Resolving FC105 SCALE Function Block STOP Faults on S7-300 CPU

Problem Summary

On a SIMATIC S7-300 CPU 313C (6ES7313-5BF03-0AB0 or comparable compact CPU with integrated digital/analog I/O), a ladder program that calls FC105 (SCALE) to normalize an analog input value into a 0–100% engineering range sends the CPU into STOP mode. The red SF (System Fault) LED illuminates, and Step 7 Monitor reports "Module is in operating mode STOP". Network 1 of the program operates correctly; the fault only appears when Network 2 containing FC105 is downloaded or scanned.

This fault pattern has three root causes, all of which must be verified before the CPU returns to RUN:

1. The FC105 block is not present in the S7 project on the CPU (or only on the programming device).
2. The analog input address PIW xxx referenced by FC105 is not configured in HW Config.
3. The CPU is missing the diagnostic organization blocks (OB 121 / OB 122) that would otherwise allow it to continue running and just log the access error.

CPU 313C Integrated I/O Address Map

The CPU 313C family (313C, 313C-2 DP, 313C-2 PtP, 314C-2 DP/PtP) integrates 24 digital inputs, 16 digital outputs, 4 or 5 analog inputs (AI), and 2 analog outputs (AO) directly on the CPU module. The I/O addresses are not Q0.0 / I0.0 by default. They follow the SIMATIC S7-300 compact-CPU address convention and depend on the slot in HW Config:

The exact PIW/PQW start address is shown in HW Config → double-click the CPU 313C → "Addresses" tab, and again on the integrated I/O sub-module row. Tutorials that show Q0.0 as the first digital output of a CPU 313C are either using a 314/315 rack CPU, or have explicitly remapped the address by changing the integrated I/O start address in HW Config. Do not assume Q0.0 is valid on a stock CPU 313C.

FC105 (SCALE) Block Technical Reference

FC105 is a Siemens standard library function that converts an integer input (IN) into a real output scaled between a defined lower limit (LO_LIM) and upper limit (HI_LIM). It is the direct sibling of FC106 (UNSCALE), which performs the inverse operation.

Inputs / Outputs of FC105:

Scaling formula (unipolar, BIPOLAR = 0):

OUT = ((IN - 0) / (27648 - 0)) * (HI_LIM - LO_LIM) + LO_LIM

Scaling formula (bipolar, BIPOLAR = 1):

OUT = ((IN - (-27648)) / (27648 - (-27648))) * (HI_LIM - LO_LIM) + LO_LIM

FC105 resides in the Standard Library → TI-S7 Converting Blocks (or in STEP 7 V5.x: Libraries → Standard Library → TI-S7 Converting Blocks → FC105). The block must be copied from this library into the Blocks folder of the S7 project on the offline PC, compiled, and downloaded into the CPU. FC105 is a function (FC), not a function block (FB), so it does not require an instance DB.

Root Cause 1 — FC105 Not Loaded in CPU

The most common cause of a STOP on first use of FC105 is that the block is present in the STEP 7 project on the programming device but was never downloaded to the CPU. When the OB1 scan reaches the FC105 call and cannot resolve the block number, the CPU's operating system reports an "OB not loaded / FC not loaded" event and transitions to STOP (provided the relevant error OB is also absent — see Root Cause 3).

Diagnostic buffer evidence:

• Event ID 16#4570 — "Error in FC" or "Block not found"
• Event ID 16#4302 — "STOP due to missing/incorrect OB"
• Block type: FC, block number: 105
• Registered event: "FC105 not loaded"

Fix: In SIMATIC Manager, open the Blocks folder of the S7 program, confirm FC105 is listed, right-click → Download (or use PLC → Download). If FC105 is missing from the project, navigate to Libraries → Standard Library → TI-S7 Converting Blocks, drag FC105 into the project's Blocks container, save/compile, then download.

Root Cause 2 — Unconfigured PIW Address

If the IN input of FC105 is wired to an analog input (e.g., PIW 756) that does not exist in the hardware configuration, the CPU raises an I/O access error at runtime. With a stock CPU 313C and default addressing, the four integrated analog channels typically sit at PIW 752, 754, 756, 758 (verify in HW Config). If HW Config has been edited, deselected, or never had the integrated AI sub-module enabled, the address is unassigned.

Diagnostic buffer evidence:

• Event ID 16#2942 — "I/O access error when reading"
• Byte-level access violation on the configured address byte
• Error class: B (peripheral access)

Fix (HW Config approach):

1. Open HW Config in the STEP 7 project.
2. Double-click the CPU 313C row → confirm "Integrated I/O" is enabled and the analog input sub-module row is present.
3. Open the Addresses tab and note the start address of the integrated AI.
4. For each analog channel, set the measurement type (voltage 0–10 V, current 0–20 mA, 4–20 mA, RTD, etc.) and the diagnostic/interrupt behavior.
5. Save, compile, and download the hardware configuration to the CPU (Stop → Download → Run).

If 4–20 mA is desired (the most common industrial loop), the channel must be set to "Current (2DMU)" / 4-wire transducer, 4–20 mA. The numeric input value at 4 mA is 0, at 20 mA is 27648. Negative values (broken wire with 4–20 mA sensors) appear in the underrange area and are clamped to -1 (16#FFFF) by FC105's bipolar logic if BIPOLAR=1 is not used.

Fix (alternative wiring): If the application does not actually need a real field input while commissioning, wire FC105's IN to a free, configured analog input or to a constant 16#0000 during testing. A simple "smoke test" verifies the FC105 logic is intact before chasing the hardware address.

Root Cause 3 — Missing Error OBs (OB 121 / OB 122)

An S7-300 CPU only continues to run after a programming or I/O access error if the corresponding error OB is loaded. The mapping is:

A field engineer who has been "bitten" by a PLC that won't keep running can add empty OB 121 and OB 122 stubs to the project, download them, and the CPU will simply log the error in the diagnostic buffer and continue. This is a powerful diagnostic technique, but it is not a fix — it only reveals the underlying cause. Always resolve the root cause and only then consider whether the error OBs should remain as a safety net or be removed.

To add empty OBs: Blocks → right-click → Insert New Object → Organization Block, type OB 121 (or 122), confirm, save, compile, download. The empty stub contains only the OB's end statement and is enough to satisfy the operating system.

Reading the S7-300 Diagnostic Buffer

The diagnostic buffer is the single most useful first-stop tool when the SF LED is lit.

1. With STEP 7 online to the CPU, open the Blocks folder.
2. Press Ctrl + D, or select PLC → Diagnostic/Setting → Diagnostic Buffer.
3. The newest event is at the top. Click the topmost entry and read the Event ID, Block type/number, and the supplementary information bytes.
4. For deeper analysis, click "Open Block" on a programming error event to open the offending block at the exact line that raised the error.
5. Click "Help on Event ID" for Siemens' built-in description of any listed event.

Common event IDs you will encounter during this type of fault:

Step-by-Step Resolution Procedure

1. Confirm the SF LED state and read the diagnostic buffer (Ctrl + D). Note the topmost event ID and the referenced block number.
2. Check the Blocks folder in the online view (PLC → Accessible Nodes or PLC → Monitor/Modify). If FC105 is shown with a red dot or absent, it is not on the CPU.
3. Open the offline S7 project on the PG. If FC105 is in Libraries → Standard Library → TI-S7 Converting Blocks but not in the Blocks folder, drag it in.
4. Compile the program (Station → Save and Compile).
5. Download the Blocks folder to the CPU (PLC → Download). The CPU will transition to STOP briefly and then to RUN if the download is clean.
6. Verify the analog input wiring in HW Config. Ensure the integrated AI sub-module is enabled, the measurement type is set to the actual loop type, and the start address matches the PIW used in FC105.
7. Add OB 121 and OB 122 if you want continued-run protection while troubleshooting a non-critical I/O error.
8. Cycle the CPU to RUN and monitor the FC105 RET_VAL for 16#0000. Non-zero return values indicate out-of-range IN (e.g., bipolar signal interpreted as unipolar, or a wire break).

Verification Checklist

• SF LED is off; DC 5V and RUN LEDs are green.
• Diagnostic buffer top entry is "RUN" (or a benign informational entry).
• OB1 is being scanned: in Monitor, Network 1 (if digital) and Network 2 (FC105) both show "RLO = 1" in their contact and coil conditions.
• FC105's OUT tracks the input: with IN = 0, OUT equals LO_LIM; with IN = 27648, OUT equals HI_LIM (unipolar, 4–20 mA sensor at 20 mA yields 100% if HI_LIM = 100 and LO_LIM = 0).
• Force or simulate a value: temporarily wire IN to a known constant and confirm the scaling math.

Worked Example — Scaling a 4–20 mA Pressure Transducer to 0–100%

Field device: pressure transmitter, 0–10 bar, 4–20 mA loop.

CPU 313C channel: integrated AI 0, configured in HW Config as 4–20 mA, 4-wire, address PIW 752.

FC105 call (LAD, Network 2):

      IN        = PIW 752   // 0..27648 over 4..20 mA
      HI_LIM    = 1.0E+2    // 100.0  (% or bar, depending on application)
      LO_LIM    = 0.0
      BIPOLAR   = FALSE
      RET_VAL   = MW 100    // Check for 16#0000
      OUT       = MD 104    // REAL result, e.g. 0.0 .. 100.0

Math check: at 12 mA (mid-scale), IN = 13824, OUT = (13824 / 27648) * 100 = 50.0. This is the value the HMI should display as 50% / 5.0 bar.

Common Pitfalls and Field Notes

• Q0.0 / I0.0 assumption. Compact CPUs do not start their integrated I/O at zero. Always check the HW Config addresses, not a tutorial copied from a rack CPU.
• Block in PG only. STEP 7 keeps a project database on the programming device; downloading the S7 program does not auto-download standard library blocks unless they are copied into the project's Blocks folder.
• PIW vs IW. FC105 expects a peripheral input word (PIW) at the IN parameter. Using the process-image input word (IW) at the same address will still work, but PIW is the canonical choice because it bypasses the I/O update cycle and reflects the most recent value.
• FC105 vs FC106 confusion. FC105 is for IN → OUT scaling (analog input to engineering units). FC106 is the reverse (engineering units to analog output value). Using FC106 on an input signal will produce nonsense output.
• OB 1 scan order. FC105 is called in OB1 on each scan. If the analog module is in diagnostic mode and the input is faulted, the IN value will be 16#7FFF (or 16#8000 for some Siemens modules). FC105 does not check for that; it scales whatever value it sees.
• Step 7 V5.x vs TIA Portal. On a CPU 313C programmed from TIA Portal, the FC105/FC106 equivalents are still usable but Siemens recommends the modern SCALE / NORM_X instructions in SCL or LAD. The diagnostic procedure and event IDs are functionally identical.

Reference Documentation

For further detail on FC105, the integrated I/O address map of the CPU 313C, and S7-300 diagnostic event IDs, consult the official Siemens manual set:

• SIMATIC S7-300 Automation System, CPU 313C / 314C Technical Data (manual entry on the Siemens Industry Online Support portal)
• SIMATIC Standard Library for STEP 7 V5.x — Converting Blocks (FC105 / FC106)
• STEP 7 V5.x Programming with LAD/FBD and STL — Diagnostic Buffer
• S7-300/400 System and Standard Functions — Organization Blocks (OB 121, OB 122)
• S7-300 CPU 313C / 314C Integrated I/O Addressing

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