Configuring LOGO! 8 AM Tag Overlays in TIA Portal HMI Tags

Overview

Siemens LOGO! 8 (and the 8.3 generation, device family 0BA8) exposes a flat 16-bit-tag namespace to the rest of the S7 world. When you connect a LOGO! 8 base module to a TIA Portal project that also contains a SIMATIC HMI (for example a KTP700 Basic PN or a Comfort Panel), the HMI tag table only surfaces a subset of the controller's address space. Digital inputs (I), digital outputs (Q), digital flags (M), and the first 851 words of variable memory (VW0 .. VW850) are visible. Anything the LOGO! program writes to an analog marker (AM) lands in the VM area above VW850, so it does not appear in the standard HMI tag import wizard, and the typical "Import LOGO! tags" tool button in TIA Portal does not bring the AM names across.

This article explains why AM tags are hidden from the LOGO! program itself, how to make them visible to a KTP700 (or any LOGO! 8-compatible HMI) using the AT tag overlay concept, and where parameter VM mapping versus network analog inputs/outputs fits into a robust setpoint-transfer architecture. Concrete addresses for the 0BA7 (LOGO! 8) and 0BA8 (LOGO! 8.3) generations are tabulated, and a complete commissioning procedure is provided at the end.

LOGO! 8 Memory Architecture

The LOGO! 8 base module ships with a fixed, contiguous VM (Variable Memory) block that the LOGO! Soft Comfort programming tool exposes as separate logical groups in the block editor:

The "M" flag area is a flat bit pack: M1..M8 occupy V0.0..V0.7 of VW0, M9..M16 occupy V1.0..V1.7 of VW0, and so on. The "AM" area, in contrast, is a flat word pack; AM1 is always a complete 16-bit INT and cannot be decomposed into eight digital bits by the LOGO! runtime. TIA Portal inherits this asymmetry because the LOGO! 8 device description in the TIA device library tells the HMI tag import tool only about the M-flag packing.

Why AM Tags Do Not Surface in the TIA Portal HMI Tag Table

There are three distinct reasons, and they matter because each one dictates a different recovery path:

1. Firmware restriction in the LOGO! program. The LOGO! runtime only reads and writes the VM area from VW0 through VW850 for its own block diagram. The LOGO! Soft Comfort editor literally cannot place a function block on an address above VW850. The first analog marker AM1 therefore lives at VW952 in the 0BA7 and at VW1118 in the 0BA8, both outside the program-accessible range. This is by design: AMs are scratch-pad words for analog values that the LOGO! holds for the Web Server and for S7 partners.
2. Different logical types. Even when the LOGO! project is imported into TIA Portal as a "LOGO! 8" device, the device description does not advertise AM symbols. The HMI tag import wizard only picks up I, Q, AI, AQ, M, NI, NQ, and the parameter-VM-mapped range. AM symbols exist only inside the LOGO! Soft Comfort (LSC) project file; they never travel through the TIA Portal symbol table.
3. Bit-vs-word typing. Digital flags (M) are bit-addressable; analog markers (AM) are word-addressable. They are two separate worlds and cannot be cross-cast inside the LOGO! runtime. A flag bit cannot be queried as an analog flag word, and an analog marker cannot be decomposed into eight digital flags. TIA Portal respects this constraint and will refuse to expose AM symbols even if you force a custom tag declaration.

The AT Tag Overlay Technique

TIA Portal offers a feature called the AT tag overlay, described in the SIMATIC S7-1200 manual collection as a way to "access an already-declared block tag with an overlaid declaration of a different data type" (see Accessing a tag with an AT overlay). The same syntax is supported on LOGO! 8 device objects inside TIA Portal because the LOGO! 8 compiler shares the S7-1200/1500 ST frontend.

An AT overlay is a second symbol that points at the same physical memory location as a first symbol, but exposes the data with a different data type. For HMI integration this is exactly what you need: declare the AM location once as WORD (because the VM is a 16-bit word) and overlay it as INT or REAL for the HMI to scale it into engineering units.

Syntax in a LOGO! 8 data block (DB) or in a Program block tag list:

// Primary tag – the raw VM word
AM1_RAW : WORD;       // backed by VW1118 on 0BA8 (VW952 on 0BA7)

// AT overlay – interpreted as a signed 16-bit integer
AM1_INT : INT AT AM1_RAW;

// Optional second overlay – interpreted as a real (engineering units)
AM1_EU  : REAL AT AM1_RAW;

The HMI tag table then binds AM1_INT or AM1_EU to a numeric I/O field, while the LOGO! program continues to write the value into AM1 via the analog-marker output of a math function block. TIA Portal keeps both overlays synchronized with the same VM word, so there is no copy step and no additional scan-time penalty.

Parameter VM Mapping for Persistent Setpoints

Parameter VM mapping is the LOGO! Soft Comfort feature that lets a function-block parameter (a setpoint, a threshold, a time-base) be backed by a word in the VW0..VW850 range. Because the parameter is stored in the LOGO!'s retained EEPROM, the value survives a power cycle, and because the parameter is in the program-accessible VM range, it can be written from a TIA Portal HMI tag as well.

Typical mapping:

The HMI tag import wizard will surface these parameter-VM-mapped words because they sit inside VW0..VW850. Use this mechanism whenever the value is a setpoint rather than a measured actual. The setpoint is retained across power cycles, and the LOGO! runtime applies it directly to the function block on every cold start.

Network Analog Inputs and Outputs as a Bridge

If you must move analog values into or out of the LOGO! that are not setpoints (for example a calculated actual value that originates in a remote S7-1200, or a recipe selection index that needs to be passed into a math block), use the network analog input (NAI) and network analog output (NAQ) blocks. These blocks live in the VM area above VW850 and are visible to the LOGO! program because the Net-IN/Net-OUT blocks are firmware-aware.

Step-by-Step: Exposing an AM Value to a KTP700 Basic PN

The following procedure moves an analog value written into AM1 on a LOGO! 8.3 (0BA8) onto a numeric I/O field on a KTP700 Basic PN connected via Ethernet to the LOGO! base module's onboard PROFINET port. Adjust the addresses for 0BA7 as noted.

1. Identify the AM address. In LOGO! Soft Comfort, open the project and select the analog marker AM1 in the block diagram. Note the firmware generation: 0BA7 = VW952, 0BA8 = VW1118.
2. Open TIA Portal and load the LOGO! device. In the project tree, double-click Devices & Networks > LOGO!_8 [LOGO! 8.3]. Open Program blocks > Main (or the equivalent data block where you want to declare the AT overlay).
3. Declare the primary tag. In the tag declaration table of the data block, add:
   AM1_RAW : WORD; // physical location VW1118 (0BA8) or VW952 (0BA7)
4. Declare the AT overlay. In the same table, on the next line, add:
   AM1_VALUE : INT AT AM1_RAW; // signed 16-bit scaled value
   If the engineering-unit value is a REAL, add a second overlay:
   AM1_EU : REAL AT AM1_RAW;
5. Wire the LOGO! program to AM1. In LOGO! Soft Comfort, connect the output of the function block that produces the analog value to AM1. Save and download to the base module.
6. Open the KTP700 HMI tag table. In TIA Portal, navigate to HMI_1 > HMI tags > Default tag table. Click Add new tag.
7. Configure the HMI tag. Name: AM1_Value. Connection: LOGO!_8 connection_1. Address: %DB1.DBX0.0 if you placed the tag at offset 0 in DB1, or use the symbolic name "Main".AM1_VALUE if your HMI tag table supports symbolic binding. Data type: Int. Length: 1 word. Acquisition mode: Cyclic continuous, 500 ms.
8. Add the I/O field. On the HMI screen, drag an I/O field onto the canvas. Set the process tag to AM1_Value. Configure display format to match the engineering unit (for example, one decimal place for °C).
9. Compile and download. Compile the LOGO! device first, then compile the HMI. Download LOGO! first, then the KTP700.
10. Verify online. On the KTP700, tap the I/O field; the value should update at 2 Hz. In TIA Portal, open Online & diagnostics on the LOGO!, expand Watch table, and confirm that VW1118 (or VW952) tracks the function-block output.

KTP700 HMI Integration in TIA Portal

The KTP700 Basic PN, KTP700 Comfort, and the newer KTP700 Basic DP are all certified LOGO! 8 HMI partners. The integration points to verify:

• Connection type. Use S7 connection (not TCP/IP open), with the LOGO! as the only S7 server on the subnet. The LOGO! 8.3 firmware accepts up to 8 simultaneous S7 connections.
• Rack/slot. Leave the LOGO! 8 at rack 0, slot 0; the HMI tag wizard reads this automatically.
• Tag import limits. The wizard imports all tags below VW850 plus NI/NQ/NAI/NAQ. It will not import AM symbols. Expect 0 to 64 tags depending on the project size.
• Cyclic polling. Default 500 ms is fine for temperature and pressure displays. Drop to 200 ms only for closed-loop control loops displayed on the HMI.
• Recipe handling. If you store setpoints as recipes on the KTP700, bind the recipe element to the parameter-VM-mapped word (VW100..VW106 in the example), not to an AM location. Recipes read/write setpoints only.

Firmware and Hardware Revisions: 0BA7 vs 0BA8

If your project must run on both 0BA7 and 0BA8, use parameter VM mapping (VW0..VW850) for everything, because the addresses are identical across both generations. AM addresses differ and would force two branches in the HMI tag table.

Troubleshooting Matrix

Best Practices and Field Tips

• Default to parameter VM mapping for any setpoint the operator can change. You get retention in EEPROM, you get the address inside the program-accessible range, and the HMI tag wizard handles it for free.
• Use AM only for display values and for values that the LOGO! computes internally. AM is the right home for an actual temperature, a filtered pressure, a duty-cycle percent, or any value that the HMI or web server reads but the LOGO! operator should not be able to overwrite.
• Reserve the network analog area for S7 partner traffic. Do not double-purpose it as scratch pad; the LOGO! 8 firmware treats NAI/NAQ words as reserved for partner communication.
• Document the AT overlay in the DB header comment. Because the overlay shares memory with the primary tag, a future engineer could overwrite one and not realize they clobbered the other. A one-line comment per overlay ("shared with AM1 at VW1118, do not write to this tag directly") prevents field-site confusion.
• Keep one AT overlay per AM, not per scaled unit. If the AM holds an INT in counts and the HMI needs a REAL in °C, declare AM1_RAW : WORD, AM1_INT : INT AT AM1_RAW, and let the HMI do the linear scaling in the I/O field properties. Two AT overlays on the same word is supported but adds compile-time checks for nothing.
• Compile and download LOGO! before the HMI. The HMI tag import wizard reads the LOGO! device's compiled tag list; if the LOGO! device is stale, the HMI tags will point at outdated addresses.

FAQ