Online Editing S7-300/400 in STEP 7 V5.5: Safe Value Changes

1. Overview: What "Online Editing" Actually Means on S7-300/400

Siemens SIMATIC S7-300 and S7-400 CPUs (programmed with STEP 7 V5.5) support online modifications to the user program, data blocks, and monitored variables while the CPU remains in RUN. The critical engineering point is that not every change is safe to apply online. The most common operations an automation engineer or process operator will perform online fall into four categories:

• Editing values in Watch / Variable (VAT) tables without downloading to the CPU.
• Force of inputs and outputs from a Force table.
• Download of logic blocks (OB/FB/FC/DB) to the running CPU.
• Download of hardware configuration (HW Config) – only supported in RUN on S7-400 CPUs with Configuration in Run (CIR) or on an H-CPU redundant pair.

Modifications to a running CPU that violate a programming error (e.g., call a non-existent DB, divide by zero, access a missing I/O) will trigger a STOP unless the corresponding error OB is present and downloaded. The first section of this reference is therefore the prerequisite for every other online change.

2. Prerequisites Before Any Online Change

Before opening STEP 7 V5.5 and downloading to a live pharmaceutical process, verify each of the following. Each is a precondition for a no-stop online change.

1. Identify the CPU family and firmware. S7-300 CPUs (312, 314, 315, 317, 319) and S7-400 CPUs (412, 414, 416, 417, 410) behave differently for hardware CIR. Confirm with PLC > Module Information > Diagnostic Buffer.
2. Confirm the load memory type. S7-300 can use RAM (volatile) or MMC (Flash, non-volatile, mandatory on newer -3PN/DP CPUs). S7-400 uses a memory card (RAM or Flash) or, on the H-CPU, a paired card. Online block changes are written to work memory immediately and to load memory on the next backup, but the difference matters for cold-restart behavior.
3. Verify error OBs are present and loaded. Open S7 Program > Blocks offline and confirm that all needed error OBs exist (see Section 3).
4. Connect the programming PG/PC via MPI, PROFIBUS, or Industrial Ethernet and reach the CPU through Accessible Nodes or a configured online interface. Use the Siemens Online Support entry for STEP 7 V5.5 SP2 for the supported interface catalog.
5. Take a backup of the offline project (File > Archive) and a CPU backup (PLC > Upload Station to PG) before any modification. In a GxP environment this backup is your audit record.

3. Mandatory Error OBs for Online Editing

STEP 7 generates an OB for every error class that can otherwise drop the CPU into STOP. The following OBs must be present in the offline program and downloaded to the CPU before you start online block editing.

OB121 and OB122 are the two OBs most often not present in legacy pharmaceutical projects. Their absence is the single most common reason that an online DB download pushes the CPU into STOP: the new logic tries to open DB 50, the old DB 50 was deleted, and OB121 does not exist, so the CPU faults.

3.1 Download Order for Safe Online Block Changes

1. Download the DBs first (in increasing number order is safest).
2. Download the FCs/FBs that call those DBs.
3. Download OBs last if any new OBs (e.g., OB121) are required.
4. Never download HW Config to a non-CIR/non-H CPU in RUN.
5. Verify in Module Information > Diagnostic Buffer after each download that the CPU remains in RUN.

4. Using Watch / Variable Tables (VAT) for Online Value Changes

A Variable Table (VAT) – known as a Watch Table in TIA Portal – is the lowest-risk tool for changing a value online. The table is held in the PG, not the CPU; nothing is downloaded to the load memory. The VAT directly reads and writes the process image and data-block areas of the working memory.

4.1 Creating and Populating a VAT

1. In the STEP 7 project, right-click Blocks and choose Insert New Object > Variable Table.
2. Name the VAT to reflect its intent (e.g., VAT_Recipe_Setpoints). In GxP contexts use a name that appears in the change-control record.
3. Add operands in the format:\pre> Address Symbol Display format Modify value DB101.DBD0 BatchSize FLOAT 1250.0 DB101.DBD4 FillRate FLOAT 42.5 M 0.1 RunEnable BOOL TRUE IW 64 PressPSI DEC 38 QW 80 ValvePct DEC 55
4. Click Monitor/Modify on the toolbar, choose Trigger (once, periodic, or on a defined point), and click Modify to write values.

4.2 Modify Trigger Settings

4.3 Operands That Can Be Modified Online

5. Force Tables: When and How to Use Force

Force is the only mechanism that overrides a physical input regardless of what the field wiring is doing. The forced value is stored on the CPU and is reasserted on every scan cycle. It is also a documented permanent state in the diagnostic buffer.

5.1 Creating a Force Table

1. Right-click Blocks > Insert New Object > Force Table.
2. Add I or Q addresses: I 0.0, IB 2, QW 4.
3. Set the force value column (0 or 1 for BOOL, DEC for bytes/words).
4. Click Force on the toolbar. The CPU enters the FORCE state; the SF/DIAG LED on the CPU lights depending on firmware.

5.2 Force Precautions

5.3 Difference Between "Modify" and "Force"

6. Data Block Initial Value vs Actual Value

This distinction is the most frequent source of "the value comes back after a restart" issues. Every DB has two value areas:

• Initial values (Startwerte) – stored in the load memory of the CPU and on the MMC/Flash card. Used as the source on a cold restart, on a memory reset, and when the DB is reloaded from the offline project.
• Actual values (Aktualwerte) – stored in the work memory of the CPU. Used by the program at runtime. They are the values visible from a VAT.

6.1 Setting a Value Permanently (Recipe Setpoint)

1. Open the DB in the Data View tab of the DB editor.
2. Change the Initial Value column (not the Actual Value column).
3. Save and download only the DB to the CPU. The new initial value is written to load memory; on the next cold restart the actual values are reseeded from the initial values.

6.2 Setting a Value Temporarily (Live Tuning)

1. Open the DB in the Data View tab.
2. Change the Actual Value column.
3. Trigger a Monitor/Modify write from the VAT instead – the value will be written to the work memory directly and survives a warm restart only if the data area is configured as retentive in the CPU properties (PLC > Properties > Retentive Memory).

7. Hardware Configuration Changes and CIR

Configuration in Run (CIR) is a Siemens feature on S7-400 CPUs (e.g., CPU 414-3, 416-3, 417-4) that lets you add or remove certain distributed I/O slaves (PROFIBUS DP / PROFINET IO) without stopping the CPU. The S7-300 family does not support CIR; any HW Config change forces a STOP.

For a step-by-step procedure see the Siemens FAQ "Configuration in RUN (CIR) for S7-400" on the Siemens Online Support portal. Key constraints:

• CIR is enabled per DP master / IO controller in HW Config.
• The CPU must be in RUN with no faults.
• Only add or remove of slaves / modules is allowed; re-parameterization of a slot is not.
• S7-400 H-CPUs (e.g., 412-5H, 414-5H, 417-5H) support online HW Config changes during redundant operation through the H system mechanism, not CIR.

8. HMI Modifications During RUN

Changes to WinCC Flexible, ProTool, or third-party HMI projects that communicate with the CPU via S7 communication do not cause the CPU to stop. The HMI station and the CPU exchange data through PUT/GET or S7 connections; the CPU treats the partner as a passive consumer. HMI tag updates, screen modifications, and alarm configuration changes are safe to push while the process is running, subject to the following:

• A full compile + download to the HMI panel is not atomic; brief loss of communication is possible during the project transfer.
• Ensure the HMI connection in the PC station / panel is not in stop state during the transfer.
• PanelView or TP/OP panels must remain powered; do not interrupt the transfer.

9. Pharmaceutical and GxP Compliance Considerations

FDA 21 CFR Part 11 and EU GMP Annex 11 treat the PLC program and the values it writes as regulated electronic records. Online changes in a running batch are therefore not just an automation concern; they are a quality concern.

10. Step-by-Step: Safely Change a Recipe Value Online

The following procedure is recommended for a 21 CFR Part 11 compliant change of a recipe parameter in a running batch on an S7-315-2 DP controlled by STEP 7 V5.5 SP2.

1. Open the offline project in STEP 7 and connect online via Accessible Nodes.
2. Open PLC > Module Information > Diagnostic Buffer and confirm no fault entries.
3. Open the Blocks folder and verify OB80, OB82, OB85, OB86, OB87, OB100, OB121, OB122 are present in the offline view.
4. Open or create the VAT for the recipe DB and enter the address, e.g., DB120.DBD0 in FLOAT format.
5. Click Monitor; confirm the displayed value matches the current setpoint.
6. Enter the new value in the Modify Value column and set Trigger = Once.
7. Click Modify. Observe the diagnostic buffer; no STOP entry must be generated.
8. Verify the HMI display updates within one OB1 cycle (typically 50–200 ms).
9. Print or save the VAT table with the change; archive the diagnostic buffer screenshot.

11. Troubleshooting Matrix

12. Field-Proven Best Practices

• Always download in the order: OBs → DBs → FBs/FCs. Reorder only with a documented impact analysis.
• Keep a "gold" project archive after each successful commissioning. Use STEP 7's archive tool with the Compress option to minimize size.
• Retentive settings in the CPU properties must match the VAT/DB strategy. If a value must survive a warm restart, declare the byte/bit range as retentive in the CPU's Retentive Memory tab.
• For recipe values, write the initial value, not the actual value. Operators should be trained on this distinction.
• Never force a safety input (E-Stop, guard door, light curtain) for convenience. The safety relay or F-CPU should be the source of truth.
• Use a dedicated programming PG with the STEP 7 project locked under a Windows account that has no operator privileges. The PG itself is a regulated asset in a GxP environment.
• Disable the "Load to MMC after every download" option during online tuning to avoid unnecessary write cycles to the Flash card (life expectancy ≈ 100,000 cycles per Siemens datasheet for S7-300 MMC).