Solving S7 Analog Output Burden Limit on 2.2 kΩ E/P Regulators

This engineering reference addresses a recurring SIMATIC S5-to-S7 migration problem: an analog output channel previously connected to a 4-wire electro-pneumatic (E/P) regulator — here, the SMC EIT 200-F102B with a measured input impedance of 2.2 kΩ — is reconnected to an S7-300 SM 332 (or S7-1500 AQ) module whose datasheet declares a 500 Ω maximum load for current outputs. The 20 mA end-of-scale signal across 2.2 kΩ would require 44 V of compliance, while the SM 332 supplies roughly 18–22 V at its output terminals. The output stage saturates, the loop current stalls near 9–10 mA, and the regulator pressure is non-linear. We explain the burden concept, calculate required compliance, compare S5 and S7 module capability, and present five field-engineered solutions that do not require replacing the field device.

Problem Overview

During a SIMATIC S5 to S7 migration, the analog output (AO) channels previously connected to a 4-wire I/P or E/P regulator must be re-terminated on the equivalent S7 module — most often an SM 332 (6ES7332-5xxx) for an S7-300 station or an AQ module (6ES7532-5xxx) for an S7-1500 station. When the engineer reads the SM 332 datasheet and notes that the maximum load for the 0/4–20 mA current output is 500 Ω, but the E/P regulator's input resistance is 2.2 kΩ, an apparent incompatibility appears. The voltage required to push 20 mA through 2.2 kΩ is 44 VDC, which exceeds the typical output compliance of an S7 current output (≈ 18–22 VDC at the terminals, depending on the backplane 24 V supply tolerance). Driving the regulator directly will either saturate the output at a non-linear value or trigger the module's wire-break / short-circuit diagnostics.

Burden Resistor Concept and Compliance Voltage

In a 4–20 mA current loop, the transmitter (or analog output) regulates current; the receiver (or field device) presents a series impedance. The transmitter must be capable of driving enough voltage across the loop to establish the commanded current. The maximum allowable loop resistance — including the receiver input impedance, wire resistance, and any series-sense burden — is the burden rating of the transmitter. Siemens TIA Portal documentation defines the burden in the context of 2-wire transducers: "The burden indicates how great the external resistance in a current loop may be. If the external resistance is greater than the specified burden, the 2-wire [transducer] can no longer reach its full output." (See TIA Portal: Burden with 2-wire transducers.) The same principle applies to an analog output acting as the loop source.

The compliance voltage (V_c) that the output stage must produce is governed by Ohm's law:

where I is the loop current (A) and R_loop is the total series resistance (Ω). For a 4–20 mA loop with a 2.2 kΩ receiver and a 100 m cable of 0.75 mm² conductors:

R_wire = 2 × 0.0172 Ω·mm²/m × 100 m / 0.75 mm² = 4.59 Ω
R_total = 2200 + 4.59 = 2204.6 Ω
V_required_max = 0.020 × 2204.6 = 44.1 V

The required compliance exceeds the SM 332 capability by approximately 22 V. The output stage will saturate well before reaching 20 mA. The table below quantifies the gap for typical current setpoints:

At 20 mA the SM 332 can develop about 22 V across its terminals, which limits the maximum current the output can push into 2.2 kΩ to roughly 10 mA. The output is in compliance saturation. The receiver will read approximately 50% of the commanded pressure setpoint, with pronounced non-linearity near the upper range.

S5 vs S7 Analog Output Capability

The original S5 system presumably drove the E/P without difficulty. The likely explanations are:

• The S5 AO was set to voltage output (0–10 V). The 2.2 kΩ input impedance is then a normal voltage-input load: 10 V / 2200 Ω = 4.5 mA, well within SM 332 voltage-output drive capability. Migrating to S7 requires reconfiguring the new AO channel for 0–10 V rather than 4–20 mA.
• The S5 AO was an older module with different compliance characteristics. Some S5 6ES5 470-4Uxxx analog output modules supplied ±10 V or 0–20 mA from an internal rail that tolerated higher-impedance current loads, at the expense of linearity and thermal dissipation. The S7 SM 332 is more rigorously specified and protective.
• The 2.2 kΩ reading came from a faulty or miscalibrated ohmmeter. Always confirm the receiver input impedance with a second instrument and the manufacturer's published datasheet.

Note: The S5 burden figure is module-version dependent; consult the specific S5 6ES5 470-x manual for the exact rating of the legacy hardware. The S7 figures are from the standard SM 332 and AQ datasheets; the HART variant 6ES7332-8TF01-0AB0 supports 750 Ω, but is still insufficient for a 2.2 kΩ load.

SMC E/P Regulator Input Characteristics

The SMC EIT 200-F102B is a 4-wire electro-pneumatic regulator with a supply pressure range of 1.5–2 bar and an output pressure range of 0.05–1 bar. Per the field information provided:

• Input signal: 4–20 mA DC
• Input impedance: 2.2 kΩ (measured)
• Supply: 24 VDC (separate, from the 4-wire pair)
• Linearity: typically ±1% of full scale

The 2.2 kΩ input impedance is unusual for a 4–20 mA current loop receiver; the conventional 250 Ω sense resistor is normally included inside the receiver to convert current to 1–5 V. A 2.2 kΩ figure suggests one of three possibilities:

1. Voltage-input variant: The device is actually configured for 0–10 V input (not 4–20 mA), and 2.2 kΩ is the typical input impedance of a high-impedance voltage-input op-amp stage. The user is migrating from a voltage S5 output and should keep the S7 AO in voltage mode.
2. Current-input with high-impedance front end: The device samples current through a small internal shunt; 2.2 kΩ is the impedance of a current-limiting / protection network on the input. In this case, the 4–20 mA signal is still expected, and the regulator must derive 8.8–44 V compliance from somewhere — typically from a 30 V or 50 V internal supply generated from the 24 V power wires.
3. Misread of a 250 Ω burden: The 2.2 kΩ reading was taken with the regulator unpowered, when the input protection network is high-impedance. With power applied, the impedance drops to its operating value (typically 250 Ω).

Wiring Topology Analysis

The 4-wire topology of the SMC EIT 200 separates power and signal onto distinct conductor pairs:

Because the E/P electronics are powered externally, the 4-wire configuration isolates the analog signal return from the power return. This is a strong indication that the input stage is not a 2-wire loop-powered device (which would share one conductor for both power and signal). Consequently, the S7 AO does not have to source operating power for the E/P; it only has to source enough voltage to push 4–20 mA through the 2.2 kΩ input stage from a 24 V backplane rail — which it cannot, hence the migration problem.

Solution Decision Matrix

Selection criteria: prefer Solution 1 if the E/P accepts voltage input (verified by impedance stability under power). Otherwise prefer Solution 2 for its high isolation (1.5 kV typical), DIN-rail mounting, and explicit 0–10 V to 4–20 mA conversion that preserves the original 4-wire field device. Solution 3 is required when the V/I converter's compliance (typically 1 kΩ) is itself insufficient for the actual loop resistance.

Solution 1: Reconfigure S7 AO for 0–10 V Output

If verification confirms the E/P has a true voltage-input front end (2.2 kΩ stable under power), the cleanest fix is to change the SM 332 output type from 4–20 mA to 0–10 V in the STEP 7 / TIA Portal hardware configuration. SM 332 modules in the 6ES7332-5xxx range support both modes on a per-channel basis. Voltage-mode output is suitable for high-impedance loads: minimum load is 1 kΩ; the 2.2 kΩ E/P input is well within spec.

Procedure in TIA Portal V17+:

1. Open the device configuration of the S7-300 station.
2. Select the SM 332 module in the rack.
3. In the Properties → Analog Outputs tab, identify the affected channel (e.g., Channel 0).
4. Set "Output type" to "Voltage".
5. Set "Output range" to "0..10 V".
6. Save and download the hardware configuration to the CPU.
7. Recompile any user program blocks that scale the AO value; the 0–10 V output uses 0 → 0 V and 27648 → 10 V (same integer range as 4–20 mA).

The wiring remains identical — only the software type changes. Verification: with output commanded to 50% (≈ 13824 decimal), measure 5.00 V ±0.05 V at the SM 332 terminals. At the E/P input, current draw is V/R = 5.00 / 2200 = 2.27 mA, which the E/P internal electronics scale to the same pressure setpoint as 12 mA into a 250 Ω burden (5 V / 250 Ω = 20 mA equivalent scaling depends on the regulator's internal transfer function — consult SMC documentation).

Solution 2: External Voltage-to-Current Converter

If the E/P is a true 4–20 mA current-input device and the S7 AO must remain in current mode for other channels, install an isolated voltage-to-current converter between the SM 332 and the field device. Recommended Phoenix Contact modules (compatible with standard DIN-rail installation in the S7 cabinet):

Configuration:

1. Configure SM 332 for 0–10 V output (the converter's input range).
2. Wire SM 332 + → MINI MCR terminal 3 (input +); SM 332 – → terminal 4 (input –).
3. Wire MINI MCR terminals 7 (+) and 8 (–) to the 4-wire E/P signal pair (E/P signal wires, NOT the 24 V power wires).
4. Set the converter DIP switches for input 0–10 V and output 4–20 mA (per Phoenix Manual UM EN MINI MCR-2-UI-UI).
5. Power the converter from the same 24 V PLC supply rail; typical current draw is 25–35 mA.

The MINI MCR-2 provides 1000 Ω of burden capability at 20 mA with the standard 24 V supply. For a true 2.2 kΩ load the converter itself will saturate at 24 V / 2200 Ω = 10.9 mA. Two remedies: (a) use the MACX MCR with a 30 V auxiliary supply, raising compliance to 30 V / 2200 Ω = 13.6 mA — still short of 20 mA; (b) use an active booster like the Knick RP-100 or WAGO 857-402 with a 48 V auxiliary, achieving 48 V / 2200 Ω = 21.8 mA — sufficient. This is the boundary between Solution 2 and Solution 3.

Solution 3: Higher-Burden Module or Field Device Replacement

For 2.2 kΩ loads that exceed the headroom of any V/I converter, the next options are module substitution or field device replacement.

Module substitution (limited applicability):

Even the 750 Ω HART variant cannot drive 2.2 kΩ at 20 mA (would need ≥ 44 V). Substituting an S7 module alone is not a solution for this specific case. This option becomes viable only if the receiver's actual impedance is ≤ 750 Ω, which would contradict the 2.2 kΩ reading. Re-verify the field-device impedance before committing to this path.

Field device replacement: When instrumentation resources permit, replace the SMC EIT 200-F102B with a current-input E/P whose input impedance conforms to the 4–20 mA standard (≤ 500 Ω, optionally with HART). Suggested replacements with low burden:

• SMC ITV0010-2 series (4–20 mA input, ≤ 250 Ω)
• Siemens SIPART PS2 (6DR50xx) with 4–20 mA input and 250 Ω input impedance
• Emerson FIELDVUE DVC6200 series (4–20 mA input, 250 Ω)
• Yokogawa YVP series E/P regulator (250 Ω) — refer to Yokogawa Analog Output Modules GS34M06H11-05E for over-range and load compliance guidance.

Replacement eliminates the burden problem but introduces mechanical rework (pneumatic tubing, mounting, calibration) and may require HART registration reconfiguration if the original device used a proprietary protocol.

TIA Portal / STEP 7 Configuration

For Solutions 1 and 2, the following TIA Portal procedure applies. Use TIA Portal V17 or later, with the appropriate HSP installed for the SM 332 module (HSP 0332 covers the standard 6ES7332-5xxx family).

1. Open project and device view. In the project tree, expand the S7-300 / S7-1500 station and double-click the SM 332 module.
2. Select the channel. In the device view, select channel 0 of the analog output group.
3. Set output type. From the dropdown, choose either:
   • "Voltage" with range "0..10 V" (Solution 1: direct drive of voltage-input E/P), or
   • "Current" with range "4..20 mA" (Solution 2: drive the external V/I converter's input stage, which itself has high input impedance; then verify the converter's input range matches the SM 332 output).
4. Enable diagnostics. Activate "Wire break" and "Short circuit" diagnostic interrupts to detect field-side faults during commissioning.
5. Compile and download. Save the configuration, compile the project, and download the hardware configuration to the CPU. The CPU will issue a STOP-to-RUN transition only after the new configuration is loaded.
6. Scale the analog value. In your user program, the AO process image uses 0 → 0% and 27648 → 100%, regardless of whether the physical output is 4–20 mA or 0–10 V. Existing FC105 / FC106 (or NORM_X / SCALE_X) blocks continue to work without modification.

For Solution 2, also configure the DIP switches on the MINI MCR before powering the converter. The Phoenix MINI MCR-2-UI-UI family uses a combination of rotary switches on the side of the housing; set input to "U" (voltage, 0–10 V) and output to "I" (current, 4–20 mA). Failure to set DIP switches correctly causes the converter to output 0–20 mA or 0–10 V regardless of the input, producing a non-linear control response.

Verification, Commissioning, and Field Caveats

Use the following procedure after hardware reconfiguration and before returning the loop to automatic control:

1. Visual inspection. Verify all four wires of the E/P are landed on the correct terminals; check for shield grounding at the cabinet end only.
2. Loop power check. With the SM 332 channel disabled (output = 0), measure the 24 VDC at the E/P power terminals. Confirm ≤ 50 mV ripple.
3. Current measurement. Insert a precision ammeter (e.g., Fluke 789) in series with the signal + wire. Command the S7 AO to 0%, 25%, 50%, 75%, 100% (values 0, 6912, 13824, 20736, 27648). Verify 4.00, 8.00, 12.00, 16.00, 20.00 mA ± 0.05 mA at the receiver for Solution 2 (or 0.00, 2.50, 5.00, 7.50, 10.00 V for Solution 1).
4. Pressure verification. With a calibrated reference manometer at the E/P output, verify the pressure endpoints and at least three intermediate points against the commanded current. Acceptance criterion: ±1% of full scale (0.01 bar at 0–1 bar output range).
5. Step response. Apply a 10–90% step and record the settling time. The SMC E/P should reach 99% of setpoint in < 2 s; if it is significantly slower, the supply pressure may be inadequate (verify 1.5–2 bar supply).
6. Fault injection. Disconnect the signal wire to confirm the S7 diagnostic interrupt (wire-break) fires. Reconnect and clear the diagnostic with ACK.

Field-proven caveats collected from S5-to-S7 migration projects:

• Read the impedance under power. A 2.2 kΩ reading with the E/P unpowered does not represent the operating impedance. Many current-input regulators present a high impedance at zero current and lower impedance when their internal supplies are active.
• Watch the voltage-mode minimum load. SM 332 in 0–10 V mode requires R_load ≥ 1 kΩ. A 2.2 kΩ load is fine; a 500 Ω load will trigger a short-circuit diagnostic.
• Don't float the MANA terminal. SM 332 channels have a common MANA return. If the field device is grounded and MANA is also grounded, a ground loop develops. Follow the wiring diagram in the SM 332 manual precisely; the diagrams distinguish 2-wire, 3-wire, and 4-wire loads with explicit MANA jumpers.
• Shield one end only. Ground the analog cable shield at the cabinet entry, not at the field device. Multiple ground points inject noise that the high-impedance (2.2 kΩ) input stage picks up easily.
• Don't use a 250 Ω sense resistor on a 2.2 kΩ input. Some legacy S5 schematics show a 250 Ω burden across the AO terminals. If the new field device already has 2.2 kΩ, adding 250 Ω in parallel reduces the input to 224 Ω, but it also creates a 5.5 V drop at 20 mA that the SM 332 must source — wasting 1.1 W of headroom for no benefit.
• Confirm the S5 was actually using 4–20 mA. A common migration error is to assume the S5 wiring is current-mode when it was voltage-mode. Check the S5 module's output type selector (DIP switch or jumper) and the original loop calibration sheet.
• Account for the converter's own compliance. A Phoenix MINI MCR-2 with a 24 V supply cannot drive a 2.2 kΩ load to 20 mA. Either supply the converter from 30 V, choose a higher-compliance conditioner, or convert the field device to voltage input.

FAQ