LOGO! Modbus: Reading Kamstrup smart-me 32-bit Float Values

Problem Overview

Field installations that pair a Siemens LOGO! 8 base module (BM) as a Modbus TCP client with a smart-me Kamstrup HAN gateway consistently report a single failure mode: the LOGO! reads a constant 0 from the energy meter while an independent Modbus master test tool (QModMaster, Modbus Poll, the Home Assistant Modbus integration, or libmodbus-based scripts) reads the same register and returns a valid IEEE-754 32-bit float. The fault is layered: three independent root causes stack on top of each other, and any one of them produces the identical 0-value symptom that the LOGO! program surfaces.

This article documents the three failure modes, the correct Network Analog Input (NAI) configuration in LOGO! Soft Comfort V8.x, the F/I converter pattern required to scale the 32-bit float into the LOGO!'s native 16-bit signed integer range, and the verification sequence using the LOGO! Online Test Data Table.

Symptom Matrix

Root Cause Analysis

The three independent failure modes are documented below in the order they should be ruled out during commissioning. Each section provides the diagnosis, the corrective action, and the verification step.

Failure Mode A — Network Address Misconfiguration

The LOGO! NAI block contains a four-field IP address (octet 1 through octet 4) and a separate port field. A single typo in any of the four octets — most commonly 198.168.0.x instead of 192.168.0.x — sends the TCP SYN to a non-existent host. The LOGO! Modbus client implementation reports the timeout to the FBD interpreter, the NAI value is never updated, and the local V-memory location retains its power-on reset value of 0.

The IP address in the LOGO! program must match the IP address that QModMaster uses to read the same smart-me module. The QModMaster connection status (green indicator) confirms the TCP path; if QModMaster reads the meter but LOGO! reads 0, the LOGO! NAI IP address field is wrong even when it looks correct at a glance.

Failure Mode B — Register Width Mismatch

A single LOGO! NAI block issues Modbus function code 0x03 (Read Holding Registers) with quantity = 1, returning a single 16-bit register. The Kamstrup meter exposes active power, voltage, current, and energy values as 32-bit IEEE-754 floats across two consecutive holding registers (4 bytes). With quantity = 1, the LOGO! receives only the first 2 bytes of the 4-byte value; the remaining 2 bytes are silently discarded by the master. The high 16 bits of an IEEE-754 float representing a normal residential active power value (e.g., 1.234 kW) often decode to a value that the LOGO! FBD interpreter rejects as out-of-range, and the NAI output is clamped to 0.

The correct pattern is to use two NAI blocks at consecutive V-memory addresses (e.g., VW0 and VW2) that read the same starting register address, then combine the two 16-bit halves into a 32-bit value using the LOGO! arithmetic block, and finally convert the IEEE-754 float to a LOGO! integer with the F/I (Float-to-Integer) converter block.

Failure Mode C — smart-me Local Modbus Server Disabled

Smart-me Kamstrup HAN gateway modules ship with the local Modbus TCP server on port 502 disabled by default in firmware revisions up to and including 1.2.x. Until the local connection is enabled through the smart-me cloud onboarding flow or the local web UI, every Modbus client (LOGO!, QModMaster, Home Assistant Modbus integration) receives a TCP RST on the SYN, surfacing as ECONNREFUSED in Linux or a connection failure dialog in QModMaster.

The smart-me setup flow includes a "Local connection test" that exercises the TCP/502 endpoint after the user enables local API access. If the test fails, the smart-me firmware keeps the local Modbus server disabled until the user re-runs the cloud configuration or until the device is power-cycled.

System Topology

Siemens LOGO! Modbus Architecture Reference

The LOGO! 8 base module integrates an Ethernet interface that supports three concurrent roles: S7 communication (PUT/GET), Modbus TCP server, and Modbus TCP client. The Modbus TCP client functionality is exposed inside the FBD program through three block families:

• NAI — Network Analog Input: reads a single 16-bit signed integer holding register (function code 0x03, quantity = 1) from a remote Modbus server.
• NAQ — Network Analog Output: writes a single 16-bit signed integer holding register (function code 0x06 or 0x10) to a remote Modbus server.
• Network Digital Input (NDI) / Output (NDQ): reads/writes a single coil (function code 0x01 / 0x05).

There is no native 32-bit read block. Two consecutive 16-bit NAI blocks must be combined in the FBD program to reconstruct a 32-bit value. The LOGO! Soft Comfort program editor allows the user to assign the V-memory target of each NAI block explicitly; this is the mechanism used to align two NAI blocks to a 32-bit-aligned address (VW0 + VW2 = VD0).

LOGO! Modbus Client Limits

Pre-Commissioning Network Verification

Before opening LOGO! Soft Comfort, verify the TCP path from the LOGO! base module to the smart-me module with a generic Modbus master. This step isolates the network from the LOGO! configuration and prevents misdiagnosis.

1. Connect a Windows PC to the same Ethernet subnet as the smart-me module and the LOGO! BM.
2. Install QModMaster (open-source) or Modbus Poll (commercial).
3. Configure the connection: TCP/IP, IP address = the smart-me module IP (commonly 192.168.0.121 for the HAN gateway default), port = 502, timeout = 1000 ms.
4. Issue a Read Holding Registers request at function code 0x03, starting address = 0, quantity = 2. The response should decode to a valid IEEE-754 float when interpreted as a 32-bit value (little-endian word order).
5. If the read returns an exception code (e.g., 0x02 ILLEGAL_DATA_ADDRESS), confirm the smart-me register map and verify that the local Modbus server is enabled.
6. If the read returns ECONNREFUSED, the smart-me local Modbus server is disabled. Enable it through the smart-me cloud onboarding or local web UI before continuing.

Record the IP address, port, and a representative register value (e.g., active power in kW) returned by QModMaster. These values are required for the LOGO! Soft Comfort NAI configuration in the next section.

Fix #1: IP Address Configuration in the NAI Block

Open the LOGO! Soft Comfort FBD program and double-click the NAI block that targets the smart-me module. The block properties dialog contains four fields in the IP Address group: IP1, IP2, IP3, IP4, plus a Port field. Enter each octet of the smart-me module's IP address in its respective field:

Save the FBD program and transfer to the LOGO! BM. Open the LOGO! Online Test view and watch the NAI block status. If the IP address is correct, the NAI block status indicator turns green within one scan cycle and the V-memory target begins updating.

Fix #2: Register Width and NAI Configuration

A single NAI block reads one 16-bit register. To reconstruct a 32-bit IEEE-754 float, configure two NAI blocks at consecutive V-memory addresses that both target the same starting holding register address on the smart-me module:

1. Add a first NAI block. Set IP Address to the smart-me module IP (see Fix #1), Port = 502, Register Address = the starting register of the 32-bit value (e.g., 0 for active power on the standard Kamstrup register map), V-memory Target = VW0.
2. Add a second NAI block with the same IP, port, but with Register Address = (starting register + 1) and assign V-memory Target = VW2.
3. Both blocks return one 16-bit register each (the first and second halves of the 32-bit value). The pair VW0 + VW2 is byte-aligned to VD0.

The LOGO! arithmetic block then combines VW0 and VW2 into a 32-bit value. Because LOGO! Soft Comfort V8.x does not provide a native 32-bit register combinator block, the combination is done in two steps: scale VW2 by 65536 and add VW0 to produce a 32-bit integer in the range 0–4294967295, then convert to a LOGO! integer range with the F/I (Float-to-Integer) block.

NAI Configuration Table — Active Power Read

Note that for many Kamstrup register maps the low word is held at the lower Modbus register address and the high word is at register address+1 (little-endian word order). The LOGO! F/I converter expects the 32-bit IEEE-754 representation in the LOGO!'s internal format; if the float reads backwards (e.g., 1.234 decodes as -1.923e-18), swap the V-memory targets of NAI #1 and NAI #2 or insert a word-swap using the arithmetic block (multiply VW0 by 65536 and add VW2 instead).

Register Width Diagram

Data Type Conversion with F/I and I/F Blocks

The LOGO! Soft Comfort FBD library provides two converter blocks that bridge the gap between IEEE-754 32-bit floats and LOGO!'s native 16-bit signed integer range (-32768 to +32767):

• F/I (Float-to-Integer): accepts a 32-bit IEEE-754 float on input EN and a scaling gain on input G. Output Q produces round(input_float × gain), clamped to the LOGO! 16-bit signed range. The block truncates silently if the input exceeds the integer range.
• I/F (Integer-to-Float): the inverse: accepts a 16-bit signed integer on EN and a scaling gain on G. Output Q produces (input_integer ÷ gain) as a 32-bit IEEE-754 float.

Scaling Gain Calculation

The F/I converter is the workhorse for displaying meter values on the LOGO! built-in display or for driving an analog output. The gain G is selected to map the expected meter range into the LOGO! 16-bit signed range:

The integer output of the F/I block can then be displayed on the LOGO! built-in text display, written to an NAQ block (16-bit write to a SCADA), or routed through additional scaling for use in a control loop.

F/I Scaling Diagram

Byte Order and Word Swap

The Kamstrup meter transmits 32-bit IEEE-754 values with the low word at the lower Modbus register address (little-endian word order, the Modbus standard). The LOGO! F/I converter assumes the IEEE-754 representation is byte-swapped to LOGO! internal order. If the resulting float reads backwards (e.g., 1.234 decodes as -1.923e-18, or reads 0.0 for a non-zero meter load), the word order in the LOGO! is reversed.

To diagnose, write the combined 32-bit raw value to VD0 and observe it in the Online Test Data Table as a hexadecimal word pair:

• Correct little-endian word order (Kamstrup default): VD0 = VW0 + (VW2 × 65536). The arithmetic block computes VW2 × 65536 + VW0 and writes the result to VD0. The F/I block then reads VD0 and produces the correct float.
• Reversed (big-endian word order): VD0 = VW2 + (VW0 × 65536). Swap the arithmetic operands and re-test.

A quick field check: if the LOGO! reads 0.0 for all loads but QModMaster reads the expected value, the arithmetic operands are likely inverted. Reverse the operand order and confirm.

Kamstrup smart-me Modbus Register Map

The smart-me Kamstrup HAN gateway exposes the underlying Kamstrup meter register map through a thin Modbus TCP wrapper on port 502. The exact register map depends on the smart-me firmware revision and the connected meter type (single-phase, three-phase, or three-phase with neutral). The typical register layout for a single-phase Kamstrup meter exposed through the smart-me gateway is:

The exact register layout for the smart-me gateway is published in the smart-me developer documentation and is subject to firmware revision. The LOGO! program should be parameterized against the live register map returned by QModMaster rather than against a printed table; if a register returns an exception code 0x02 (ILLEGAL_DATA_ADDRESS), the meter does not support that quantity and the NAI block must be removed or re-mapped.

Modbus Exception Codes

smart-me Local Modbus Server Activation

Smart-me Kamstrup HAN gateway modules ship with the local Modbus TCP server disabled by default. The local server is independent of the smart-me cloud API and must be enabled through one of three paths:

1. Cloud onboarding flow: during the initial smart-me setup, the user is prompted to enable the local connection. Selecting "Enable local Modbus" writes a configuration bit to the device and reboots the local Modbus server on port 502.
2. Local web UI: browse to http://<smart-me IP> from a PC on the same subnet, log in with the device credentials (default username admin, password on the device label), navigate to Settings → Local API → Modbus TCP, and toggle Enable. The change applies after a device reboot (~30 s).
3. Cloud API configuration endpoint: the smart-me cloud exposes a REST endpoint that toggles the local Modbus server. This path is reserved for OEM integrators; the endpoint URL and authentication token are documented in the smart-me developer portal.

Online Test Data Table Verification

The LOGO! Soft Comfort Online Test Data Table is the primary verification tool for confirming that the LOGO! Modbus client is correctly reading the smart-me module. The procedure is:

1. Connect LOGO! Soft Comfort to the BM via Ethernet (Tools → PC ↔ LOGO! Connection, or the online button in the toolbar).
2. Open the Tools → Online Test view, or press F8.
3. Switch to the Data Table tab. The table supports byte, word, and double-word address entries: VB0, VW0, VD0, etc.
4. Add the following entries to the watch list:
   • VW0 — low 16 bits of the active power read
   • VW2 — high 16 bits of the active power read
   • VD0 — combined 32-bit view
   • VW4 — output of the F/I converter
5. Observe the values update at the LOGO! scan rate. If VW0 and VW2 both change in step with the live meter load, the NAI blocks are reading correctly. If VW0 and VW2 are both stuck at 0, the NAI blocks are not receiving responses (network or register address issue). If VW0 changes but VW2 is stuck at 0, the high register address is out of range or the byte order is reversed.

Verification State Machine

Troubleshooting Matrix

Verification Procedure

After completing the configuration, follow this end-to-end verification sequence to confirm that the LOGO! is reading the Kamstrup smart-me module correctly:

1. Network path: from a PC on the same subnet, run ping 192.168.0.121 (replace with the smart-me IP). Reply time should be < 5 ms on a wired Ethernet segment.
2. Modbus TCP endpoint: with QModMaster, connect to the smart-me IP on port 502 and read holding register 0, quantity = 2. Confirm a valid IEEE-754 float response. Record the value.
3. LOGO! NAI status: in LOGO! Soft Comfort Online Test Data Table, add VW0, VW2, VW4. Within 5 seconds, all three should show non-zero values that change as the meter load changes.
4. F/I converter output: confirm that VW4 (F/I output) matches the expected scaled value. For active power = 1.234 kW with gain = 1000, expected VW4 = 1234.
5. Display test: configure a LOGO! display message to show VW4. Confirm the displayed value matches the QModMaster value scaled by the F/I gain.
6. Stress test: run the program for 24 hours. Confirm VW4 remains non-zero and updates at the scan rate. Investigate any NAI timeout events by checking the smart-me cloud for connection logs.
7. Failover test: power-cycle the smart-me module. The LOGO! should automatically re-establish the Modbus TCP connection within one scan cycle and resume updating VW0 / VW2 / VW4.

Frequently Asked Questions