WinCC Flex Multiplex Tags and Alarm Acknowledgment on MP277

1. Overview

WinCC Flexible 2005 (and the parallel 2008 SP1/SP2/SP3/SP4 line that followed) is the HMI configuration suite that targets the SIMATIC MP 277 multitouch panel family as well as the smaller TP 177, OP 177 and Mobile Panel 177 devices. Two of the most common integration questions on these projects are:

1. Whether multiplex tags (the WinCC Flex form of indirect addressing) can be bound to objects other than the Symbolic I/O field, such as command buttons or bar displays.
2. How to wire the Acknowledgement PLC property of a Discrete Alarm so that a single HMI tag can acknowledge many alarms without producing a "property value invalid" message in the editor.

This reference consolidates the verified behaviour, the editor constraints, the Power Tag licensing pitfalls, and the migration impact of the Siemens Product Phase-Out announcement for WinCC flexible dated 01.10.2022 into a single engineering document.

2. Multiplex Tag Syntax in WinCC Flexible

A multiplex tag in WinCC Flexible is Siemens' implementation of runtime indirect addressing. The online help describes it as: "In multiplexes, a type of indirect addressing, the tag used is first determined at runtime. A list of tags is defined for the multiplex tags. The relevant tag is selected from the list of tags in runtime. The selection of the tag depends on the value of the index tag."

Despite the wording, the WinCC Flexible implementation differs from the original ProTool behaviour. In ProTool the index selected a tag from a predefined list. In WinCC Flex the index is used to compute the PLC memory address itself at runtime. The most common and fully supported syntax is the variable DB number, fixed offset form:

Address of IndirectTag = DB [IndexTag] DBW 20

Where:

• IndexTag – Integer (INT) tag that holds the DB number to be read or written.
• 20 – Fixed byte offset (here, word 20 = byte 20/21) inside whichever DB the index currently points to.

The same construction works for the bit form: DB [IndexTag] DBX 4.0, for byte form DBB, and for double-word form DBD. The index tag may legally hold a negative number; the value is passed through to the S7 communication layer as-is, so a negative index will cause an access error in runtime and must be clamped by the PLC program.

2.1 Power Tag consumption

Each multiplex tag counts as 1 Power Tag for licensing purposes, regardless of the size of the index range it can resolve to. This is different from ProTool, where each selectable target in the list incremented the tag count. The distinction matters when the editor reports tag count warnings during compilation.

3. Binding Multiplex Tags to Buttons, Bar Displays and Other Objects

The editor's symbolic I/O field has dedicated wiring for multiplex tags – it presents a property page that lists the candidate tags and a separate index tag field. For all other object types (Command Button, Bar, Gauge, Numeric I/O field, Date/Time field, Graphic I/O field) the multiplex tag appears in the tag picker as a single entry identical to a normal tag, and is wired through the normal Property > Process Value / Tag dialog.

Common reason a multiplex tag is ghosted out in the tag picker for a button:

4. PLC Acknowledgment for Discrete Alarms

A discrete alarm in WinCC Flex has three optional PLC-side properties:

• Trigger tag – the bit that raises the alarm.
• Acknowledgement tag – the bit the HMI sets in the PLC when the operator acknowledges the alarm.
• State / status tags – bit groupings that report "came", "went", "acknowledged" to the PLC.

For the Acknowledgement PLC property to be accepted in the editor, it must be an array element of the same array tag used for the trigger. The HMI polls the alarm bits and the ack bits in the same cycle, so it forces the engineer to group them into one contiguous block to minimise poll overhead.

4.1 Required bit layout

Common engineering convention is to place the ack bits 100 words (1600 bits) above the trigger bits inside the same array tag. The MP 277 polls the entire 16-bit word containing the ack bit, so it will set the correct bit without disturbing the other trigger bits that share the word.

4.2 Resolving "Property value invalid"

The "property value invalid" message in the Acknowledgement PLC field is almost always caused by one of the following:

1. The ack tag is a different array tag (or a scalar BOOL tag) than the trigger tag. Change it to the same array tag name and select a different array index.
2. The ack bit number is outside the array size. If the trigger array is 50 bits, the highest valid ack bit is bit 150 (trigger 50 + 100 word offset mapped to bit 50 + 1600). Extend the array or move the ack bits lower.
3. The tag type is BOOL, not an array of BOOL. In the tag editor, create the alarm tag as a contiguous array element, e.g. Alarm_array[0..255], then assign the single bit inside the discrete alarm configuration.
4. The data block on the S7 side does not exist or has not been downloaded. Build the S7 project first, then rebuild the HMI tags from the PLC connection.

5. Power Tag Licensing and the "One Screen Per Machine" Trap

WinCC Flex 2008 Power Tags are the licensed concurrent-acquisition tags. On the MP 277 the base licence ships with 256 Power Tags, expandable in 256/512/2048 increments. Multiplex tags help consolidate screen logic, but if used naively they still consume Power Tags at the same rate as a fully expanded screen:

"If you've got 20 odd, electrically identical machines you want to visualise, it makes sense just to create one screen rather than 20. However, if they've each got 20 IO points, then with Multiplex addressing, you've used up 400 Power Tags (as you would have with 20 different screens). Instead, create 20 dummy variables which you address in the Screen and map the I/O in the PLC according to the address of the machine to be viewed. That way you've saved 380 Power Tags and the load on the CPU is pretty well negligible."

5.1 Recommended pattern

1. Create 20 internal HMI tags of the same type (e.g. IO_intern_01..20).
2. Place the 20 internal tags on the screen exactly as if they were real I/O.
3. In the S7 PLC, copy the data block of the currently selected machine into the internal DB that the HMI polls. Use the index tag (machine number) to drive the SFC / FB that does the copy.
4. The HMI never sees the machine index; it always polls the same internal DB. Power Tag count remains equal to the screen's tag count (20), not 20 × N.

6. Script Behaviour with Multiplex Tags

WinCC Flex scripts can read and write multiplex tags, but with one documented limitation: the value at the memory location will only be read at the next update cycle of the addressed tag. The implication is:

• Do not change the index and then read the resolved value of the multiplex tag within the same script – the value returned will be stale (one cycle behind).
• If a script must react synchronously, write the data to the target DB directly using an absolute tag, not the multiplex tag.
• Scheduled scripts (poll-cycle based) are the recommended execution point for index changes that will be picked up by the next cycle.

7. Step-by-Step: Configuring a Multiplex Tag on a Command Button (MP 277)

Prerequisites

• WinCC Flexible 2008 SP4 (or 2005 for the original MP 277 firmware < V12.0.0.0) installed.
• STEP 7 V5.5 project with the S7-300/400 station containing a DB whose number is held in a PLC INT tag (e.g. MW 100).
• MP 277 device version selected in the WinCC Flex project tree.

Procedure

1. Open the Tags editor, right-click → New Tag. Name it IndexTag, type INT, connection = the S7 connection to the PLC, address = MW 100. Acquisition cycle = 1 s.
2. Create the target tags. For three conveyors:

   Conveyor_1.Section.Start : BOOL, DB31.DBX 0.0
   Conveyor_2.Section.Start : BOOL, DB32.DBX 0.0
   Conveyor_3.Section.Start : BOOL, DB33.DBX 0.0

3. Create a new internal tag ConveyorStart with type BOOL. In its properties enable Multiplexing. Set the index tag to IndexTag. Map the three conveyor tags into the multiplexing list. Because WinCC Flex uses indirect addressing, only one Power Tag is consumed regardless of list length.
4. Place a Command Button on the screen. In its Events > Press action use the function SetBit with the tag ConveyorStart.
5. If the button tag picker greys out ConveyorStart, switch the button to Switch mode with function InvertBit; the multiplex tag will then be selectable as the process value.
6. Compile the project. If the compile log reports a multiplex limit warning, see Section 10.
7. Download to the MP 277 via Ethernet (RFC1006 / OP/TP route). Verify in the cross-reference (Tools → Cross-Reference) that ConveyorStart is referenced by exactly one button.

8. Step-by-Step: Configuring PLC Acknowledgment for Discrete Alarms

Prerequisites

• Discrete Alarm class created in the HMI project (default classes 1–4 are sufficient).
• A BOOL array tag in the PLC that is long enough to hold both trigger and ack bits (recommended 256 bits minimum).

Procedure

1. In the STEP 7 project, create a DB (e.g. DB 100) with a BOOL array Alarm_array[0..255]. Bits 0–99 are trigger bits; bits 100–199 are ack bits.
2. In the HMI tag editor create the HMI tag Alarm_array of type Array of BOOL, length 256, address = DB100.DBX 0.0 with the array option enabled.
3. Open Discrete Alarms → New. In Trigger tag select Alarm_array; in Trigger bit enter 1.
4. In the Acknowledgement PLC property select the same tag Alarm_array and enter 101 in Ack bit. The "property value invalid" message will clear.
5. Add a second alarm. Trigger tag = Alarm_array, trigger bit = 2. Ack tag = Alarm_array, ack bit = 102.
6. Compile. The discrete alarms editor will accept both rows because the ack bit now lies inside the same array.
7. Download to the MP 277 and trigger Alarm 1 from the PLC. Acknowledge the alarm in runtime; verify that bit 101 of DB100 is set by the HMI, then cleared by the PLC acknowledgement logic.

9. Verification

10. Troubleshooting Matrix

11. Migration to TIA Portal / WinCC Comfort

Because WinCC flexible was phased out on 01.10.2022 per Siemens Support entry 109810654, new projects must use WinCC Comfort / Advanced / Professional in TIA Portal. The two concepts covered here map cleanly:

Use the TIA Portal migration tool ("Migrate project") on a copy of the WinCC Flex project. The tool preserves multiplex tags and the alarm bit layout, but verify the array sizes because the migration tool occasionally truncates very long arrays to 256 elements.

12. Frequently Asked Questions