Updating PID_Compact Parameters from HMI on S7-1200/1500

1. Overview: PID_Compact and HMI Parameterization

PID_Compact is the standard PID controller instruction supplied with the S7-1200, S7-1500, and S7-1200 G2 CPU families. The instruction is part of the PID Control technology object family and is included in every TIA Portal installation without additional licensing. PID_Compact delivers continuous, pulse, or mixed output behavior from a single FB and provides built-in pretuning and fine tuning routines.

When commissioning a temperature, level, flow, or pressure loop, the integrator or operator frequently needs to adjust controller parameters (Gain, Integral time TI, Derivative time TD) after the loop is running. Three valid parameterization paths exist in TIA Portal:

1. Inspector window of the PID_Compact block (offline configuration download).
2. Online/offline edit of the instance DB (DBedit) from the project tree.
3. HMI write access to the retentive structure of the instance DB during runtime.

Only the third path lets the operator change Gain, TI, and TD from a Comfort Panel, Unified Comfort Panel, Basic Panel, WinCC Runtime, or any third-party HMI that speaks S7 communication. The configuration question is therefore: which of the two PID_Compact parameter source modes (Instance DB or Instruction) exposes the values in a way the HMI can write?

Reference: Technology object PID_Compact (S7-1200, S7-1500, S7-1200 G2).

2. Prerequisites and Compatibility

3. PID_Compact Source Modes: Instance DB vs. Instruction

PID_Compact exposes two paths for the Setpoint, Input (process value), and Output tags. The selected path determines whether the HMI can write to the value. This is documented in the SIMATIC S7-1200, S7-1500 PID control function manual (page 73 in the V21 PDF edition).

For tuning, the operator is changing controller parameters (Gain, TI, TD) and not the Setpoint, so the relevant tags live in the retentive structure of the instance DB regardless of the source mode setting. However, choosing Instance DB for Setpoint/Input/Output is the safer convention for two reasons:

1. It keeps all operator-changeable values in one DB address range, simplifying HMI tag generation.
2. It prevents the program from inadvertently overwriting an operator entry on the next OB1 cycle.

To select the source mode, open PID_Compact in the project tree, switch to the Configuration tab, and set Input / Setpoint / Output source to Instance DB. Apply and download the project to the CPU.

4. Identifying HMI-Accessible Parameters

PID_Compact is delivered as a FB 1130 (S7-1500) or FB within the global library (S7-1200). Each call creates an instance DB that holds three groups of tags:

For the common requirement of changing Gain, TI, and TD from the HMI, the absolute tag addresses inside the instance DB are:

5. Step-by-Step: Enabling HMI Writes to PID Parameters

5.1 Confirm the Instance DB Source Mode

1. Open the project in TIA Portal.
2. Expand Technology objects > PID_Compact_x [DBx].
3. Open Configuration > Basic settings.
4. Set Input, Setpoint, and Output to Instance DB.
5. Compile and download the hardware and software to the CPU.

5.2 Verify the Retain Structure Exists

1. Open the instance DB in the project tree (double-click PID_Compact_x [DBx]).
2. Confirm the path Retain.CtrlParams.Gain is visible.
3. If only the cyclic I/O is shown, the FB version is older than V2. Replace the block using Technology objects > PID_Compact > Update.

5.3 Configure the HMI Connection

1. Add an HMI device to the project (Comfort Panel, Unified Comfort Panel, or WinCC RT Professional).
2. Open Devices & Networks and connect the HMI to the S7-1200/1500 CPU. An integrated S7 connection is created automatically.
3. Right-click the CPU in the device view, choose Properties > Protection & Security, and confirm Permit access with PUT/GET from remote partner if you intend to use absolute addressing from a third-party HMI.

5.4 Create HMI Tags Pointing to the Retain Structure

1. In the HMI project tree, open HMI tags.
2. Add a new tag with these properties:

The PLC tag field accepts the symbolic address of the instance DB tag. This eliminates the need to type absolute byte offsets. TIA Portal resolves the symbol during HMI compile.

5.5 Bind the Tags to HMI Objects

1. Open the screen that should host the parameter entry.
2. Insert an I/O field and configure:

• Tag: PID_Gain (or the appropriate variable).
• Mode: Input/output.
• Format pattern: 0.000 for Gain, 0.0 s for TI/TD.
• Limits: 0.0 to 1000.0 (adjust to the process).

3. Repeat for TI and TD.
4. Add three numeric read-only displays that bind to <PID_DB>.Retain.CtrlParams.Gain, .Ti, and .Td as confirmation of the active value after the CPU accepts the write.

5.6 Compile and Download

1. Compile the HMI first (Compile > Software (rebuild all)).
2. Compile the PLC software to refresh tag consistency.
3. Download both to the target devices.
4. Switch the HMI to runtime and test the write cycle.

6. Program-Side Best Practices for Safe HMI Writes

HMI writes land directly in the retentive area of the instance DB. Without guard code, the operator can enter negative values, NaN, or values outside the actuator range. Add a validation block in OB1 or a cyclic OB before PID_Compact is called:

// ST (SCL) snippet - parameter guard
IF "PID_DB".Retain.CtrlParams.Gain < 0.0 OR
   "PID_DB".Retain.CtrlParams.Gain > 1000.0 THEN
    "PID_DB".Retain.CtrlParams.Gain := "iGain_LastValid";
END_IF;

IF "PID_DB".Retain.CtrlParams.Ti < 0.0 OR
   "PID_DB".Retain.CtrlParams.Ti > 10000.0 THEN
    "PID_DB".Retain.CtrlParams.Ti := "iTi_LastValid";
END_IF;

IF "PID_DB".Retain.CtrlParams.Td < 0.0 OR
   "PID_DB".Retain.CtrlParams.Td > 10000.0 THEN
    "PID_DB".Retain.CtrlParams.Td := "iTd_LastValid";
END_IF;

Store the last-valid values in a separate, non-retentive global DB. Update them on every cycle in which the new value passes validation. This pattern prevents a stuck actuator if the operator enters an out-of-range Gain or a negative TI.

7. Verifying the HMI Write Reached the CPU

Perform the following checks after commissioning. Each step has a pass/fail criterion.

8. Edge Cases and Field Tips

8.1 HMI Writes are Overwritten by the Program

Symptom: Operator changes Gain from 1.0 to 2.0, the value reverts within a second. Cause: a section of the user program (often a startup or HMI image handler FB) writes PID_DB.Retain.CtrlParams.Gain := 1.0 on every cycle. Move the assignment to a one-shot logic (use a rising edge from a first-cycle flag such as FirstScan in OB100).

8.2 Tag Not Visible in the HMI Tag Dialog

If the symbolic tag DBx.Retain.CtrlParams.Gain is grayed out, the HMI device is not connected to the CPU as an HMI partner. Open Devices & Networks, drag a line between the HMI and CPU ports, and recompile.

8.3 TIA Portal V15 vs. V21 HMI Behavior

Older TIA Portal versions (V15, V15.1) do not expose Retain.CtrlParams.Gain as a selectable symbolic tag. Either update TIA Portal or use the absolute address. The instance DB byte offset for Gain (REAL) is fixed at DBx.DBD64 in the V1 instruction layout. Verify the offset by inspecting the Offset column of the instance DB editor.

8.4 Pretuning Resets Operator Values

Running pretuning (Mode = 1) overwrites Gain, TI, and TD with the result of the identification step. After pretuning completes, the operator values from the HMI are lost. Save the current values to the HMI recipe before triggering pretuning if the operator must be able to roll back.

8.5 PUT/GET Blocked by CPU Protection

If a third-party HMI writes by absolute address, the CPU must allow remote PUT/GET. Navigate to CPU properties > Protection > Connection mechanisms and enable Permit access with PUT/GET from remote partner. With integrated HMI connections, this is not required.

8.6 Hot-Restart of the S7-1500

On S7-1500, the retentive data of PID_Compact is loaded from the load memory at STOP-to-RUN transition. HMI writes made while the CPU is in STOP are visible in the online DB but do not take effect on the controller until the CPU returns to RUN. This is a common confusion in commissioning.

9. PID_Compact Runtime Mode Word Reference

To let the operator switch between manual and automatic from the HMI, expose the Mode input. The valid values are:

For bump-less transfer between manual and automatic, set the manual value to the current process output before switching back to automatic:

"PID_DB".ManualValue := "PID_DB".Output;
"PID_DB".ManualEnable := TRUE;
"PID_DB".Mode := 2;  // automatic

10. SVG: Data Flow from HMI to PID_Compact

The arrow order is: HMI I/O field → HMI tag → S7 connection (PUT/GET or integrated) → Instance DB Retain.CtrlParams → PID_Compact block on the next call. The CPU only accepts the new Gain, TI, or TD at the boundary of the next PID_Compact call cycle.

11. Troubleshooting Matrix

12. Frequently Asked Questions