Configuring Siemens LMV51 Modbus Communication

1. Architecture Overview

The Siemens LMV51 is a microprocessor-based burner management controller for modulating gas/oil burners up to ~350 kW. It exposes a serial Modbus RTU interface on the X89 terminal block (RS-485) for integration into BMS or PLC networks. The Prosoft PLX31-PND-MBS4 is a PROFINET Device / Modbus Serial master-or-slave gateway. Combined with a Siemens S7-1215C (CPU 1215C DC/DC/DC, 6ES7215-1AG40-0XB0 or newer), the topology is:

Data flow: TIA Portal cyclically exchanges I/O data with the gateway on PROFINET; the gateway independently polls the LMV51 with Modbus function codes and updates the I/O image. The PLC does not need to handle the Modbus protocol directly — it only reads/writes the process image configured in the Prosoft gateway.

2. Prerequisites

2.1 Hardware

2.2 Software

3. Configuring the LMV51 Modbus Interface

By default the LMV51 ships with Modbus disabled or set to address 1. Before powering the burner, set the serial parameters on the AZL2x display:

1. Enter the parameter level on the AZL2x (see LMV51 parameter list, parameter 161 "Communication address").
2. Set parameter 161 to a unique Modbus slave address (default = 1, valid 1…247).
3. Set parameter 162 for baud rate (typically 9600 for PLX31-PND-MBS4).
4. Set parameter 163 for parity (None, Even, Odd) — 8N1 is the LMV51 default.
5. Wire the LMV51 X89 terminals: 61 = RS-485 A (+), 62 = RS-485 B (−), 63 = GND.

Reference the official Siemens LMV5x system manual (A7541) under "Bus connection / Modbus" to confirm the register table. A typical subset used in burner control programs:

4. Installing the GSDML in TIA Portal

1. Open the TIA Portal project containing the S7-1215C station.
2. In the project tree choose Options → Manage general station description files (GSD).
3. Set the source path to the folder where the unzipped Prosoft GSDML is stored. TIA Portal lists every .xml matching the GSDML schema (e.g. GSDML-V2.3-ProSoft-PLX31-PND-MBS4-20180112.xml).
4. Select the file and click Install. The device becomes available under Other field devices → PROFINET IO → Gateway → ProSoft Technology → PLX31-PND-MBS4.
5. Restart TIA Portal if a module update is requested (HSP conflict on older V13/V14 builds).

5. Adding the Gateway to the PROFINET Topology

1. Open the Devices & networks editor and switch to the network view.
2. Drag the PLX31-PND-MBS4 from the hardware catalog onto the PROFINET subnet of the S7-1215C. TIA will assign it a temporary name and IP (e.g. plx31-pnd-mbs4 / 0.0.0.0).
3. Open the gateway's Properties → PROFINET interface and assign a fixed IP address (e.g. 192.168.0.20), subnet mask 255.255.255.0 and a PROFINET device name (must match the one stored in the gateway, see step 6).
4. From the device view, drop one or both available slots — the PLX31-PND-MBS4 exposes up to two IO modules:

Compile the hardware configuration and download it to the S7-1215C. The PLC will only establish PROFINET AR (Application Relationship) with the gateway once the gateway's stored device name matches the one TIA programmed into the device.

6. Configuring the PLX31-PND-MBS4 with Prosoft Configuration Builder

Connect the Ethernet port of the PLX31-PND-MBS4 to the engineering PC, open PCB, and load the default project. Important settings:

6.1 Building the Modbus Map

Add commands in PCB's Modbus Master table. A minimal burner-control map:

When Word swap is selected, the gateway applies byte-swap so the data lands in the PLC big-endian as expected by Modbus-typed tags.

7. PLC Programming — SCL Example

The PROFINET I/O addresses are configured in step 5. With the mapping above, the LMV51 status word arrives at IW0 and the setpoint is written to QW0. The following SCL function block (TIA Portal V16, S7-1200 FW 4.4) can be used as a starting point:

FUNCTION_BLOCK "fb_LMV51_Gateway"
VAR
    // Edge bits
    bResetCmd     : BOOL;
    bRunEnable    : BOOL;
    // Process image from PLX31-PND-MBS4
    nStatus       : UINT;  // IW0
    nLockoutCode  : UINT;  // IW2
    nSetpointEcho : UINT;  // IW4
    nActualMod    : UINT;  // IW6
    nFlame        : UINT;  // IW8
    diHoursRun    : DINT;  // IW10..IW13
    // Outputs to PLX31-PND-MBS4
    nSetpoint     : UINT;  // QW0  (0..1000 = 0.0..100.0 %)
    nCmdWord      : UINT;  // QW2
END_VAR
BEGIN
    // Scale setpoint request from HMI 0.0..100.0 % to LMV51 0..1000
    #nSetpoint := REAL_TO_UINT( "HMI_Setpoint_pct" * 10.0 );

    // Build command word: bit 0 = Start, bit 1 = Reset lockout
    #nCmdWord := 0;
    IF #bRunEnable THEN #nCmdWord := #nCmdWord OR 16#0001; END_IF;
    IF #bResetCmd  THEN #nCmdWord := #nCmdWord OR 16#0002; END_IF;

    // Diagnostics: trip a fault if lockout code <> 0 and the burner is supposed to run
    IF (#nStatus AND 16#0001) = 0 AND #bRunEnable THEN
        "LMV51_Fault_latch" := #nLockoutCode;   // user-defined global
    END_IF;
END_FUNCTION_BLOCK

Call fb_LMV51_Gateway in a cyclic OB (OB1) and add an alarm to drive the S7-1215C's PROFINET diagnostic interrupt if the gateway reports a Modbus timeout (status word bit "IO-Not-OK").

8. Verification

1. PROFINET AR up? In TIA's Online & diagnostics → PROFINET diagnostics, both devices should show AR Established and no alarms.
2. Gateway LEDs: The PLX31-PND-MBS4 has separate PROFINET and Serial activity LEDs. Green on PROFINET, flickering green on the serial port means valid Modbus traffic.
3. Watch table: Force QW0 = 500 (50.0 %); the LMV51 should ramp its actuator and the actual modulation read-back at IW6 should track within the configured ramp rate.
4. Modbus trace in PCB: PCB's Diagnostics → Modbus Traffic window must show the four configured commands being answered without "CRC error" or "Timeout".
5. LMV51 AZL2x view: Navigate to parameter 0/1/2 and confirm the values match what the PLC wrote and read.

9. Diagnostics & Troubleshooting Matrix

10. LMV51 Register Map — Quick Reference

The following is a typical LMV5x Modbus map valid for BCM5.3x firmware. Refer to the LMV5x system manual (A7541) for the exact register set in your firmware version. Do not assume a register exists because it appears here — verify with the official Siemens parameter list.

11. PLC Fault Handling — Recommended Pattern

Use the PROFINET slot diagnostics to surface a clear operator message. In the S7-1215C, add a hardware interrupt OB (OB82) to react to diagnostic interrupts from the gateway:

ORGANIZATION_BLOCK "OB_82_PROFINET_Diag"
TITLE = 'PROFINET Diagnostic Interrupt'
VAR_TEMP
    t_LocalData  : OB82_STRUCT;
END_VAR
BEGIN
    // Latch raw event data, then evaluate on next OB1 scan
    "Last_PN_Diag" := t_LocalData;
    "PN_Diag_Pending" := TRUE;
END_ORGANIZATION_BLOCK

In OB1, raise an HMI alarm if the gateway went into IO-Not-OK, and write the last good values to a freeze buffer so the operator can identify when the failure occurred.

12. FAQ