reference gauge.

For a 24 VDC cabinet, skip the resistor and use an AM2 module set to 4–20 mA mode — the module does the conversion internally, and the live-zero detection is built in. The trade-off is panel space and the cost of the extension.

Either approach lets you bring a real industrial pressure signal into a stock LOGO! without a custom signal conditioner, a dedicated RTU, or a PLC upgrade.