Reading 4-20mA Sensor with LOGO! 6ED1052-1HB00-0BA6 0-10V Input

Overview

The Siemens LOGO! 6ED1052-1HB00-0BA6 is a logic module from the LOGO! 8 generation family whose on-board analog inputs are rated 0–10 V only. Field instrumentation, however, overwhelmingly uses the 4–20 mA current loop because it tolerates long cable runs, provides live-zero fault detection (3.6 mA / 21 mA NAMUR NE 43 limits), and can power two-wire transmitters over the same pair. Bridging these two worlds requires either an active signal conditioner, the AM2 / AM2 RTD expansion module, or — when no module is available — a single precision resistor that converts the loop current into a voltage that fits inside the LOGO! 0–10 V window.

This reference documents the resistor-drop method in full detail: how to choose the resistor value, why the popular 250 Ω suggestion does not work on a 0–10 V input, how to wire two-wire loop-powered and four-wire active transmitters, and how to map the resulting voltage back to engineering units inside LOGO! Soft Comfort. It applies to the following base modules:

• 6ED1052-1MD00-0BA5 / 6ED1052-1CC00-0BA5 (LOGO! Basic, 0BA5)
• 6ED1052-2MD00-0BA5 / 6ED1052-2CC00-0BA5 (LOGO! Pure, 0BA5)
• 6ED1052-1MD00-0BA6 / 6ED1052-1CC00-0BA6 (LOGO! Basic, 0BA6)
• 6ED1052-2MD00-0BA6 / 6ED1052-2CC00-0BA6 (LOGO! Pure, 0BA6)

The 0BA6 firmware exposes the additional analog channels Ai3 / Ai4 on terminals I1 and I2, which the older 0BA5 generation does not. This matters when you need more than two analog channels without adding an AM2 expansion. Refer to the Siemens Industry Online Support portal for the LOGO! 8 system manual and the LOGO! Soft Comfort V8.x help system for the latest signal conditioning blocks.

Prerequisites

Electrical Theory: 4–20 mA to 0–10 V Conversion

Ohm's law governs the conversion: V = I × R. With a 4–20 mA loop and a fixed resistor R, the voltage developed across the resistor is:

• At 4 mA: V_min = 0.004 × R
• At 20 mA: V_max = 0.020 × R
• Span: V_span = 0.016 × R

The receiving LOGO! input accepts 0–10 V. Three practical resistor choices exist:

The widely cited 250 Ω resistor (the legacy '250 Ω sense resistor' for 1–5 V HART receivers) is wrong for a LOGO! 0–10 V input because it produces only 1–5 V, leaving the upper five volts of the ADC range unused. The 250 Ω value is a leftover from 1–5 V instrumentation, where the standard is 4–20 mA across 250 Ω = 1–5 V. On a 0–10 V ADC you must use 500 Ω so that the live-zero point (4 mA) lands at 2 V and full-scale (20 mA) lands at 10 V.

Power dissipation in the resistor at full scale is:

P = I² × R = (0.020)² × 500 = 0.20 W

A 0.5 W metal-film part gives a 2.5× safety margin. A standard 1/4 W carbon-film resistor is mechanically acceptable but introduces ±5 % initial tolerance and ~250 ppm/°C drift that will show up as a wandering analog reading once the cabinet warms up.

Quantization and Resolution

The on-board analog channels on the 6ED1052-1HB00-0BA6 are 10-bit successive-approximation converters with a 0–10 V input range, yielding a least-significant bit (LSB) of:

LSB = 10 V / 1024 = 9.77 mV

With the 500 Ω resistor the sensor 4 mA step maps to 8 mV at the resistor, which is below one LSB; in practice the sensor will sit between two counts near the bottom of the range, and LOGO! Soft Comfort's hysteresis function on the analog amplifier is required to prevent the output from toggling between adjacent engineering values. For a temperature loop, a 0.5 °C dead-band is typical.

Effective resolution after mapping the 4–20 mA signal to 2–10 V is 8 V / 1024 = 7.81 mV per count. For a 0–100 °C PT100-style transmitter this is 0.098 °C per count, which is acceptable for most HVAC applications but borderline for laboratory work — switch to the AM2 RTD module 6ED1055-1MD00-0BA0 in that case.

Resistor Selection Criteria

Avoid standard 1/4 W carbon-film resistors from the local electronics shop. The reading drift caused by self-heating and ambient temperature change is the single most common field complaint with this circuit. Specify:

• Value: 500.0 Ω exactly. Do not substitute 470 Ω, 510 Ω or 560 Ω.
• Tolerance: ≤ 0.1 % (100 ppm). 0.01 % parts exist but are unnecessary.
• Temperature coefficient: ≤ 25 ppm/°C. 5 ppm/°C (Vishay Z-Foil or similar) is recommended when the cabinet runs above 40 °C.
• Power rating: ≥ 0.5 W at 70 °C ambient.
• Construction: Metal film, axial or SMD with leads. Wire-wound resistors introduce stray inductance that can resonate with sensor cable capacitance.

Identifying the 0BA6 Analog Inputs

The 0BA6 family supports four on-board analog channels instead of the two found on 0BA5:

On the 6ED1052-1HB00-0BA6 specifically (the LOGO! 8 module), inputs I1–I8 are still selectable as digital 24 V inputs. When the program references Ai1–Ai4, LOGO! Soft Comfort automatically routes the corresponding terminal to the analog path. There is no configuration switch or jumper. Refer to the LOGO! 8 system manual on the Siemens Industry Online Support portal for the full I/O list.

Wiring a 4-Wire (Active) 4–20 mA Sensor

A four-wire sensor has dedicated power terminals (often labelled +Vs / −Vs or V+ / V−) and dedicated signal terminals (often labelled I+ / I− or Sig+ / Sig−). The transmitter actively sources the loop current; the receiver only has to provide a return path.

1. Connect sensor +Vs to the +24 V DC supply.
2. Connect sensor −Vs to the supply negative (and to LOGO! M terminal).
3. Connect sensor I+ to one end of the 500 Ω resistor and to the LOGO! analog input (I7 = Ai1, I8 = Ai2, I1 = Ai3 or I2 = Ai4).
4. Connect the other end of the resistor to LOGO! M (the common terminal paired with the input you used).

Resulting loop: 24 V → +Vs → internal regulator → I+ → 500 Ω resistor → M → −Vs → supply negative. Current flows from the sensor through the resistor back to the sensor. The voltage measured at I7 / I8 / I1 / I2 with respect to M is the resistor drop: 2.0 V at 4 mA, 10.0 V at 20 mA.

Wiring a 2-Wire (Loop-Powered) 4–20 mA Sensor

A two-wire sensor receives its operating power from the same two wires that carry the 4–20 mA signal. The supply must therefore provide both the sensor electronics current and the loop current, which adds in series. Typical compliance voltage required is 12–30 V DC depending on the sensor.

1. Connect +24 V DC to the sensor's positive terminal.
2. Connect the sensor's negative terminal to the LOGO! analog input (I7 = Ai1, I8 = Ai2, I1 = Ai3 or I2 = Ai4).
3. Connect one end of the 500 Ω resistor to the same analog input terminal.
4. Connect the other end of the 500 Ω resistor to LOGO! M (common).

Resulting loop: 24 V → sensor + → sensor − → I7 / I8 / I1 / I2 → 500 Ω → M → supply negative. The full loop current passes through the resistor; the LOGO! input is high-impedance and draws no current. Voltage at the input is again 2.0–10.0 V.

LOGO! Soft Comfort Scaling Procedure

The raw analog count from the on-board input is 0–1000 for 0–10 V. With the 500 Ω resistor, the sensor's 4 mA maps to 200 counts and 20 mA maps to 1000 counts. To recover engineering units, use the Analog Amplifier (B036) or Mathematical Instruction with the linear formula:

Engineering = ((Raw − 200) / 800) × (E_max − E_min) + E_min

Example 1 — Pressure transmitter 0–10 bar at 4–20 mA:

Pressure_bar = ((Raw − 200) / 800) × 10

Example 2 — Temperature transmitter −20 to +150 °C at 4–20 mA:

Temp_C = ((Raw − 200) / 800) × 170 − 20

In FBD programming mode, drag an Analog Amplifier block onto the worksheet, connect the analog input (I7 / Ai1, etc.) to its input, and set:

• Sensor type: 0–10 V
• Sensor min: 200 (corresponds to 4 mA)
• Sensor max: 1000 (corresponds to 20 mA)
• Parameter min: −20 (or your E_min)
• Parameter max: 150 (or your E_max)
• Gain: 1.00
• Offset: 0

The block output is the scaled engineering value, ready to feed a threshold trigger, message text, display block, or TDE / HMI tag.

For a NAMUR NE 43 fault-detection check, add a parallel Threshold Trigger block configured to flag any reading below 184 counts (3.68 mA) or above 1020 counts (20.4 mA). This catches broken-wire and shorted-loop conditions within one program scan.

Alternative: AM2 Analog Expansion Module

If the sensor's maximum loop resistance is below 500 Ω, or if you need four-wire RTD / thermocouple inputs, install an AM2 module rather than use the resistor method:

Set the DIP switches on the AM2 to the 0/4–20 mA position. Wire the sensor positive to AM2 terminal I1+ or I2+, and the negative to M. No external resistor is needed. The AM2 returns 0–1000 counts for 0/4–20 mA, depending on the live-zero configuration you choose in the software.

Common Pitfalls and Errors

• Using 250 Ω because 'that's the standard sense resistor'. That standard applies to 1–5 V receivers, not 0–10 V. Always use 500 Ω on the LOGO! 0–10 V input.
• Referencing I7 as a digital input in the program. When I7 is used as a digital input in any rung, the analog path is disabled. Use the Ai1 block connector exclusively.
• Connecting the resistor between I7 and M but not breaking the ground return. If the sensor and LOGO! share a common ground elsewhere (e.g., through the 24 V return), the resistor is bypassed and the input reads 0 V regardless of sensor output.
• Substituting a 1/4 W carbon-film resistor. Self-heating and ambient drift will move the reading by 1–3 % over an hour.
• Forgetting the live-zero offset in software. Mapping raw 0–1000 counts to engineering 0–100 % without the (Raw − 200) / 800 step gives 4 mA = 20 % error, not zero.
• Sharing the LOGO! 24 V sensor supply across many loops. Each 20 mA loop draws 0.020 A. The LOGO! 24 V output is typically limited to 200 mA; budget for the sum of all loop currents plus the sensor quiescent draws.
• Exceeding the sensor's maximum loop load. A 500 Ω drop + sensor minimum compliance (often 12 V) requires at least 22 V supply. At 24 V you have 2 V margin; at lower supplies the loop will not reach 20 mA.

Troubleshooting Matrix

Verification Procedure

1. Power up the LOGO! with the sensor disconnected. Confirm input reads 0 counts.
2. Connect a precision 4 mA current source (or a calibrator) to the input terminals. Confirm reading is 200 ±5 counts.
3. Apply 12 mA. Confirm reading is 600 ±5 counts.
4. Apply 20 mA. Confirm reading is 1000 ±5 counts (or 999 with 0.5 % tolerance).
5. Disconnect the calibrator and connect the actual sensor. With sensor at zero process variable, confirm reading matches sensor datasheet live-zero (typically 4 mA → 200 counts).
6. Apply a known process variable (e.g., 5 bar with a 0–10 bar transmitter). Confirm LOGO! displays 5 bar ±0.1 bar after scaling.
7. Leave the system powered for 1 hour; re-check reading at the same process variable. Drift should be ≤ 0.2 % of full scale (≤ 2 counts for 10-bit resolution).
8. Simulate a broken wire by disconnecting the sensor signal lead. Confirm the NAMUR NE 43 threshold trigger block activates within one scan.

Field-Commissioning Notes

• Wire the resistor inside the LOGO! cabinet on a terminal block, not on the sensor head. Vibration and temperature on the sensor head accelerate drift on cheap resistors.
• Use shielded twisted-pair cable (e.g., Belden 8761 or equivalent) for runs over 3 m. Ground the shield at the LOGO! cabinet end only; floating the shield at the sensor end invites common-mode noise.
• When two or more sensors share the LOGO! M terminal, run each loop's return wire back to M independently. Daisy-chained returns add resistance that shifts the calibration.
• If the cabinet temperature exceeds 50 °C, derate the resistor by 50 % or specify a 1 W metal-film part. Reference Vishay CMF55 or Yageo MFR-25FBF52 series for confirmed 25 ppm/°C and 0.1 % tolerance at 1 W.
• For HART-compatible transmitters, the 500 Ω resistor is also the HART communication burden. Verify the transmitter supports 500 Ω minimum load before commissioning; otherwise HART data will not pass through.
• When using the LOGO! on-board 24 V sensor supply (terminal U+ / U−), confirm the supply can source the loop current plus all other sensor loads. The 6ED1052-1MD00-0BA6 supplies 200 mA; a single 20 mA loop uses 10 % of the budget.

Related Module Variants

The 6ED1052-1HB00-0BA6 is the LOGO! 8 (0BA8) generation. Earlier and later modules follow the same analog input rules but with different MLFB suffixes:

For LOGO! 8 (0BA8) modules, the analog resolution is 12 bits (0–4095 counts) instead of 10 bits (0–1000), giving 4× the resolution and 2.4 mV per LSB. The same 500 Ω resistor and (Raw − 819) / 3276 scaling formula applies with the new count range.

FAQ