Resolving S7-300 CPU 315-2DP ET200S Analog Output Failure

1. Problem Summary

A field service scenario on a Siemens SIMATIC S7-300 CPU 315-2DP station driving a distributed ET 200S rack produced two intertwined symptoms that engineers regularly confuse for a single fault:

1. The SCL source blocks in the PLC cannot be re-uploaded to the engineering station in editable form; an Upload station to PG operation returns STL (Statement List) listings that are functionally unreadable.
2. Channel 2 of the analog output module 6ES7135-4FB00-0AB0 (a 2 AO U/I module on the ET 200S) collapses to 0 V the moment a forced value at the I/O addresses PQW120 and PQW122 rises above 7000 (INT), while channel 1 tracks the forced value without issue.

The two root causes are different but they are correlated. The SCL recovery problem is a STEP 7 V5.x tool-chain limitation, not an automation fault. The AO collapse is hardware-side and is almost always a backplane power-budget deficiency at the ET 200S, a damaged output stage, or a load-side fault on channel 2 alone. Resolving the symptom requires three actions in sequence:

1. Add a second PM-E power module to the ET 200S station to raise the backplane current budget.
2. Modify the HWConfig in the original STEP 7 V5.x project (or in an Upload station to PG replica) and download only the hardware configuration back to the CPU.
3. Validate the program blocks remain intact after the hardware download and re-test the analog output at PQW122.

2. Affected Hardware Components

The exact MLFB of the CPU 315-2DP in the field can be verified from the on-line diagnostic buffer (Module Information > Identification) before any hardware modification is committed. The MLFB determines firmware dependencies for the HWConfig GSD revision.

3. Why SCL Source Cannot Be Uploaded

STEP 7 V5.x (SIMATIC Manager) stores SCL blocks in two fundamentally different ways:

The SCL compiler translates every SCL statement into MC7 instructions, the native code executed by the CPU. When the engineer triggers PLC > Upload Station to PG, the CPU returns the MC7 image; the STEP 7 Sources tab will only display STL. STL decoded from SCL is structurally valid but contains expression trees that are functionally illegible (nested temporary variables, generated CASE ladders, and so on) and cannot be recompiled back to SCL.

Consequence: the only reliable path to recover SCL source is the offline backup archive. If the customer does not have an EKB (STEP 7 project archive) or recent source backup, the SCL logic is effectively lost. The remaining option is to read the STL, comment it, and rewrite the function in SCL or LAD/FBD for any maintenance change.

4. Root Cause Analysis: ET 200S Power Budget vs. AO Channel-2 Collapse

The fact that the field symptom is channel 2 only is a strong indicator that the root cause is not the user program. The STEP 7 hardware diagnostics block SFB 52 / SFB 54 or the diagnostic interrupt raised by the AO module is the first place to look on-line.

4.1 Power Module Behavior in the ET 200S

Each PM-E module on the ET 200S backplane supplies a finite amount of current and power to the modules to its right up to the next PM-E or terminator. For a single-PM station that contains, for example:

• 2 AO modules (6ES7135-4FB00-0AB0),
• 4 AI modules,
• Several digital modules,

the cumulative backplane current draw can exceed the 1 A typical limit of a single 6ES7138-4CA01-0AA0. When the analog output stage of channel 2 demands the additional current required to drive the higher DAC count above 7000 INT (approximately 2.53 V on a 0–10 V range, or about 4.83 V on a 1–5 V range), the backplane voltage sags and the output stage shuts down or latches to 0 V to protect itself.

4.2 Output Range Mapping of 6ES7135-4FB00-0AB0

The 7000-INT threshold where the symptom appears is a function of the load and the power supply, not a function of the DAC itself. A healthy output stage in a properly powered station will track the full 0–27648 INT range linearly.

4.3 Diagnostic Decision Matrix

5. STEP 7 V5.x: Download Hardware Without Touching the User Program

This is the central procedure the customer needs. STEP 7 V5.x provides two distinct download paths, and the choice of path determines whether the user program is preserved.

5.1 Prerequisites

1. The customer must have a STEP 7 V5.x project that contains the on-line configuration of the S7-300 station. If only the SCL source was on the PC, the project tree must be rebuilt via PLC > Upload Station to PG.
2. The CPU's operating mode selector must be in RUN-P or STOP. Hardware download to a CPU in RUN is rejected.
3. The PC must have a PROFIBUS or MPI connection to the CPU (USB MPI adapter, CP 5611, CP 5613, or Ethernet routing via the CP 343-1 Lean / Standard).
4. The HWConfig must show the current configuration of the ET 200S. If the PC project is stale, the system will refuse to download without first doing an upload.

5.2 Step-by-Step Procedure

1. Open the S7 project in SIMATIC Manager. Open HW Config by double-clicking the Hardware object under the SIMATIC 300 station.
2. If the offline configuration does not match the actual ET 200S topology, click PLC > Upload Station to PG from inside HWConfig. The tool will read the current PROFIBUS topology and rebuild the racks.
3. Insert a new PM-E module into the ET 200S rack. Drag PM-E from the hardware catalog onto the slot immediately to the right of the existing PM, or wherever the engineering topology requires. The catalog entry is SIMATIC ET 200S > Power Modules > PM-E 24 VDC.
4. Save and compile the HWConfig: Station > Save and Compile (Ctrl+S). The compiler will report the bus current budget for the ET 200S in the diagnostic window. Verify the configuration is accepted (no red entries).
5. Click the Download to Target button in the HWConfig toolbar (the icon resembling a PLC receiving a downward arrow). Confirm the target CPU in the dialog.
6. In the Select Node Address dialog, choose the MPI/PROFIBUS/Ethernet route that reaches the CPU. Click OK.
7. STEP 7 will warn that a download to a CPU in RUN requires a STOP→RUN transition. Accept the STOP transition. The CPU performs an overall reset on the hardware data area but the user program in load memory and work memory is preserved.
8. After the download completes, place the CPU back in RUN-P. The SCL blocks remain online and continue execution.

6. ET 200S Power Module Sizing

Each module in the ET 200S draws a defined backplane current. The cumulative current must be less than the PM-E supply limit, and the cumulative power must be within the PM-E thermal budget.

6.1 Power Budget Example

For a station with 1 IM151, 1 PM-E, 2 AO, 4 AI, 8 DO, 8 DI, the cumulative backplane current is:

This is well under the 1 A PM-E limit. However, additional considerations apply:

• Each AO module can drive up to ±10 V into a 1 kΩ load. The output stage current draw from the 24 V supply rises linearly with the output voltage and load current.
• The DC/DC converter inside the AO module has a fixed efficiency curve. At higher output demands, more current is drawn from the PM-E, not less.
• If the field wiring has a partial short on channel 2 (a few kΩ leakage to ground), the AO module's internal current limit activates, the output stage overheats, and the channel folds back to 0 V.

6.2 Recommended Action for the Field Case

1. Insert a second PM-E module (6ES7138-4CA01-0AA0) into the ET 200S at the position immediately to the right of the AO module, splitting the load into two sub-segments.
2. Save and compile the HWConfig. STEP 7 reports the per-segment current draw; both should be below 0.7 A to leave thermal headroom.
3. Download only the hardware configuration from inside HWConfig (see Section 5). Do not issue a full project download.

7. Detailed Diagnosis of the Channel-2 0 V Collapse

If adding the second PM does not resolve the channel-2 symptom, the failure is in the module itself or the wiring. Use the following procedure:

1. De-energize the 24 V supply to the ET 200S. Open the front connector of the 6ES7135-4FB00-0AB0 module.
2. Inspect the front connector terminals for the channel-2 output (terminal 1 = ANA+ Ch0, terminal 2 = ANA- Ch0, terminal 3 = ANA+ Ch1, terminal 4 = ANA- Ch1 on the 6ES7135-4FB00). Look for discoloration, deformation, or signs of contamination.
3. Measure the loop impedance from terminal 1 to terminal 2 with an ohmmeter (channel 0/1 disconnected from the load). A reading of less than 10 kΩ indicates a partial short on the field side.
4. Power the station. With the load still disconnected, force PQW122 to 27648 in Monitor/Modify. Measure the voltage at terminals 1 and 2 with a DMM. A healthy output should read 9.95–10.05 V on the 0–10 V range.
5. If the output is still 0 V, the output stage of channel 2 is damaged. Replace the module with a 6ES7135-4FB01-0AB0 (the current production successor) or a compatible variant.

8. SCL Source Recovery — Practical Options

Since the on-line upload cannot return SCL, the recovery options are limited. Choose in the order below:

1. Customer's EKB or project backup: The fastest path. The customer must locate the last archived STEP 7 project (.s7p or .zip export) that contains the SCL source. Restore and recompile.
2. Plant historian or HMI project: If the SCL implements scaling or interlocks that are mirrored in the HMI tag database, the HMI export can serve as a cross-reference.
3. Reverse engineering from STL: Read the STL with comments. For each FB, identify input/output interface, scan the temporary variables, and rewrite the equivalent SCL block. This is the least reliable path and must be unit-tested before deployment.
4. Vendor support request: If the system integrator is known, request the source archive under the original contract terms.

For long-term serviceability, the customer should adopt a source-controlled archive strategy: every SCL/FB/FC/DB change is committed to a version-controlled folder on the engineering server, with periodic backup to off-line media.

9. Replacement Module and Firmware Considerations

The 6ES7135-4FB00-0AB0 was superseded by 6ES7135-4FB01-0AB0. Engineers should be aware of the following migration notes:

HWConfig in STEP 7 V5.5 SP4 or later recognizes both MLFBs. If the engineering station is older than V5.5 SP2, install the HSP (Hardware Support Package) from the Siemens support site before attempting to add the module.

10. Commissioning and Verification Checklist

1. Open the project in STEP 7 V5.x. Confirm the CPU is in STOP.
2. Open HWConfig. Verify the ET 200S topology matches the field installation (slots, MLFBs, PM-E count).
3. Compile. STEP 7 must report zero errors and zero warnings.
4. Click Download to Target from inside HWConfig only.
5. After the download, place the CPU in RUN-P. Verify the CPU diagnostic buffer shows no configuration errors (SF LED off).
6. Open PLC > Monitor/Modify. Confirm the SCL blocks (FCs, FBs, DBs) are still online and have the expected timestamps.
7. Force PQW120 to 0, 13824, 27648 and verify channel 0 output: 0 V, 5.0 V, 10.0 V on the 0–10 V range.
8. Force PQW122 to the same values. Verify channel 1 output matches within the module accuracy spec (±0.5 % of full scale).
9. Disable the force and let the user program write PQW122. Verify the analog output follows the program logic.
10. Record the final HWConfig (Export to TXT) and archive with the SCL source as part of the customer documentation set.

11. Common Pitfalls and Field Notes

• Wrong download path: Issuing a download from the SIMATIC Manager project tree wipes the user program. The HWConfig Download to Target button is the only path that preserves the program.
• Memory reset on STOP transition: Some CPU 315-2DP firmware versions (< V2.0) perform a full memory reset on STOP→RUN transitions. The user must check the CPU's firmware via Module Information > Identification. If the firmware is older, a STOP transition erases work memory and the program must be reloaded.
• Force values vs. program values: The 7000-INT threshold observed via Monitor/Modify > Force may differ from the threshold when the user program writes the value. If the symptom only appears with force, the issue is in the forcing mechanism, not the module.
• Watchdog on the AO module: Some 6ES7135-4FB00 modules built before 2010 have a known output-stage latch-up issue. Reference the Siemens FAQ on the module.
• Front connector torque: Screw-terminal front connectors must be torqued to 0.6–0.8 N·m. Loose connections on the AO output cause intermittent 0 V readings.
• Shield grounding: The analog cable shield must be terminated at the cabinet gland, not at the field device, to prevent ground-loop induced errors.

12. Frequently Asked Questions