S5 to S7 Migration: Lauer PCS900 HMI Memory Mapping Reference

1. Overview: The SIMATIC S5 + Lauer PCS900 to S7 Migration Challenge

Many European machine builders installed Lauer PCS900 operator panels on top of Siemens SIMATIC S5 PLCs between the late 1980s and the early 2000s. The PCS900 was a compact, rugged operator terminal with 20 freely labelable function keys, 40 indicator LEDs (a green and a yellow LED per key), and a numeric/alphanumeric LC display. It connected to the S5 via PROFIBUS-DP using the Lauer PCSpro configuration software, and was treated as a DP slave on the S5 side.

When the S5 is replaced with a SIMATIC S7-300, S7-400, S7-1200, or S7-1500, two problems surface at the same time:

1. The S5 program must be re-hosted on the S7 CPU. The original S5 STL or LAD cannot run on S7 directly; you must either use the Siemens S5 to S7 converter (migration tool in the STEP 5 / STEP 7 / TIA Portal family), rewrite from the documentation, or rebuild from scratch.
2. The HMI tag database must be re-mapped. The PCS900 did not use symbolic tags — it exchanged raw bit/byte/word values with the S5 over a fixed System Data area that PCSpro allocated inside the S5 memory. If you do not extract that mapping before the S5 is decommissioned, you cannot rebuild the operator panel without disassembling the machine's I/O behavior.

2. PCS900 Hardware Architecture and I/O Model

The PCS900 panel is documented in the original Lauer manual (publication P991_e) and the PCSpro configuration manual. The relevant facts for migration are:

The PCS900 is purely a passive I/O terminal over PROFIBUS-DP. There are no symbolic tags. Every key, every LED, and every value is a fixed offset inside a contiguous data block that PCSpro allocates inside the S5 flag (Merker) area, the S5 peripheral I/O area, or an S5 data block. The integrator chooses the start address during PCSpro commissioning; PCSpro then tells you the resulting offsets.

3. PCSpro Configuration Software and the System Data Area

PCSpro is the Lauer Windows configuration tool used to:

• Inscribe the F-key labels (printed to foil or rendered on the LC).
• Define the LED text and behavior.
• Allocate the System Data area in the PLC.
• Compile the PROFIBUS-DP slave configuration and GSD.

The System Data area is the single block of PLC memory that the PCSpro project exchanges with the panel. Inside it you will find three logical sub-areas:

The exact start address and bit packing is not standardized. Each PCSpro project specifies:

• Base area type: Merker (M / F), Data Block (DB), or Peripheral (P).
• Base address: e.g. MW 100, DB 50 DBW 20, or PQW 64.
• Bit offset within the first byte, if the base area does not start on a byte boundary.

4. SIMATIC S5 Memory Model Relevant to PCS900 Mapping

The PCS900 almost always exchanges data through the S5 Merker (flag) area or an S5 data block. The S5 memory model you need to recognize in the original documentation is:

The example in the source question — "M0.1 connect with start button" — refers to Merker 0.1, i.e. bit 1 of byte 0 of the flag area. This is the most common location for the PCS900 status bits because the S5 CPU update of the flag area happens at OB1 cycle boundary and the panel sees a stable image.

5. PCS900 System Data Area Layout in the PLC

Below is a typical (illustrative) layout that an integrator might have configured in PCSpro. Use this as a template; the real layout is whatever is documented in the original PCSpro printout.

6. Extracting the Existing PCSpro Project

Before the S5 is decommissioned, perform the following archive procedure.

1. Locate the original PCSpro installation media and license. PCSpro runs on Windows (XP / 7 / 10 with compatibility mode) and requires a parallel-port or USB-to-serial dongle for some panel variants.
2. Connect the configuration PC to the PCS900 service port using the original Lauer RS-232 programming cable (sub-D 9-pin). The service port is on the rear of the panel.
3. Launch PCSpro and select Upload project from panel. PCSpro reads back the full configuration including the System Data area specification.
4. Export the System Data area as a printed report. Use File → Print project documentation. The printout contains every F-key to bit assignment and every value to word assignment in plain text.
5. Save the project file with a versioned name, e.g. PCS900_MachineLine3_Rev04.pcs. Store it in the machine's engineering archive alongside the S5 program floppy / EPROM image.
6. Photograph the panel front showing the foil inscriptions on the function keys. This gives you the human-readable meaning of each F-key (e.g. F03 = "Start", F04 = "Stop", F11 = "Acknowledge fault").

7. Mapping PCS900 Tags to SIMATIC S7 Addresses

After you have the PCSpro printout, the migration to S7 follows this transformation table.

7.1 Byte-swap and word-order rules

S5 stores a 16-bit word with the high byte at the lower address. S7 also stores a 16-bit word with the high byte at the lower address. This part is the same. The confusion arises when you read an S5 MW and then re-create the same logical value in S7. If the PCSpro printout says "Process value 1 is at MW 100, 16-bit integer, big-endian word", the S7 equivalent is "MW 100, INT, big-endian word" — direct copy.

Where the byte order changes is when the PCSpro packs two values inside one word, e.g. low byte = status nibble, high byte = value. In that case the PCSpro printout shows two byte addresses that you must rebuild explicitly on the S7 side.

7.2 Symbol table reconstruction

Create a STEP 7 / TIA Portal symbol table that mirrors the PCSpro assignment. Example fragment for the table above:

// Symbol table excerpt (STEP 7 / TIA Portal)
HMI_F01       BIT   M0.0     // PCS900 key F01 status
HMI_F02       BIT   M0.1     // PCS900 key F02 status
HMI_F03       BIT   M0.2     // PCS900 key F03 status (Start)
HMI_LED01_G   BIT   M10.0    // PCS900 LED F01 green
HMI_LED01_Y   BIT   M10.1    // PCS900 LED F01 yellow
HMI_Disp1     INT   MW100    // PCS900 display value 1
HMI_Disp2     INT   MW102    // PCS900 display value 2
HMI_Set1      INT   MW200    // PCS900 entry value 1
HMI_Set2      INT   MW202    // PCS900 entry value 2

This symbol table becomes the tag interface you then bind to the new Siemens HMI in WinCC (TIA Portal) or to a third-party HMI.

8. Selecting and Configuring a Replacement HMI

The PCS900 is end-of-life. Modern replacements fall into three categories.

If you keep the PCS900, you must keep the original PROFIBUS slave GSD and configure the S7 as DP master. For an S7-300, use CP 342-5 or the integrated DP port of the CPU 31x-2 DP. For an S7-1200, add CM 1243-5. For an S7-1500, use CM 1542-5. The DP master config is loaded into the S7 using STEP 7 HW Config or TIA Portal device configuration. The I/O area assigned to the PCS900 slot becomes the new home of the System Data area — for example, the PCS900 might occupy input bytes 0 to 7 and output bytes 0 to 7, so the panel's status bits land in IB 0 and the LED control bits live in QB 0.

9. Step-by-Step Migration Workflow

1. Archive the existing project. Upload the PCSpro project, print the System Data report, photograph the front foil, archive the S5 program (LIST / INAW STL on floppy, EPROM dump, or STEP 5 archive).
2. Convert or rewrite the S5 program to S7. Use the Siemens S5 → S7 converter for a first-pass translation, then manually clean up any unsupported instructions. Address all S5 absolute accesses against the new S7 tag names.
3. Decide HMI replacement strategy. Either keep the PCS900 (replace only the PLC) or fit a new Siemens panel.
4. If keeping the PCS900: configure the S7 as DP master; install the PCS900 GSD in TIA Portal; map the panel's input/output bytes to the addresses the PCSpro project allocated. Mirror the original Merker assignments into the S7 Merker area or into a dedicated DB so that the rest of the S7 program can stay unchanged.
5. If replacing the panel: create the symbol table from section 7.2; build the WinCC / TIA Portal screen layout matching the original 20-key philosophy; bind each F-key tag to the original S5/S7 Merker or DB bit. Render the foil or label the new keys with the same inscriptions.
6. Compile and download the S7 program and the HMI configuration. If using TIA Portal, compile as a single project so the HMI tag table binds at compile time.
7. Run an off-line simulation. Use PLCSIM (S7-300/400) or PLCSIM Advanced (S7-1500) and the WinCC Runtime simulator to verify that every F-key toggles its expected bit and every LED responds.
8. Power down the S5 line. Replace the CPU and any affected I/O. Power up the S7 and the HMI.
9. Run the on-line commissioning checklist. See section 10.

10. Verification and Commissioning Procedure

After the swap, perform the following checks before returning the line to production.

11. Troubleshooting Matrix

12. Related S5 / S7 Migration Notes

• STEP 5 program conversion to STEP 7 is supported by the Siemens S5 to S7 converter for STL / LAD / CSF. Not every FB and not every FB parameter block is convertible. Plan to rewrite the process-specific FBs by hand.
• PROFIBUS-DP master configuration in TIA Portal lives under Device configuration → Properties → PROFIBUS interface → Operating mode → DP master. The PCS900 GSD must be installed first via Options → Manage device description files (GSD).
• Symbolic versus absolute addressing in S7: take the opportunity of the migration to introduce a symbol table. The PCS900 was absolute-only; S7 can be either. Future-proof the next migration by storing every panel-related tag in a dedicated DB (e.g. DB 900 "HMI_Interface") instead of scattering bits through the Merker area.
• Retain the original PCSpro printout in the machine documentation folder for the lifetime of the equipment. If a future technician re-uses the machine as a spare-parts donor, the HMI mapping is part of the asset value.