Configuring Automatic Screen Change via PLC Tag on KTP400 Basic

Overview

This reference covers automatic HMI screen changeover on a Siemens SIMATIC KTP400 Basic panel driven entirely by PLC tag values from a SIMATIC S7-1200 CPU programmed in TIA Portal. The typical use case is an operator-guidance sequence: screen 1 displays "Place product in tool", sensor I0.0 detects the part, screen 2 displays "Press start button", and the cycle continues without any HMI button press.

Two engineering-grade implementations are documented end-to-end:

1. HMI tag event method - uses the WinCC Basic system function ActivateScreenByNumber (or ActivateScreen) wired to the value-change event of an HMI tag that mirrors a PLC data block integer.
2. Job mailbox method - uses the standardized Siemens area pointer 70 ("Job mailbox") to issue job number 51 (ActivateScreenByNumber) from the PLC program.

The first approach is preferred on KTP400 Basic because it requires no VBS script (Basic panels do not run VBS) and minimal PLC ladder logic. The job mailbox approach is the documented Siemens alternative when the project is being ported between Comfort/Professional panels and Basic panels, or when a single screen number has to be set from multiple PLC sources.

Throughout this document, the example PLC hardware is a CPU 1214C DC/DC/DC (firmware V4.4 or later) and the example HMI is a 6AV2 123-2DB03-0AX0 KTP400 Basic mono PN or 6AV2 123-2MA03-0AX0 KTP400 Basic color PN, programmed with WinCC Basic inside TIA Portal V16. Behavior on TIA Portal V13 SP1 and later is identical for the screen-change functions.

Terminology note: an "HMI input tag" in Siemens TIA Portal is any tag whose value is read from the PLC into the HMI (contrast with the HTML <input> form element documented at MDN Web Docs: <input>, which is unrelated). The screen-change mechanism described below uses an input tag whose value the HMI consumes to decide which screen to display.

Prerequisites

• TIA Portal V16 or later (V13 SP1 minimum) with STEP 7 Basic and WinCC Basic installed.
• SIMATIC S7-1200 CPU, firmware V4.1 or later (CPU 1211C / 1212C / 1214C / 1215C / 1217C).
• SIMATIC KTP400 Basic (4-inch mono or color PN variant), PROFINET interface.
• Configured PROFINET connection between the CPU and the HMI in the TIA project (Devices & Networks editor) with the HMI in the same subnet and the S7-1200 set as the IO controller.
• HMI screens named and numbered in the project; for the example: Screen_1 = "Place product", Screen_2 = "Press start", Screen_3 = "Cycle running".
• PLC data block DB_HMI containing a global ScreenNo tag of data type Int (or UInt) with start value 1. Sensor wired to I0.0.
• For the job mailbox method: HMI connection area pointer "Job mailbox" enabled in the HMI connection properties.

Architecture: PLC-Driven Screen Sequencing

For the requested sequence, the engineering target is a state machine that the PLC owns and that the HMI only visualizes. The PLC increments or assigns a screen number based on the process inputs and outputs, and the HMI panel simply renders the screen that matches the current value.

The most robust pattern uses a single integer tag shared by both controllers and one HMI tag event that fires whenever the integer changes. This avoids polling delays and ensures deterministic screen transitions even when the PLC updates the tag multiple times in quick succession.

The basic data flow is:

1. The PLC updates DB_HMI.ScreenNo whenever a transition condition becomes true (e.g., I0.0 rising edge).
2. The HMI tag Tag_ScreenNumber is connected to DB_HMI.ScreenNo through the standard HMI tag configuration (acquisition mode: "Cyclic continuous", acquisition cycle: 1 s by default; tighten to 100 ms if the operator-guidance sequence is time-critical).
3. The HMI tag event "Value change" fires and calls ActivateScreenByNumber with the new tag value as the screen number parameter.

Screen numbers are assigned by the engineer in TIA Portal under screen properties (right-click screen → Properties → General → Screen number). They are not necessarily the order in the project tree. Recording the intended screen number on each screen is recommended for maintainability.

PLC Tag Design and Screen Number Tracking

To prevent the screen from being re-activated on every HMI tag refresh (or on every PLC scan if the HMI is polling the tag very fast), the PLC must guarantee that the screen number tag only changes when the screen should change. Three practical implementations are described below.

The most common implementation is the explicit-assignment variant. The PLC moves a constant into the screen-number tag from the step-transition code:

// FB_Sequence - STEP 10: Place product
IF "I0.0" THEN                       // Sensor detects part present
    "DB_HMI".ScreenNo := 2;          // Activate screen 2 ("Press start")
    State := 20;
END_IF;

// STEP 20: Operator presses start
IF "I0.1" AND NOT "M_StartMem" THEN  // Edge detection on start button
    "DB_HMI".ScreenNo := 3;          // Activate screen 3 ("Cycle running")
    State := 30;
END_IF;
"M_StartMem" := "I0.1";

When using this approach, the HMI tag event fires only on a value change (1→2, 2→3), not on repeated identical writes. If the PLC must write the same value multiple times (e.g., to re-show screen 3 after a fault), include a one-step "bounce" through an intermediate value or use the job mailbox method described below.

Step-by-Step: Configure ActivateScreenByNumber via HMI Tag Event

1. Create the PLC data block. In the S7-1200 program, add a new global data block (e.g., DB_HMI). Add a tag ScreenNo of type Int with the start value 1. Compile and download the PLC program.
2. Add the HMI tag. In the TIA project tree, expand the KTP400 Basic and open "HMI tags". Add a new tag:
   • Name: Tag_ScreenNumber
   • Connection: the configured S7-1200 connection
   • PLC tag: DB_HMI.ScreenNo (link via the PLC tag dialog)
   • Data type: Int (must match the PLC tag)
   • Acquisition mode: Cyclic continuous
   • Acquisition cycle: 1 s (default); reduce to 200 ms for fast sequences
3. Open the tag event editor. Right-click Tag_ScreenNumber in the HMI tags list and choose "Properties". Switch to the "Events" tab.
4. Add the value-change event. Click "Add" in the Events table. For the event, choose "Value change" from the drop-down. For the function list, expand "Screen" → "Activate" and select ActivateScreenByNumber.
5. Bind the parameter. The system function ActivateScreenByNumber takes a single input parameter (screen number). Click the input field and bind it to Tag_ScreenNumber itself. This ensures the function receives the current value of the tag every time it changes.
6. Set the start screen. In the HMI project tree, right-click the KTP400 Basic → "Properties" → "Screens" → "Start screen". Select "Screen_1" (or whichever screen corresponds to screen number 1).
7. Compile and download. Compile the HMI project fully (right-click the panel → "Compile" → "Software (all)"). Download to the panel via PROFINET or Ethernet. Confirm the project is set to "RT" (Runtime) mode on the panel.
8. Test with the PLC tag table. In the PLC project online view, open the data block DB_HMI and use the "Monitor/modify" function to write 2 to ScreenNo. The HMI should switch to screen 2 within one acquisition cycle. Write 3 to switch to screen 3.

Alternative: Job Mailbox (Area Pointer 70) Method

The job mailbox is the documented Siemens interface for triggering HMI functions from the PLC without any configuration on the tag-event side. It is particularly useful when:

• Multiple PLC programs need to trigger the same screen change.
• The screen number to be activated is computed dynamically (e.g., from a recipe) and would be cumbersome to bind to a single HMI tag.
• The project must work identically across WinCC Basic, Comfort, and Professional panels.

The job mailbox is one of the standard PROFINET/S7 area pointers. For S7-1200 + KTP400 Basic, it is configured as follows.

1. Configure the area pointer. In the HMI project, right-click the connection to the S7-1200 and choose "Properties" → "Area pointers". Enable the "Job mailbox" pointer. The default DB number is the user-defined DB_HMI; the default start address is 0. The pointer occupies 4 bytes.
2. Accept the DB on the PLC side. The pointer write target must exist in the PLC. Accept the default DB number suggested by TIA Portal (or change it to your project DB) and compile the PLC.
3. Job mailbox byte layout. The job mailbox is a 4-byte buffer:

   Byte	Name	Meaning	Example for ActivateScreenByNumber(2)
   Byte 0	Job number	Function code	51 (= ActivateScreenByNumber)
   Byte 1	Parameter 1 (low)	First parameter (screen number for job 51)	2
   Byte 2	Parameter 1 (high) / Parameter 2 (low)	Word/byte depending on job	0
   Byte 3	Parameter 2 (high) / Parameter 3	Reserved	0

4. Trigger the job from the PLC. Use the standard pattern of writing job 51 to the pointer and then resetting it to 0:

   // Trigger screen change to ScreenNo via job mailbox
   "DB_HMI".JobMB[0] := 51;                            // Job number 51
   "DB_HMI".JobMB[1] := UINT_TO_BYTE("DB_HMI".ScreenNo); // Screen number
   "DB_HMI".JobMB[2] := 0;
   "DB_HMI".JobMB[3] := 0;
   

5. Reload and test. Compile both projects, download to PLC and HMI, and verify with the PLC monitor.

Verification and Commissioning

The following acceptance test confirms the configuration before handing the system over to production:

1. Power up the PLC and the HMI. Confirm that the HMI starts on the configured start screen (screen 1).
2. Force I0.0 ON using the PLC monitor. Within one acquisition cycle, the HMI should switch to screen 2.
3. Force I0.1 ON briefly (edge). The HMI should switch to screen 3.
4. Force DB_HMI.ScreenNo directly to 2, then to 1, then to 2 again. Confirm the HMI follows the values in order and does not skip.
5. Disconnect the PROFINET cable for 5 seconds. Confirm that when the connection re-establishes, the HMI re-synchronizes the screen number from the current PLC value (no manual reload required).
6. Check the HMI diagnostics buffer for any communication errors and the PLC diagnostics buffer for any PROFINET IO faults.
7. Cycle power on the panel only (PLC remains running). Confirm the panel re-acquires the current screen number from DB_HMI.ScreenNo on startup instead of reverting to the start screen.

Troubleshooting

KTP400 Basic Hardware and Firmware Constraints

Frequently Asked Questions