1. Problem Statement
An S7-1500 CPU 1516-3 PN/DP (typical order number 6ES7516-3AN02-0AB0) is programmed in TIA Portal V18.1 and downloaded successfully, but the CPU does not transition to RUN when the operator clicks the green "play" icon in the project toolbar. The user accepts the "Run CPU" prompt, the dialog reports a successful action, yet the PLC folder in the project tree remains yellow, the CPU_1 icon shows the yellow STOP badge, and any attempt to monitor logic in OB1 produces grey lines instead of solid green ones. In TIA Portal terms, grey power-rail / contact elements in the network monitor mean the CPU is not executing the cycle (logic is not being processed), not that the program is failing.
This article walks through the root cause, the diagnostic sequence, and the verification steps that bring the CPU back to a healthy RUN state without re-downloading the project.
2. Symptoms Observed
The following symptoms, all reported together in the same machine, form a coherent fault picture:
- Project downloads and goes online without error in TIA Portal V18.1.
- Clicking the green "Start CPU" / "Run CPU" toolbar icon prompts for confirmation; clicking OK produces no change.
- The CPU folder icon in the project tree remains yellow (STOP) instead of turning green (RUN).
- In OB1 online monitoring, every network element displays in grey, indicating the OB is not being processed.
- The
RUNLED on the CPU front panel is OFF; theSTOPLED is ON (yellow/orange). - Writing a forced value from the watch table returns an error indicating the CPU is in STOP.
| Indicator | RUN (healthy) | STOP (fault) |
|---|---|---|
| RUN LED | Solid green | Off |
| STOP LED | Off | Solid yellow |
| ERROR LED | Off (or briefly flashing during startup) | Flashes red on configuration fault |
| MAINT LED | Off | May flash yellow if maintenance demanded |
| Project tree icon | Green without overlay | Yellow |
| OB1 network monitor | Green / blue wires | Grey wires, no flow |
| Cyclic OB time-stamp | Updates continuously | Frozen at last good value |
3. Root Cause Analysis
The CPU refuses to enter RUN because the physical mode selector switch located behind the front-panel display of the S7-1500 CPU is in the STOP position. Even when the TIA Portal software issues a "Start CPU" command, the firmware of the S7-1500 enforces a hardware-level rule: if the mode selector is set to STOP, the CPU will remain in STOP and will silently reject any software RUN request. This is a deliberate design choice to prevent unauthorized remote starts on commissioned machines.
The S7-1500 mode selector has three positions:
| Position | Function | Behavior |
|---|---|---|
| RUN | Run program | CPU executes the user program; remote RUN/STOP allowed |
| STOP | Stop program | CPU does not execute user program; remote STOP honored, remote RUN refused |
| MRES | Memory reset | Triggers a full memory reset; only active momentarily while held |
Because the operator originally left the selector in STOP (the default shipping position from Siemens), every click of the TIA Portal "Run CPU" button was accepted at the dialog level but rejected at the firmware level.
4. Hardware: The Physical Mode Selector
The mode selector on the 1516-3 PN/DP is a small rotary / toggle switch recessed into the front of the CPU. To access it:
- Power off the CPU is not required, but recommended for mechanical safety.
- Flip up the front-panel display. The display is hinged at the top; it lifts upward and locks into a viewing position.
- Below or behind the display, the selector switch is visible. It is a small rotary knob or three-position toggle (depending on the CPU variant).
- Move the switch to RUN. The CPU immediately begins executing the user program; the RUN LED turns green; the STOP LED turns off; the project tree icon turns green.
On 1516-3 PN/DP variants with the full graphic display (firmware V2.0 and later), the display itself also shows the current operating mode and can be used to navigate to "PLC > Operating Mode > Start" or "Stop", which performs the same logical action as the physical switch — except the physical switch retains priority. If the selector is in STOP, the display's "Start" menu item will report "Operation not permitted: mode selector in STOP".
5. Software Path: Online & Diagnostics in TIA Portal V18.1
Before toggling the hardware, the operator should always inspect the diagnostics buffer. The buffer records every operating-mode transition, every STOP reason, and every diagnostic event in chronological order. It is the single most useful tool for differentiating a hardware-blocked STOP from a program-fault STOP.
- In the TIA Portal project tree, right-click the CPU device (e.g.
PLC_1 [CPU 1516-3 PN/DP]). - Choose Online & > Online & Diagnostics.
- In the working area, expand the Diagnostics folder.
- Click Diagnostics buffer.
- Click the Update (refresh) icon to fetch the current buffer from the CPU.
The buffer shows timestamped entries such as "Operating mode change from STOP to RUN requested", "STOP due to mode selector", or "CPU remains in STOP because user program is inconsistent". Each entry has an icon that conveys severity:
| Icon | Meaning | Typical cause |
|---|---|---|
| Green check | Informational event | Mode change, download, restart |
| Yellow triangle | Warning | Maintenance demanded, parameter mismatch |
| Red circle | Fault | Hardware defect, program error, I/O fault |
When the mode selector is in STOP, the buffer entry reads roughly: "STOP requested by mode selector switch" with timestamp matching the operator's last click. This is the unambiguous signature of the failure mode described in this article.
6. CPU Display Operation
The S7-1500 1516-3 PN/DP is delivered with a 6 cm full-graphic display. The display is the operator's fastest way to confirm the current state without a PC. The default menu structure is:
- Overview: CPU name, IP address, operating mode, project name.
- Diagnostics: Diagnostics buffer summary, alarm display, module status.
- Settings: Date/time, IP address, operating mode, protection, display language.
- PLC: Run/Stop, memory reset (MRES), firmware update, reset to factory settings.
Navigation is performed with the four keys around the display:
| Key | Function |
|---|---|
| OK | Confirm selection / open submenu |
| ESC | Cancel / one menu level up |
| Up / Down arrows | Scroll / change parameter |
| Left / Right arrows | Navigate tabs / change parameter |
| F1 / F2 / F3 / F4 | Soft keys; context dependent |
To start the CPU from the display, navigate to PLC > Operating mode. If the physical selector is in RUN, the display will accept the command and the CPU will start. If the selector is in STOP, the display returns an error.
7. Step-by-Step Resolution Procedure
- Verify online connection. In TIA Portal, click the "Go online" button. The status bar at the bottom of the window should show the CPU's IP address and the green "online" indicator.
- Open the diagnostics buffer. Right-click the CPU > Online & Diagnostics > Diagnostics buffer. Confirm that the most recent entry reads "STOP requested by mode selector" or similar.
- Inspect the physical selector. Without powering off, lift the front-panel display and visually inspect the small mode switch.
- Move the selector to RUN. Toggle or rotate the switch to the RUN position. Listen for a relay click and watch for the RUN LED to illuminate green within 1–2 seconds.
- Confirm via display. The display's "Overview" screen should now show "Operating mode: RUN".
- Confirm via project tree. The CPU folder icon in TIA Portal should turn green.
- Refresh the diagnostics buffer. A new entry "Operating mode change from STOP to RUN" should appear at the top of the buffer.
- Monitor OB1. Open Main OB1 in the project, click the glasses icon "Monitoring on/off", and verify that the network elements are now colored green or blue (true/false) and update in real time.
- Verify cyclic OB execution. The "Cycle time" and "Last cycle" fields in Online & Diagnostics > Cycle time should be updating continuously.
8. Verification of Healthy Operation
After the selector is moved to RUN, perform these verification checks:
| Check | Expected result | Pass criterion |
|---|---|---|
| RUN LED | Solid green | No flicker, no orange |
| STOP LED | Off | Off |
| ERROR LED | Off | Off |
| Display overview | "Operating mode: RUN" | Matches |
| Project tree icon | Solid green | No yellow overlay |
| OB1 monitor | Green/blue wires updating | At least one cycle per second |
| Watch table | Modify value, PLC writes | Successful write acknowledgement |
| Cycle time | Monotonically increasing | Stable, no overruns |
| Diagnostics buffer | Last entry "STOP to RUN" | No new error entries |
9. Secondary Causes That Can Block RUN Mode
If the mode selector is already in RUN and the CPU still refuses to start, work through this matrix of secondary causes:
| Cause | Diagnostic buffer signature | Fix |
|---|---|---|
| Missing or faulted OB (e.g. OB82, OB83, OB86 not loaded but hardware fault requires it) | "STOP caused by missing/incorrect OB" | Load the required OB, or acknowledge the hardware fault |
| Hardware configuration mismatch | "Configuration error: module not present" or "module parameter error" | Compare online vs. offline hardware; re-detect topology |
| PROFINET / PROFIBUS station failure | "IO device failure" or "DP slave failure" | Check cables, replace device, or re-assign device name |
| Password / access protection set to RUN-protected | "Start command rejected due to access protection" | Enter correct password or adjust protection level |
| CPU in commissioning / firmware update mode | "Operating mode: commissioning" | Wait for firmware update to complete, then restart |
| Memory reset in progress | "Memory reset active" | Wait ~30 s; do not power off |
| Firmware mismatch with TIA Portal version | "Firmware version not supported" | Update firmware or downgrade project to compatible version |
| Time-of-day synchronization loop with NTP server unreachable | Rare; firmware V2.6+ may refuse RUN | Disable NTP or fix server reachability |
6ES7516-3AN02-0AB0 ships with firmware V2.9; TIA Portal V18.1 fully supports it. If you are working with a 1516-3 PN/DP variant that is firmware V3.0 or higher, TIA Portal V18 may refuse the download with "Firmware not supported", at which point TIA Portal V19 (or newer) is required.10. Prevention and Engineering Best Practices
To prevent recurrence and to accelerate commissioning of similar cells:
- Document the switch position. Add a sticker to the inside of the cabinet door noting the selector position expected after commissioning (typically RUN).
- Lock the display cover with the captive screw once the switch is set, so vibration cannot move it.
- Use the display's PIN-protected "Operating mode" menu rather than the physical switch for day-to-day RUN/STOP commands. This prevents accidental switch movement.
- Enable protection level "Complete protection" after commissioning to require a password for online changes — but understand that the mode selector still overrides software commands.
-
Set up an HMI start button on a Siemens Comfort Panel or WinCC Runtime that issues the "Start CPU" tag via
SET_CP/PG/PC start CPUfunction — but only if the physical selector is in RUN. - Always read the diagnostics buffer first when the CPU fails to start. This is faster than guessing between hardware, software, and project errors.
11. Useful TIA Portal V18.1 Diagnostic Features
Beyond the diagnostics buffer, the following online tools are worth knowing for future S7-1500 commissioning:
| Tool | Location | Use |
|---|---|---|
| Diagnostics buffer | Online & Diagnostics > Diagnostics | Event history with timestamps |
| Cycle time | Online & Diagnostics > Cycle time | OB1 execution duration, overflow count |
| Memory | Online & Diagnostics > Memory | Load memory, work memory, retentive usage |
| Online & compare | Right-click CPU > Compare | Offline vs. online project divergence |
| Force table | Project > Add new > Force table | Force I/O values; use with caution |
| Watch table | Project > Add new > Watch table | Read/write tags in real time |
| Trace | Project > Add new > Trace | Time-based signal recording |
12. Related Documentation
For further reading, refer to the official Siemens documentation:
- S7-1500 Automation System System Manual — covers CPU operation, mode selector, and diagnostics.
- SIMATIC S7-1500 CPU 1516-3 PN/DP Manual — device-specific pinout, LED descriptions, and order numbers.
- SIMATIC S7-1500 Display Manual — full keypad reference and menu tree.
- Diagnostics of S7-1500 CPUs Application Note — explains each diagnostics buffer event class.
- TIA Portal V18 Release Notes — compatibility and firmware matrix.
Frequently Asked Questions
Why does the S7-1500 CPU ignore my software "Run" command from TIA Portal?
The physical mode selector on the front of the CPU is in STOP. The firmware enforces hardware priority: if the selector is STOP, the CPU will not enter RUN regardless of any software command. Flip up the display, move the switch to RUN, and the CPU starts within 1–2 seconds.
Why are all my OB1 networks grey in online monitoring instead of green?
Grey networks indicate the OB is not being processed, which means the CPU is in STOP. Once the CPU enters RUN, the elements turn green (logic true) or blue (logic false) and update each cycle. If networks remain grey after RUN, check the diagnostics buffer for OB loading errors.
How do I open the diagnostics buffer in TIA Portal V18.1?
Right-click the CPU device in the project tree, choose Online & Diagnostics, then click the Diagnostics folder and select Diagnostics buffer. Click the refresh icon to pull the current buffer from the CPU.
Can I start the S7-1500 CPU from the front-panel display?
Yes. Navigate to PLC > Operating mode on the display and select Start. This works only if the physical mode selector is in RUN. If the selector is in STOP, the display returns "Operation not permitted".
Which TIA Portal version supports the CPU 1516-3 PN/DP?
TIA Portal V18 and later support the 1516-3 PN/DP with firmware V2.x. CPUs with firmware V3.0 or higher require TIA Portal V19 or newer. Verify the order number and firmware version before commissioning.