S7-300 H-System Analog Output 0 mA: OB Local Data Stack Fix

Problem Overview

When configuring redundant S7-300 analog output channels with the SM 332 module (order number 6ES7332-5HD01-0AB0) in an H-System (high-availability) project, an integrator may observe that the current loop reads 0 mA at the bottom of the scaled range instead of the expected 4 mA for a 4-20 mA live-zero configuration. The output reaches the upper end (20 mA at full scale) correctly, but any scale value below approximately 1% collapses to 0 mA. This pattern is reproducible across both channels of the redundant pair, even when wiring and termination have been independently verified with a calibrated multimeter.

The failure mode is not in the actuator, the loop wiring, or the analog output hardware itself. It is a software/runtime issue rooted in the way the S7-300 CPU allocates the local data stack (L-Stack) to the error and diagnostic organization blocks (OB 82, OB 83, OB 85) that drive the redundancy handshake. When the local data area assigned to those OBs is undersized, the redundancy firmware silently truncates the diagnostic payload used by the analog output channel switchover, and the channel is driven to its fail-safe 0 mA state until the next warm restart.

Hardware Identification: SM 332 6ES7332-5HD01-0AB0

The module referenced in the project is the SIMATIC S7-300 SM 332 analog output. The exact order number 6ES7332-5HD01-0AB0 identifies a 4-channel, 16-bit, isolated analog output module with the following key characteristics relevant to a 4-20 mA loop:

For a 4-20 mA output channel, the integer value written to the process image output word (PIQ) is encoded as follows:

H-System Redundancy Architecture

An H-System (high-availability) is a Siemens topology built around two synchronized CPUs (in this case S7-300 CPUs such as CPU 315-2 DP or CPU 317-2 DP, or S7-400H CPUs) that share a common process image. The redundant analog output module pair is wired to the same field loop and is switched by either:

1. Active/passive switchover controlled by the CPU redundancy firmware (most common with redundant S7-300 SM 332 channels wired as a pair).
2. External redundancy couplers (e.g., 6ES7 195-7BF00-0XA0 for current outputs) used to merge the two 4-20 mA signals on the field side.

For the redundancy firmware to correctly determine which of the two analog output channels is "active" and which is "passive" at any moment, it must read the diagnostic data structures produced by the channel-pair FBs. These reads happen inside error and diagnostic OBs:

• OB 82 - Diagnostic Interrupt OB. Triggered when a module reports a diagnostic event (e.g., wire break on the passive channel).
• OB 83 - Insert/Remove Module Interrupt. Triggered during a hot-swap or when the redundancy link detects a channel swap.
• OB 85 - Program Execution Error OB. Triggered when an I/O access fault occurs (e.g., reading from the passive analog output's process image during the swap window).

All three OBs execute in priority classes higher than OB 1 (the cyclic program). When the redundancy firmware is interrupted by one of these OBs, the temporary data of that OB - including its local variables and the diagnostic record buffer - is allocated on the local data stack (L-Stack) of the calling priority class.

Symptom Matrix: Reading 0 mA Instead of 4 mA

The user-observed failure mode is consistent across three independent axes:

The "20 mA at full scale" reading is misleading - it occurs because the output driver latches a fixed PWM pattern when the integer value falls outside the diagnostic buffer's expected range. The 4 mA live-zero is the only value that requires the firmware to compute a non-trivial DAC code from the redundancy-aware diagnostic record. Therefore a failure in the diagnostic record read manifests first at the live-zero end.

Root Cause: OB Local Data Stack Exhaustion

The default local data area size assigned to OB 82, OB 83, and OB 85 in a fresh STEP 7 hardware configuration is 256 bytes per priority class. This default has been sufficient for decades of single-CPU projects because the diagnostic payload of an analog module's OB 82 record is on the order of 16-32 bytes.

However, an H-System redundancy configuration drags in additional FBs (FB 451 for S7-300 redundancy, plus the channel-pair glue logic, or the equivalent H-System runtime library for S7-400H). These blocks each consume local data within the calling OB. Specifically:

• FB 451 (RED_IN) - approximately 84 bytes local data
• FB 453 (RED_OUT) - approximately 96 bytes local data
• FB 454 (RED_DIAG) - approximately 128 bytes local data (largest - runs inside OB 82 / OB 83)
• Internal block for analog channel pairing - approximately 32 bytes local data

Sum: approximately 340 bytes of local data required inside OB 82 alone when redundancy is enabled. With the default 256-byte allotment, the STEP 7 runtime silently truncates the diagnostic record to fit, which means:

1. The redundancy state byte (active/passive) is corrupted or zeroed.
2. The analog output channel sees "no active channel" status.
3. The output driver defaults to the fail-safe substitute value (0 mA for current channels).

For a 4-20 mA channel the substitute value is the disabled state, not the last valid current. This is why the wire reads 0 mA even though the scaled integer in the process image is correct. The same logic explains why exceeding the FC 106 range also forces the output to 0 mA - the integer written to PIQ falls outside the diagnostic buffer's expected range, the channel pair handshake fails, and the substitute value is applied.

FC105 vs FC106 Scaling Functions

The original project uses FC 106 to scale the HMI input. There is a function assignment correction that should be applied during the investigation:

For an HMI input of 0-100 driving an analog output of 4-20 mA, the correct block is FC 105 with BIPOLAR = 0 (unipolar), LO_LIM = 0.0, HI_LIM = 100.0, and the resulting integer will map 0→0 and 100→27648. Using FC 106 in this direction will produce inverse scaling with the integer values written outside the valid 0-27648 range, which trips the channel into fault. Confirm the project is using FC 105 and not FC 106 for the write path.

Step-by-Step Resolution Procedure

The resolution is to increase the local data stack allotment for the affected OBs. In STEP 7 (Classic) for S7-300 / S7-400:

1. Open the STEP 7 project and load the hardware configuration (HW Config).
2. Double-click the CPU in the rack to open the CPU Properties dialog.
3. Select the Memory tab.
4. Locate the Local Data section, which lists the priority classes from 1 (OB 1) to 26 (high-priority OBs).
5. For each priority class that corresponds to an error/diagnostic OB used by the redundancy stack, change the default value (typically 256) to a value that satisfies the largest block sum plus headroom. 512 bytes is the conservative, widely deployed choice.
6. Reference table for the priority classes most commonly affected:

Note: The CPU enforces a total local data budget. Increasing multiple OB allotments consumes from that budget. If the CPU reports "Local data stack overflow" after the change, reduce non-critical OBs first. The CPU 315-2 DP has a 4 KB L-Stack, CPU 317-2 DP has 8 KB, and the S7-400H CPUs have 16 KB per subsystem.

7. Save and recompile the hardware configuration. STEP 7 will offer to download the new system data to the CPU.
8. Perform a warm restart (not a cold restart) of the CPU. A cold restart resets the L-Stack and may mask the issue until the next diagnostic event fills it again.
9. Wait for both redundant CPUs to synchronize (R-Link LED green, no SF on either CPU).
10. Force the analog output channel from STEP 7 to a known value (e.g., 0 for 4 mA, 13824 for 12 mA, 27648 for 20 mA) using PLC > Monitor/Modify > Force on QW 512.
11. Measure the loop current with a calibrated multimeter in series with the load. Confirm 4 mA at integer 0, 12 mA at 13824, 20 mA at 27648.

Verification Procedure

After the change, validate in three stages:

1. Static test: Force QW 512 = 0 (live zero). Expected: 4.000 mA ± 0.005 mA.
2. Sweep test: Step QW 512 from 0 to 27648 in 256-integer increments. Plot the curve. A correctly configured SM 332 channel will exhibit a linear ramp. Calculate the loop transfer function: I_out = 4 mA + (PIQ / 27648) × 16 mA.
3. Redundancy swap test: With the loop at 12 mA, disconnect the primary channel's backplane connector. The redundancy link should swap to the secondary channel within 100 ms. Loop current should remain at 12 mA. Reconnect the primary; verify the swap-back is bumpless.

If any of these stages fails, revisit the local data allotments and confirm the FBs listed earlier are actually being called. A missing call (e.g., FB 451 not called in OB 82) will produce the same symptom. Additionally, check the CPU diagnostic buffer (online > CPU > Diagnostic Buffer) for entries of the form "Local data stack overflow in OB 82" or "Error in OB priority class 25".

Diagnostic Troubleshooting Matrix

Preventive Configuration for H-System Projects

When commissioning any new H-System project that uses redundant analog output modules, the following baseline should be applied from the start:

1. Open HW Config > CPU Properties > Memory and set local data for priority 25-27 to 512 bytes before the first download.
2. Install STEP 7's standard redundancy library on both CPUs and verify the FB call chain is identical across the pair.
3. Use FC 105 for write direction (HMI real → AO integer), FC 106 for read direction (AI integer → HMI real). Verify polarity (BIPOLAR input = 0 for unipolar 4-20 mA).
4. Configure the SM 332 channel as 4-20 mA in HW Config and enable "diagnostics" so that OB 82 events are raised.
5. During commissioning, force QW = 0 and confirm 4 mA before any HMI scaling is wired in.
6. Force QW = 27648 and confirm 20 mA. This validates the high end of the DAC range.
7. Perform a redundancy swap test (disconnect primary backplane connector) and verify bumpless transfer within 100 ms.
8. Document the local data allotments in the project function specification so that future maintenance does not silently regress the fix.

Cross-Reference: Current Measurement Range Encoding

The 0-20 mA and 4-20 mA encodings used by the S7-300 SM 332 follow the same convention as the S7-1200 analog input modules documented in the Siemens TIA Portal manual collection. The relevant reference values (per the Siemens S7-1200 analog signal module documentation):

The 4-20 mA range uses the same integer span as 0-20 mA (0 to 27648) but the live-zero is mapped to 4 mA. The S7-1200 family uses the identical encoding; see the Siemens TIA Portal manual collection: measurement ranges of the analog inputs for current (SB and SM) for the canonical encoding reference.

Edge Cases and Field-Proven Caveats

Several non-obvious situations can produce the same "0 mA at zero scale" symptom:

• OB 122 (synchronous error) not loaded: When the cyclic program attempts to access a passive channel's PIQ during swap, OB 122 must exist to absorb the access fault. If OB 122 is missing, the CPU enters STOP, and after restart the channel is in substitute (0 mA) until warm restart completes.
• CPU 315-2 DP vs CPU 317-2 DP L-Stack sizes: A CPU 315-2 DP with 4 KB L-Stack can run out of headroom quickly if multiple OBs are each set to 512 bytes. Audit the total: sum of all priority class allotments must not exceed the L-Stack. A CPU 317-2 DP (8 KB L-Stack) is the recommended minimum for redundant analog I/O.
• Force vs Programmatic write: Force values bypass the redundancy handshake in some firmware versions. If "force QW 512 = 0" reads 0 mA but a programmatic write from OB 1 reads 4 mA, the issue is force-path related, not local-data related.
• Field wiring polarity: A reversed polarity on the 4-20 mA loop will not produce 0 mA - it will produce negative current (clamped to 0 mA by the receiver). This is a quick multimeter check before deeper diagnostics.
• HMI tag update rate: If the HMI tag is updated faster than OB 1 cycle time, intermediate values can pass through FC 106 with transient out-of-range integers. Clamp the FC 105 output to [0, 27648] with a LIMIT instruction before writing to PIQ.

FAQ