SITRANS LR560 4-20mA Loop Wiring to PLC Analog Input

SITRANS LR560 Overview and Identification

The SITRANS LR560 is a 2-wire 24 GHz FMCW (Frequency Modulated Continuous Wave) radar level transmitter designed for continuous, non-contact measurement of bulk solids in silos, bins, crushers, and stockpiles. The 4-20 mA HART variant accepts loop power from the host system on the same two conductors that return the process variable. This guide covers the 7ML5440-xxxxx-xA (mA / HART) order code variant and is consistent with Siemens operating instructions A5E34647946.

Reference: SITRANS LR560 (mA/HART) Operating Instructions, A5E34647946.

Prerequisites and Required Documentation

Before opening the LR560 enclosure or landing conductors, gather the following items on the bench and verify each is current to the project revision:

• Operating instructions A5E34647946 (PDF on a phone or tablet is acceptable; printed copy preferred for cold-weather sites).
• 24 V DC regulated power supply capable of delivering ≥ 22 mA continuous plus cold-start inrush margin (Siemens 6EP1332, Phoenix Contact QUINT, or equivalent).
• Shielded twisted-pair (STP) instrument cable, 2-core, 0.5 to 1.5 mm² (20 to 16 AWG) with a drain wire and overall foil + braid shield.
• PLC analog input module (4-20 mA, sink or source type) with channel diagnostic available. The AI module must include a 250 Ω internal burden for HART communication.
• 2.5 mm slotted and Pozidriv (PZ1) screwdrivers, ferrule crimper, wire stripper calibrated to 8 mm strip length.
• Multimeter with mA DC scale and 4-20 mA loop test mode; HART communicator (Rosemount 475, Emerson TREX, or Siemens PDM) for setup.
• Cable gland sized to the cable OD, plus a spare shield termination ring.

Power and Signal Loop Concept

The 4-20 mA HART model of the LR560 is a 2-wire loop-powered device. The same two conductors that power the radar electronics also carry the analog process signal. A 24 V DC source feeds current into the loop; the transmitter regulates that current between 4 mA (level = empty / 0% URV) and 20 mA (level = full / 100% URV) as a function of measured distance to the bulk-solid surface. The host controller measures the loop current via an internal sense element, typically a 250 Ω precision burden resistor that converts 4-20 mA to 1-5 V for the analog-to-digital converter.

The 4 mA lower limit preserves a small bias current that powers the LR560 electronics, the 24 GHz oscillator, the signal processor, and the optional LCD. The 16 mA span between 4 mA and 20 mA represents 0% to 100% of the calibrated level range. The 4 mA bias also serves as a "device alive" indicator: a steady 4 mA with no HART response usually means the device is unconfigured, while a steady 0 mA means the loop is broken.

Terminal Layout Inside the LR560 Housing

Open the LR560 enclosure by loosening the lid lock screw and unscrewing the upper housing cover. The display module (if installed) is hinged and lifts out of the way. The terminal block sits on the lower electronics stack, below the display pivot.

On the same board you will find jumpers or DIP switches for write protection, fail-safe direction (3.6 mA or 22 mA on fault), and HART enable. The default for the mA/HART variant is fail-safe low (3.6 mA) and write protection disabled. Local display operation uses the four-button keypad on the hinged module; menu navigation is documented in chapter 4 of the operating instructions.

Wiring the 2-Wire 4-20 mA Loop

1. Isolate the 24 V DC supply and the PLC analog input module. Lock-out/tag-out the cabinet and verify zero energy with a multimeter.
2. Open the LR560 housing cover. Lift the display module out of the way if present. Identify terminals 1 (+) and 2 (-) on the label.
3. Strip 8 mm of insulation from each conductor of the shielded twisted-pair cable. Apply insulated ferrules to stranded conductors to prevent stray whiskers from shorting adjacent terminals.
4. Connect the loop positive conductor (red) to terminal 1 (+) of the LR560 terminal block. Torque the screw to 0.5 to 0.6 N·m.
5. Connect the loop negative conductor (blue or black) to terminal 2 (-) of the LR560 terminal block. Tighten to the same torque.
6. Terminate the cable shield at the LR560 external ground stud using the supplied cable gland shield ring. Do not bond the shield to terminal 4 inside the device; doing so will create a ground loop on shielded runs over a few meters.
7. Route the cable through the cable gland. Tighten the gland to the point where the rubber grommet grips the jacket without crushing it. Pull-test the cable to confirm it cannot be pulled out of the gland.
8. Reinstall the display module (if hinged aside) and close the housing cover. Torque the lid lock screw and re-engage the safety pin if equipped.

Connecting to a PLC Analog Input

The PLC side of the loop must include a sense element to convert the 4-20 mA signal into a voltage the analog-to-digital converter can read. The most common implementation is a 250 Ω precision resistor inside the AI module, which converts 4-20 mA to 1-5 V. For HART communication, the resistor must remain in the loop (HART uses 1200/2200 Hz FSK keyed onto the same current), and the burden value must be at least 230 Ω to keep the FSK amplitude above the demodulation threshold.

Examples of compatible Siemens AI modules:

For non-Siemens controllers, choose an AI channel configured as 4-20 mA with a 250 Ω internal burden or external 250 Ω resistor across the input. The S7-1200 SM 1231 modules and the ET 200SP AI 4xI 2-wire modules accept the LR560 directly with no external resistor. Allen-Bradley 1756-IF8H, 1769-IF8, and CompactLogix 5069 modules also accept 2-wire 4-20 mA inputs with the on-board 250 Ω burden enabled in Studio 5000.

Voltage Drop and Loop Load Calculation

The LR560 has a defined terminal voltage range. The supply must be high enough to drive the worst-case loop current (20 mA at full level) through the cable resistance, the AI module sense resistor, and any other series element, while still leaving the LR560 within its operating window.

Use the following equation:

For HART 7 communication, the total loop resistance (including the 250 Ω sense resistor) must be between 250 Ω and 600 Ω. Above 600 Ω, the HART modem will not have enough signal swing to demodulate the FSK signal; below 250 Ω the FSK modulation is shorted by the source.

Worked example for a 100 m cable run using 1.0 mm² copper (loop resistance ≈ 36 Ω/km round trip):

This leaves 6.9 V of headroom above the LR560 minimum and is well within HART compliance. If a longer cable run is required, derate by increasing conductor cross-section or use a higher supply voltage (e.g. 26-28 V DC if the PSU can be adjusted). Do not exceed 30 V DC at the LR560 terminals.

For a 500 m run with 1.0 mm² copper the round trip becomes 18 Ω and the LR560 terminal voltage falls to 24 - (20 mA × 268 Ω) = 24 - 5.36 = 18.64 V, still healthy. A 1 km run with 0.75 mm² (50 Ω/km) yields 50 Ω round trip, terminal voltage 24 - 5.5 = 18.5 V - still healthy because the sense resistor dominates. The 250 Ω sense resistor sets most of the loop drop; cable resistance is secondary until runs exceed a few hundred metres.

For installations where the LR560 is mounted on a structure with poor local grounding, the shield should be bonded to the LR560 ground stud and left floating at the cabinet end. This star-grounding topology prevents ground-loop currents that would otherwise add a DC offset to the loop and bias the level reading by 1-3% of span.

Cable Selection and Shielding

Siemens recommends a shielded, twisted-pair instrument cable with the following characteristics:

• Conductor cross-section: 0.5 to 1.5 mm² (20 to 16 AWG). Larger is acceptable for long runs but does not improve noise immunity beyond 1.5 mm².
• Shield coverage: ≥ 80% braid or foil/braid combination. Foil-only shields are acceptable for short runs inside cabinets.
• Capacitance: ≤ 100 pF/m between conductors; lower capacitance aids HART communication over long runs by reducing the RC time constant seen by the FSK signal.
• Voltage rating: ≥ 300 V.
• Jacket: PVC, PE, or LSZH suitable for the ambient and chemical exposure. Use PUR or TPE in oily environments.

Route the cable in a dedicated conduit or cable tray, separated from VFD power cables, motor feeders, and any conductor carrying > 1 A. Maintain at least 200 mm (8 in) of separation from 480 V three-phase feeders; cross perpendicular to power conductors if the path must intersect. Ground the shield at one end only, at the cabinet (PLC) end, using a low-impedance bond to the panel backplate. Bond the LR560 end of the shield to the external ground stud on the housing. If the run is less than 5 m, the shield can be bonded at both ends without significant offset error.

First-Power Commissioning

1. With the loop wired, verify polarity at the LR560 terminals with a multimeter before energizing. Terminal 1 should be positive with respect to terminal 2.
2. Close the LR560 housing. Torque the cover and engage the lock screw.
3. Confirm the 24 V DC PSU is in the OFF state. Connect the loop conductors to the PLC AI module terminals per the module's wiring diagram (e.g. AI+ on terminal 2, AI- on terminal 3 for SM 1231).
4. Power the 24 V DC PSU. The LR560 runs a self-test (the display will cycle if equipped with a display, and the local HART master sees a 0x00 status code change during power-up).
5. On the PLC, configure the AI channel for 4-20 mA, 2-wire mode. Scale the engineering units in the PLC program to match the silo height, e.g. 4 mA = 0.0 m, 20 mA = 25.0 m (or whatever the calibrated empty and full distances are).
6. Read the loop current with a clamp meter or by breaking the loop at the test terminal. At empty silo, the LR560 should output between 3.8 and 4.2 mA; at full silo, between 19.8 and 20.2 mA, less a small offset for damping.

If the LR560 has an integral display, enter the Quick Start wizard via the keypad. The menu path is Menu > Quick Start > Unit (m, ft, %, %-volume, mA, in) > Empty Calibration (distance from sensor face to empty level) > Full Calibration (distance from sensor face to full level) > Dielectric Range (low, medium, high for the bulk solid) > Output Damping (10 to 30 s typical for solids applications; 60 s for cement silos with heavy dust). A typical LR560 Quick Start on a fly-ash silo is: Unit = m, Empty = 18.0 m, Full = 1.0 m, Dielectric = low (dry fly ash), Damping = 30 s.

Save the configuration by selecting Apply in the Quick Start menu. The LR560 will exit configuration mode and resume normal measurement within 2 to 5 seconds, with the configured damping time controlling the rate of approach to the final value.

Verification

1. Loop voltage check: with the loop energized and the LR560 at 20 mA, measure the voltage at the LR560 terminals. It must lie between 12 V and 30 V. The Siemens operating instructions list a "Voltage low at the LR560" diagnostic that triggers if the terminal voltage falls below the minimum. Two causes are listed in the manual: (1) voltage outside specification due to cable drop or undersized supply, and (2) hardware defect.
2. Loop current check: break the loop at the test terminal or use a clamp meter. Confirm 4 mA at empty and 20 mA at full after a level change. Hold the reading for 60 seconds to verify damping is engaged.
3. HART communication check: connect a HART communicator (or SIMATIC PDM) across the 250 Ω sense resistor on the loop. The device should report short tag (HART 5) or long tag (HART 7), PV (primary variable = level), SV (distance), TV (echo strength), QV (temperature), and device status with no active alarms.
4. PLC scaling check: write the PV to a real-time tag in the PLC (e.g. DB10.DBX0.0 as REAL) and verify the engineering unit conversion matches the silo level. The raw count from a 13-bit SM 1231 channel at 4 mA should be 0; at 20 mA, 27648.
5. Damping and step response: introduce a simulated step (e.g. trip the silo level set point, or use a level simulator / half-clamp on the antenna to create a false echo) and confirm the PV in the PLC tracks within the configured damping time ±10%.
6. Fail-safe test: induce an echo-lost condition (cover the antenna with a metal plate) and verify the LR560 drops the loop to 3.6 mA (or 22 mA, depending on the fail-safe configuration). The PLC should treat the 3.6 mA / 22 mA current as a fault and switch to backup control or alarm.

Troubleshooting Common Wiring Issues

The Siemens operating instructions list the following common wiring-related faults and remedies. Each row maps the symptom to a probable cause and an actionable test.

Reference: SITRANS LR560 (mA/HART) Operating Instructions - "Voltage low at the LR560" diagnostic and remedies on the troubleshooting pages.

Spares, Firmware, and Field Tips

Common spare parts to keep on the truck or in the panel shop:

• LR560 terminal block spare (Siemens spare A5E34468020 or equivalent). The push-in spring terminals can be replaced in the field without desoldering.
• Cable gland and shield termination ring sized to the actual cable OD used.
• 250 Ω 0.1% precision burden resistor as a HART communication reference; a 500 Ω substitution is the most common commissioning error on third-party systems and breaks HART.
• Pre-terminated 5 m tail of shielded twisted-pair cable with ferrules for bench-testing the LR560 before it goes up the silo.

Firmware: the LR560 supports field firmware updates via SIMATIC PDM (PC) or a 475/TREX handheld over the HART interface. The current firmware revision can be read via the menu path Menu > Diagnostics > Device > Firmware revision. Recent firmware revisions have improved false-echo handling in dusty solids applications and reduced the time-to-first-measurement after power-up. Always check the Siemens Product Support page for the latest revision compatible with the device's hardware (HW) rev before flashing; mixing a newer FW with an older HW can lock out specific diagnostic features.

Field tips that have saved return trips:

• Always read the loop voltage at the LR560 terminals, not at the PSU. A 24 V supply at the panel can fall to 18 V at the silo on a long, undersized run.
• Tag both ends of the cable with the same circuit number. A reversed conductor pair on a multi-pair cable is a common cause of 4 mA pinned output.
• Apply 1-2 wraps of self-amalgamating tape around the cable gland threads after tightening; this prevents water ingress in outdoor silo applications and slows corrosion of the brass gland body.
• Photograph the LR560 wiring and HART configuration before leaving site. The image is invaluable for remote support and for the as-built documentation package.

FAQ