Siemens S7-1200 CPU Start/Stop Without Mode Selector Switch

Siemens S7-1200 CPU Start/Stop Without Mode Selector Switch

The Siemens SIMATIC S7-1200 programmable controller family does not include a physical RUN/STOP/MRES selector switch on the CPU housing, unlike the S7-200 (RUN/TERM/STOP) and S7-300 (RUN/RUN-P/STOP/MRES) predecessors. This is a deliberate design choice that reduces cost, removes a mechanical wear item, and shifts all mode transitions into software. The change has practical consequences for commissioning and operation: any field engineer who needs to start or stop the S7-1200 CPU without a TIA Portal station must rely on alternate procedures (user-program instructions, web server, HMI, peer PLC, or carefully configured power-cycle behavior).

This reference documents the design rationale, the LED behavior that replaces the selector, six proven field methods for placing the S7-1200 in RUN or STOP, and a verification matrix. It targets engineers migrating from S7-200 or S7-300 to S7-1200 and integrators who operate machines without a programming device on the shop floor.

S7-1200 Mode Selector: Design Background

The S7-200 (CPU 221 through CPU 226, plus the later CPU 224 XP and CPU 226 XM) and the S7-300 (CPU 312 through CPU 319, plus the F-CPU variants) both include a 3-position key-operated mode selector on the CPU front panel. The S7-200 selector exposes RUN, TERM, and STOP. The S7-300 selector exposes RUN, RUN-P, STOP, and MRES (memory reset). The MRES position drives a cold-restart sequence that clears work memory and the retentive flags.

When Siemens developed the S7-1200 (announced 2009, firmware V1.0), the design team removed the selector entirely. The SIMATIC S7-1200 Programmable Controller System Manual states that all operating-mode transitions are handled via software. The reasons documented in Siemens literature and confirmed by the platform's behavior include:

• Mechanical selectors are a service item. Key switches wear, accumulate contamination, and require replacement in dirty industrial environments. Removing the switch removes a scheduled spare.
• Component cost. The selector and its cut-out are eliminated from the bill of materials. The saving applies to every S7-1200 model (CPU 1211C, 1212C, 1214C, 1215C, 1217C).
• Housing sealing. A selector penetrates the front cover. Eliminating the through-hole increases the achievable IP rating and reduces radiated EMC leakage.
• Software-only features. Password-protected STOP, role-based operating-mode change, and remote command execution require a software surface; a physical switch cannot express them.
• Memory reset (MR) by selector is replaced by TIA Portal. The MR function is preserved, but only via the engineering tool. This forces a deliberate, audited reset action.

The result is that the CPU's mode is entirely the responsibility of the engineering tool, the user program, or auxiliary hardware such as HMI, peer PLC, or web server.

Comparing S7-200, S7-300, and S7-1200 Mode Controls

The S7-1200 protection scheme is documented in the S7-1200 System Manual, chapter "Protection and Security". Setting the protection level to "Full protection (no access from HMI)" disables even legitimate HMI requests for mode change until the password is supplied.

S7-1200 LED Status Indicators

Without a selector, the S7-1200 communicates operating state through its front-panel LEDs. The exact pattern depends on firmware version, but the most recent system manual covers all states back to V4.0.

CPU Startup Configuration in TIA Portal

Before any field method works reliably, the CPU's startup behavior must be configured. Navigate Project tree > Devices & Networks > [CPU] > Device view > Properties > General > Startup. The available options are:

1. No restart (stay in STOP) — Default for CPUs delivered with newer firmware. After a power cycle, the CPU powers up into STOP and waits for an explicit RUN command from TIA Portal or one of the methods below. Recommended for safety-critical applications where no uncontrolled machine motion is allowed after power restoration.
2. Warm restart - RUN — After power cycle, the CPU executes the configured startup OB(s) (OB100 by default) and then enters RUN. Used when unattended operation is required after a power disturbance.
3. Warm restart - Mode before power off — After power cycle, the CPU returns to the mode it was in before power was removed. If the CPU was in STOP before power-off, it returns to STOP. If RUN, it returns to RUN.

Additional parameters under the same node:

• Startup time — S7-1200 permits up to 10 seconds of startup OB execution. If startup OB does not finish within the window, the CPU transitions to STOP and posts a diagnostic event.
• Parameter assignment of outputs in STOP — Selectable: outputs freeze (hold last value) or output 0 V. Selection affects field behavior during STOP.
• Monitoring time for parameter assignment by the CPU (100 ms default) — Maximum time the CPU waits for I/O modules to acknowledge configuration.

Mode-Change Decision Flow

Method 1: Power Cycle With Configured Startup Mode

Use case: The CPU was placed in STOP by an operator action and must be brought back to RUN without a PC.

Procedure:

1. In TIA Portal, set Device configuration > Properties > General > Startup to "Warm restart - RUN" (or "Warm restart - Mode before power off").
2. Download the hardware configuration to the CPU.
3. Disconnect 24 VDC power from the CPU for at least 3 seconds (permits internal NVRAM hold capacitors to discharge).
4. Re-apply 24 VDC.
5. Watch the RUN/STOP LED. The CPU flashes green/yellow during startup OB, then solid green in RUN.

The CPU executes startup OB100 (if programmed), then transitions to RUN. This method requires no PC, no HMI, and no field cabling beyond the power supply. It is the simplest field-startup technique and is appropriate for unattended cabinet installations.

Constraints:

• Requires that no fatal error is pending. If a fatal diagnostic exists in the diagnostic buffer, the CPU remains in STOP after power-up. Clear the diagnostic from TIA Portal first.
• Cannot put the CPU into STOP from the field via this method. Use STP, web server, HMI, peer PLC, or simply turn off power (note: power-off does not generate a clean STOP event in the diagnostic buffer).
• Resetting the CPU to factory defaults is not possible via power cycle. A factory reset requires TIA Portal (Online > Reset to factory settings) or a SIMATIC Automation Tool command. The legacy S7-1200 IP Tool did not provide this function.

Method 2: STP Instruction in User Program

The S7-1200 instruction set includes the STP instruction (Stop CPU). It is available in LAD, FBD, and SCL. The instruction evaluates the EN input on a rising edge and immediately transitions the CPU to STOP.

Ladder Logic (LAD) implementation:

Network 1:
      Stop_Request    STP
    |--| |------------(STP)--|
       (rising edge detects via P_TRIG or contact on differential tag)

Structured Control Language (SCL) implementation:

IF "Stop_Request" THEN
    STP();
END_IF;

Restart after STP: Once STP executes, the only ways to return to RUN are:

1. Power cycle, with startup mode set to "Warm restart - RUN" or "Mode before power off" (the latter returns to RUN if the CPU was running before STP).
2. Command from TIA Portal (Online > CPU operating panel > RUN).
3. Command from web server (firmware V4.0+, see Method 3).

Constraints:

• Some firmware versions require password authentication to permit STP execution. Check Device configuration > Properties > Protection & Security > CPU protection. If set to "Full protection", STP from the user program still executes (it is local code), but a remote trigger via PUT/GET or web server is denied.
• The diagnostic buffer records an "Operating mode transition by user program" event with the time stamp and the OB number in which STP executed.

Method 3: Web Server (Firmware V4.0 and Later)

The S7-1200 web server is the simplest field method because it uses any standard browser and requires no proprietary client software. It was introduced in firmware V4.0 and significantly expanded in V4.2 (user-defined pages) and V4.4 (improved security).

One-time setup in TIA Portal:

1. Device configuration > Properties > Web server > Enable web server.
2. Select "Permit access only via HTTPS" for security (recommended).
3. Define users under Users & Roles. At minimum, create one operator-level user with the "Operating mode" privilege.
4. Compile and download to the CPU.

Field operation:

1. Connect a laptop, tablet, or smartphone to the CPU's PROFINET port (directly with a patch cable, or via the network).
2. Open a browser and navigate to https://<CPU_IP_address> (default HTTPS port 443).
3. Log in with the operator user.
4. Navigate to the "Operating mode" or "CPU" page.
5. Click "RUN" or "STOP" and confirm the action.

The CPU honors the command within 5 seconds. The web page displays the current state, so the operator has positive feedback that the change took effect.

Browser and device compatibility:

• Tested with Microsoft Edge, Mozilla Firefox, Google Chrome, Apple Safari.
• Mobile browsers (iOS Safari, Android Chrome) render the standard pages correctly.
• JavaScript must be enabled.

Constraints:

• Firmware must be V4.0 or later. For older firmware, upgrade or use Methods 4-6.
• The default "Everyone" user has read-only access. Without an operator user configured, the mode-change controls are greyed out.
• The web server page loads only over TCP/IP. The PROFINET network must be reachable from the operator's device. If a router or firewall separates the two, ensure port 443 is open.
• HTTPS requires a valid certificate. The CPU ships with a self-signed certificate; modern browsers will warn the first time. Install the certificate on the operator device for clean operation.

Method 4: External HMI Panel

A SIMATIC HMI (Comfort, Unified, or Basic panel) connected via Ethernet or PROFIBUS can issue RUN/STOP commands. The most common implementation is the SetPLCMode function block in WinCC / TIA Portal HMI configuration.

TIA Portal HMI configuration steps:

1. Add an HMI connection from the panel to the S7-1200 in Devices & Networks.
2. In the HMI screen, drag a button onto the canvas.
3. Configure the button's "Click" event to call the system function "SetPLCMode" with the requested mode (RUN or STOP).
4. Add a status indicator that reads the S7-1200's operating-mode tag.
5. Compile and download the HMI project.

For non-Siemens HMIs (Beijer Electronics, Pro-face, Red Lion, AutomationDirect):

• Configure an Ethernet driver (S7-1200 native protocol, Modbus TCP with MB_SERVER, or EtherNet/IP with the appropriate adapter block).
• Map a button tag to the mode-control area.
• Note that S7-1200 mode control via Modbus or EtherNet/IP requires writing to internal tags exposed by the protocol block; direct operating-mode control is not part of those protocols.

Constraints:

• The HMI must have the S7-1200 in its known device list. Modern TIA Portal panels support S7-1200 from WinCC V13 onward.
• Operator authority must be assigned under HMI user administration. The mode-change button should be behind a "Maintenance" or "Service" login level so production operators cannot inadvertently STOP the line.
• The HMI's connection must be active. A disconnected HMI cannot issue commands.

Method 5: SIMATIC Automation Tool

The SIMATIC Automation Tool (SAT) is a free Siemens utility for commissioning and diagnostics. It runs on a Windows laptop without a TIA Portal license and supports S7-1200, S7-1500, ET 200S, ET 200MP, and ET 200pro stations over Ethernet.

Procedure:

1. Download and install SIMATIC Automation Tool from Siemens support. Version 4.x supports current S7-1200 firmware.
2. Connect the laptop to the same Ethernet subnet as the CPU(s).
3. Launch SAT and click "Discover". The tool sends a broadcast and lists all reachable stations with their IP, MAC, and firmware version.
4. Select the target CPU in the device table.
5. Right-click > "Operating mode" > choose RUN, STOP, or Memory Reset.
6. Confirm the action. SAT writes to the CPU and reports success or failure.

SAT supports bulk operations. A service laptop can place 50 stations in RUN simultaneously, which is useful for line-startup procedures in large plants.

Constraints:

• SAT supports only S7-1200 firmware V4.0 and later. Earlier firmware may not respond.
• Network discovery uses UDP broadcast on the local subnet. If the laptop is on a different VLAN or subnet, the broadcast is dropped. Add the network manually by entering the IP address.
• Some corporate firewalls block the discovery broadcast. In those cases, use the manual IP entry or route the broadcast.
• SAT requires a USB or network connection to the laptop. The legacy S7-1200 IP Tool (a smaller utility for IP assignment only) does not include mode control.

Method 6: S7 Communication PUT From Peer CPU

A peer S7-1200 (or S7-1500, S7-300) can place the target S7-1200 in RUN or STOP using the PUT instruction. The peer writes to the target's mode-control data block.

One-time setup on the target CPU:

1. Device configuration > Properties > Protection & Security > Connection mechanisms > enable "Permit access with PUT/GET communication from remote partners".
2. Add the peer CPU's IP address to the access list (firmware V4.2+). Without the access list entry, the PUT request is rejected.
3. Download the configuration to the target CPU.

Peer CPU programming (S7-1200 example):

// Peer CPU: PUT block in OB1
REQ    := TRUE;                  // Always or on condition
ID     := 1;                     // Local connection ID
DONE   := "put_done";            // Output flags
ERROR  := "put_error";            // Output flags
STATUS := "put_status";           // Output word
ADDR_1 := 'CPU_STOP_REQUEST';    // Remote tag: 1 = STOP, 0 = no action
SD_1   := "stop_command";        // Local tag containing the value
LEN    := 1;                     // Length in bytes

The peer CPU sets stop_command = 1 to put the target in STOP, or 0 to put it in RUN (depending on the bit mapping defined in the project's data block).

Constraints:

• S7-1200 firmware must be V4.2 or later to expose mode-control data to PUT/GET with the access list feature.
• The peer CPU must be in the access list. Without it, the PUT request returns error W#16#8311 (connection partner not authorized).
• PUT/GET runs over the S7 protocol on TCP port 102. Network firewalls must permit this.
• This method requires at least one CPU with the capability to execute PUT. It is the right approach when a master S7-1200/S7-1500 controls several slave S7-1200 stations, such as in a multi-machine cell.

Field Commissioning Procedure

When commissioning a new S7-1200 machine with no PC present after the engineering phase, follow this sequence:

1. Pre-commissioning (PC side, in office or staging area):
   • Open TIA Portal project for the CPU.
   • Set Device configuration > Properties > General > Startup to "Warm restart - RUN" or "Mode before power off".
   • Enable Web Server (firmware V4.0+) and configure at least one operator user with the "Operating mode" privilege.
   • If an HMI is in the cabinet, add the SetPLCMode function to a button on the operator screen.
   • Compile and download the project to the CPU.
   • Verify in TIA Portal: Online > Diagnostics > Operating mode shows RUN. Confirm the RUN/STOP LED is green solid.
   • Test the web server from the engineering laptop: open https://<CPU_IP>, log in, toggle to STOP, confirm the LED turns yellow, then toggle back to RUN.
   • Document the IP address, the operator user credentials, and the chosen field-start method in the O&M manual.
2. Site power-up (no PC):
   • Apply 24 VDC. CPU powers up, executes OB100, transitions to RUN.
   • Confirm RUN/STOP LED is green solid.
3. Field mode change (no PC):
   • Operator uses HMI button, web browser, or peer CPU as documented in the O&M manual.

Verification Checklist

Troubleshooting Matrix

Operating-Mode States and Timing

The "STOP after diagnostic" state is the most confusing for first-time operators. The LED is yellow (suggesting a normal STOP), but it flashes (indicating the STOP was caused by a fault, not by a user command). The diagnostic buffer must be cleared before the CPU will accept a RUN command via web server or HMI. A power cycle does not clear the buffer.

Comparison: Field Start/Stop Methods by Available Hardware

Safety Considerations

An S7-1200 STOP event is not a Safety Integrity Level (SIL) function. In a safety-critical machine, the S7-1200 must be paired with a safety controller (such as a SIMATIC S7-1200F variant, an S7-1500F, or a dedicated safety relay such as the Sirius 3SK2) that independently removes power from hazardous motion. The S7-1200's "standard" STOP must never be the sole means of stopping a hazardous machine.

Per ISO 13849-1 and IEC 62061, safety functions require a defined category (B, 1, 2, 3, or 4) and a Performance Level (PL a through PL e) or Safety Integrity Level (SIL 1 through SIL 3). The standard S7-1200 STOP via STP or web server does not meet these requirements. Always use the F-variant CPU with PROFIsafe communication and a Safety program in the TIA Portal Safety Administration for any SIL-rated function.

Firmware-Specific Notes

The current production firmware for S7-1200 is V4.6 or later (depending on the model and date of manufacture). Newer firmware is functionally compatible with the methods described; older firmware may require upgrade or method substitution. Always check the project compatibility in TIA Portal before firmware changes.

CPU Part Numbers and Variants

The S7-1200 family includes six CPU variants. All of them share the same mode-control behavior.

The exact order-number suffixes (xx, yyy) depend on the supply type (AC/DC/RLY, DC/DC/DC), the country certification, and the firmware version. Refer to the Siemens catalog or the product label for the precise MLFB. The F-variant CPUs add safety functions and the PROFIsafe protocol, but their mode-control behavior is identical to the standard CPUs.

Related Design Choices on S7-1500 and S7-200 SMART

The S7-1500 family (CPU 1511, 1512, 1515, 1516, 1517, 1518) also ships without a mode selector. The S7-1500 adds a CPU display with a touch-driven MODE selector that performs RUN, STOP, and MRES functions. The display is a service-only interface; the operating mode can still be changed from TIA Portal, the web server, an HMI, or the user program.

The S7-200 SMART (6ES7288 ...) includes a physical 3-position selector (RUN / TERM / STOP), which differs from the S7-1200 design. The S7-200 SMART is positioned as the lower-cost replacement for the S7-200 and retains the older physical interface.

Engineers migrating between platforms should review this difference carefully. A panel layout that expects a S7-200 SMART selector must be redesigned for the S7-1200 (or S7-1500) with a software-mode operator interface.

Frequently Asked Questions