S7-1500 OB100 Startup Block Complete Initialization for TIA

1. Overview: What OB100 Does in an S7-1500 CPU

The S7-1500 organization block OB100 (Startup) is a one-shot routine that the operating system of the CPU calls exactly once immediately after a startup event, before the first scan of the main cyclic organization block OB1 begins. It is the Siemens equivalent of the First Scan bit (S:FS) used in RSLogix 5000 / Studio 5000 for Allen-Bradley ControlLogix, but instead of a single boolean flag, the entire OB body is executed top-to-bottom a single time. This makes OB100 the canonical location to bring the PLC's user program into a deterministic, known-good state before any cyclic logic runs.

OB100 belongs to the startup class of organization blocks. Per the official Siemens documentation, the startup OB called by the CPU depends on the type of restart that occurred:

• OB100 – Complete restart (warm restart): default startup OB for S7-1500 and most S7-300/400 CPUs. The non-retentive portion of M, DB, IEC timers/counters, and process image is cleared; retentive data is preserved.
• OB101 – Hot restart: only available on specific S7-400 CPUs with hot-restart-capable firmware. Execution resumes from the point of interruption. Not used on S7-1500.
• OB102 – Cold restart: all RAM is initialized; even retentive data is cleared. Used on S7-400 / WinAC RTX. On S7-1500, a cold restart is effectively a power-cycle with no project backup retained.

For a modern S7-1500 CPU such as the CPU 1515-2 PN (6ES7515-2AM02-0AB0) or CPU 1516-3 PN/DP (6ES7516-3AN02-0AB0) running firmware V2.9 or later in TIA Portal V17/V18/V19/V20/V21, OB100 is created automatically when a new project is generated, but its body is left empty by default. The programmer is expected to populate it with whatever initialization the application requires.

2. Prerequisites

3. OB100 Execution Lifecycle in an S7-1500 CPU

Understanding when OB100 runs is critical to writing correct initialization code. The lifecycle for a complete restart is:

1. Power ON, mode selector change from STOP → RUN, or CPU command Complete restart from TIA Portal.
2. CPU clears all non-retentive M, DB instance data (static), and process image of inputs/outputs.
3. CPU evaluates retentive flags: M bits declared retentive in the PLC tag table, DB variables marked Retain, and IEC counters/timers flagged retentive are preserved.
4. CPU clears the non-retentive portion of the process image outputs (PIQ) to zero.
5. CPU calls OB100 once.
6. CPU writes outputs from the PIQ to the physical outputs.
7. CPU enters RUN mode and begins cyclic execution of OB1 (or the cyclic interrupt OB assigned priority 1).

If OB100 is not present in the project, the CPU simply skips step 5 and proceeds to cyclic execution. If OB100 exists but is empty, it still runs as a one-shot no-op. If OB100 raises an unhandled error, the CPU transitions to STOP.

4. Creating OB100 in TIA Portal – Step-by-Step

1. Open the S7-1500 station in the project tree.
2. Expand Program blocks.
3. Double-click Add new block.
4. Set the type dropdown to Organization block (OB).
5. From the OB family list, select Startup; the dropdown shows OB100 by default.
6. Choose a programming language: LAD, FBD, STL, or SCL. SCL is recommended for complex initialization with arrays and loops.
7. Click OK. TIA Portal generates the OB100 skeleton with the startup information local variables (OB100_START_EV_ID, OB100_STRTUP_INF, OB100_PRIORITY, OB100_OB_NUMBR).
8. Add the initialization code in the OB body.
9. Compile the project and download to the target CPU.

5. Startup Information Available in OB100

Every call of OB100 receives a 20-byte OB100_STARTUP_INFO temporary variable visible in the block's interface. Useful fields:

Reading OB100_STRTUP_INF lets the same OB100 react differently to a power-on versus a download-restart versus a manual mode-switch restart — useful for skipping warmup delays after a download.

6. Typical Initialization Tasks for OB100

Engineering teams conventionally use OB100 to perform the following kinds of work. The exact list is process-specific.

7. Sample Code – LAD, FBD, and SCL

7.1 LAD example – Reset and initialize

// OB100 (Startup) – LAD
// Brings process into a deterministic state at complete restart

Network 1:  // Reset process-level latched alarms
      SET
      R   "HMI_DB".Alarm_Latch_1       // reset warning latch
      R   "HMI_DB".Alarm_Latch_2
      R   "HMI_DB".Alarm_Latch_3
      R   "HMI_DB".Fault_Latch_Pump

Network 2:  // Initialize non-retentive DB counters
      L   0
      T   "Process_DB".Cycle_Counter       // INT
      T   "Process_DB".Reject_Counter       // INT
      T   "Process_DB".Batch_Number         // INT

Network 3:  // Default values
      L   100
      T   "Recipe_DB".Setpoint_Temp         // default setpoint, °C
      L   50.0
      T   "Recipe_DB".Setpoint_Flow         // REAL default

Network 4:  // Move state machine to safe state
      L   0                              // 0 = SAFE_INIT
      T   "State_DB".State_Current
      S   "State_DB".Safety_Lockout        // force manual reset after power-on

Network 5:  // Mark first scan in a flag for cyclic OB1 to read
      SET
      S   "Sys_DB".First_Scan_Done         // will be reset in OB1 after use

7.2 SCL example – Conditional warmup based on startup reason

// OB100 (Startup) – SCL
IF OB100_STRTUP_INF = 16#0001 THEN                 // manual restart via mode switch
    "Process_DB".Warmup_Seconds  := 30;            // long warmup – drives are cold
    "Process_DB".Auto_Mode_Allow := FALSE;
ELSIF OB100_STRTUP_INF = 16#0002 THEN              // automatic restart after power on
    "Process_DB".Warmup_Seconds  := 60;            // longer warmup after power loss
    "Process_DB".Auto_Mode_Allow := FALSE;
ELSIF OB100_STRTUP_INF = 16#0003 THEN              // restart via HMI / SFC
    "Process_DB".Warmup_Seconds  := 10;            // short warmup – just after download
    "Process_DB".Auto_Mode_Allow := FALSE;
END_IF;

7.3 STL example – For legacy ports from S7-300 projects

// OB100 (Startup) – STL
      SET
      R     M      200.0          // reset operator message queue
      R     M      200.1
      R     M      200.2
      L     0
      T     DB10.DBW    0         // recipe slot = 0
      T     DB10.DBW    2         // batch count = 0
      SET
      S     M      250.7          // "Restart_Active" for OB1 to read

8. Alternative: System Memory Byte and First-Scan Bit

Siemens offers a second mechanism that resembles the Allen-Bradley S:FS pattern: System Memory Bytes. Unlike Allen-Bradley's single first-scan bit, the S7-1500 system clock memory byte and first-scan bit are enabled in the CPU properties.

To enable this in TIA Portal:

1. Open Device view, select the CPU.
2. In the Properties inspector go to System & clock memory.
3. Check Enable system memory byte; default address is MB1. The highest bit (M1.7) is the first-scan bit — it is TRUE only on the first cyclic scan of OB1 after a startup.
4. Check Enable clock memory byte; default address is MB0.

Use the first-scan bit for one OB1 cycle initialization (e.g. preset a counter value to 0). Use OB100 for one block execution initialization (preset dozens of tags, evaluate startup reason, send WRREC, etc.). Many engineers use both: the heavy lifting in OB100, and a final fine-tune inside the first OB1 cycle gated by M1.7.

9. Retentive vs Non-Retentive Variables – What OB100 Should Reset

Understanding the CPU's retentive behavior avoids redundant writes in OB100.

Configure retentive ranges in Device view → CPU properties → Retentive memory. For S7-1500 CPUs the default is retentive from MB0 to MB15, and one or two DBs may be flagged retain in the DB properties. Adjust per process requirements.

10. Diagnosing and Verifying OB100 Execution

1. Compile and download the project.
2. Switch the CPU to STOP, then to RUN.
3. Open Online & diagnostics → Diagnostics → Cycle / Time; the diagnostic buffer will list the OB100 call timestamp and a startup reason code matching OB100_STRTUP_INF.
4. Open Watch table with the variables OB100 should set. Add the watch table to the Trigger condition: Once at startup or On transition to RUN. This forces TIA Portal to capture the values from the first scan.
5. Force the CPU to STOP → MRES → RUN (memory reset) to confirm the same initialization occurs even with retentive data cleared.
6. Pull the 24 V supply to simulate a real power loss; confirm warmup timers and latched-alarm resets still fire correctly.

To inspect the startup information in real time, monitor the OB100 temporaries with a Monitor with trigger session. Because the temporaries are valid only inside OB100, set the trigger condition to On start of OB for OB100 to capture the values during the single execution.

11. OB100 vs Cyclic OB1 – Comparison

12. Troubleshooting Matrix

13. Best-Pattern Template for a New Project

Use this SCL skeleton in OB100 to give every S7-1500 project a consistent, safe startup:

// OB100 – SCL – Project-wide startup template

// 1. Reset non-retentive process counters
FOR i := 1 TO 16 DO
    "Proc_DB".Counters[i]  := 0;
END_FOR;

// 2. Apply safe defaults
"Proc_DB".Auto_Mode_Allow    := FALSE;
"Proc_DB".Safety_Lockout     := TRUE;        // require manual reset
"Proc_DB".Warmup_Seconds     := 30;

// 3. Re-arm communication status
"Comm_DB".PN_C1_Healthy      := FALSE;
"Comm_DB".PN_C2_Healthy      := FALSE;

// 4. Write module parameters (per requirement)
//    WRREC variant example:
//    ret := WRREC(REQ := TRUE, IOID := 0, LADDR := 288, RECORD := "IO_DB".Cfg_Rec);

// 5. Single-cycle re-init pulse for cyclic OB to read
"Sys_DB".First_Scan_Done     := TRUE;

Pair this with a one-line consumption in OB1:

// OB1 – Network 1
IF "Sys_DB".First_Scan_Done THEN
    // 1-shot init that requires PIQ to be available
    "Proc_DB".Drive_Handshake  := TRUE;
    "Sys_DB".First_Scan_Done  := FALSE;
END_IF;

14. Cross-Platform Notes (Allen-Bradley and Beckhoff Equivalent)

15. Standards, Manuals, and Official References

The following Siemens official documentation is the authoritative source for OB100 behavior, startup information, and configuration:

• Startup organization blocks (OB 100, OB 101 and OB 102) – TIA Portal Documentation

Additional official references (TIA Portal help portal, Function Manual "S7-1500 Automation System", and the Program Lifecycle / Organization Blocks chapter) describe the OB100 temporary interface and the startup event class codes shown in section 5. Always cross-check the local Siemens Industry Online Support portal for the firmware version of your specific CPU before relying on any timing value.