Configuring Modbus TCP/IP on Siemens LOGO! 8 (0BA8): Setup Guide

Overview

The Siemens LOGO! 8 logic module (order suffix 0BA8) introduced native Modbus TCP/IP support starting with hardware revision FS04 and firmware 8.1.x. Earlier revisions (FS01-FS03) cannot speak Modbus on either the Ethernet or the RS-485 port, and no LOGO! 0BA8 variant can speak Modbus RTU natively on the RS-485 interface. To bridge RTU devices you must use an external Modbus RTU-to-Modbus TCP/IP gateway that exposes the RTU slaves as Modbus TCP servers on the LOGO!'s Ethernet network.

This reference covers three deployment topologies:

1. LOGO! 8 as a Modbus TCP server (slave) exposing its Variable Memory (VM) area to a supervisory controller, HMI, or SCADA.
2. LOGO! 8 as a Modbus TCP client (master) polling remote Modbus TCP servers such as frequency drives, energy meters, or I/O couplers.
3. LOGO! 8 communicating with RTU field devices through a third-party Modbus RTU-to-TCP converter.

The procedures reference LOGO! 8 System Manual (Siemens Support entry 109751606) and the Siemens Industry Online Support portal for firmware/binary downloads.

LOGO! 8 Hardware and Firmware Compatibility Matrix

The capability to communicate with Modbus TCP/IP is gated by both device generation and firmware status (FS). Verify the exact FS designation printed on the side of the module or read it from the diagnostics web page before commissioning.

Prerequisites

Before starting, confirm the following hardware and software stack:

• LOGO! 8 base module with firmware status FS04 (8.1) or FS01 (8.2) or later. Read the FS code from LOGO! > Diagnostics > Product Information on the device or from the device's built-in web server (default IP 192.168.0.1, user LOGO, password LOGO).
• LOGO!Soft Comfort (LSC) V8.1.0 minimum for 8.1 hardware, V8.2.0 minimum for 8.2 hardware. Siemens publishes free updates on the Siemens Industry Online Support portal under entry ID 109751606 (LOGO! 8 system manual) and the LOGO! Soft Comfort product page. Always install the matching service pack before commissioning.
• Ethernet switch (100 Mbit/s minimum) and Cat5e/Cat6 cabling terminated to TIA-568B.
• Static IP plan — LOGO! 8 does not support DHCP on its Ethernet port by default; reserve a unique IPv4 address plus the supervisory controller IP.
• For RTU integration: a Modbus RTU-to-TCP gateway such as Wago 750-362, Moxa MGate MB3170, or Anybus X-gateway with a documented register-mapping configuration tool.
• PC running LSC and an Modbus TCP client test utility (Siemens recommends Modbus Poll, QModMaster, or diagslave) for verification.

Network Architecture

LOGO! 8 occupies one Ethernet port and accepts up to 8 concurrent Modbus TCP/IP connections combined with up to 8 S7 connections. A typical topology is shown below.

+----------------------+         +-------------------------+
|   SCADA / HMI / PLC  |  TCP    |  Ethernet Switch        |
|   Modbus TCP Client  +---------+  (192.168.0.0/24)      |
+----------------------+         +----+----------------+--+
                                      |                |
                                      |                |
                              +-------+----+   +-------+----+
                              | LOGO! 8   |   | LOGO! 8    |
                              | .10 (srv) |   | .11 (cli)  |
                              +-----+-----+   +-----+------+
                                    |                |
                                    |                | (gateway traffic)
                              +-----+------+  +------+-------------+
                              | Expansion  |  | RTU/TCP gateway    |
                              | DM8 / AM2  |  | e.g. Wago 750-362 |
                              +------------+  +-------+-----------+
                                                   | RS-485 (A/B/GND)
                                              +----+--------+
                                              | VFD / Meter|
                                              +-------------+

Recommended port assignments:

Configuring LOGO!Soft Comfort for Modbus TCP/IP

1. Set the LOGO! IP Address

1. Open LOGO!Soft Comfort, choose Tools → Ethernet Connections.
2. Click Add Server Connection. The dialog exposes the LOGO!'s IP and a read-only view of currently configured connections.
3. For online configuration, connect the PC to the LOGO! via the same switch. From Tools → Transfer → Ethernet → Set Ethernet Address enter a free static IP, e.g. 192.168.0.10, subnet 255.255.255.0, gateway 192.168.0.254.
4. Click Apply to LOGO!. The device reboots (about 10 s) and responds on the new address.

2. Enable the Modbus Server (LOGO! as Slave)

Starting with firmware 8.1 / LSC 8.1, Modbus TCP server functionality is enabled by default and requires no additional activation bit. The server exposes the LOGO!'s Variable Memory (VM) area as holding registers, input registers, coils, and discrete inputs. The address mapping is fixed by Siemens and cannot be remapped:

The VM area is the same memory that LSC programs address symbolically with the Network Input/Output, Analog Flag, Shift Register, and Open Connector blocks. Each boolean VM bit maps 1:1 to a Modbus coil; each 16-bit VM word maps 1:1 to a Modbus register.

3. Add a Modbus Client Connection (LOGO! as Master)

LOGO! 8 can act as a Modbus TCP client to poll up to 8 remote servers. The configuration is done in the LSC diagram editor using the Network Analog Input (NAI) and Network Digital Input (NDI) function blocks, or via the Modbus Master table introduced in LSC 8.2.

1. Place an NAI block on the FBD diagram (drag from the toolbar → Special → Network Analog Input).
2. Double-click to open the configuration dialog.
3. Set IP address to the remote Modbus server, e.g. 192.168.0.50.
4. Set Port to 502.
5. Choose Read Holding Register (FC03) or Read Input Register (FC04).
6. Set Starting Register, Number of Registers, and the Sample Time (minimum 100 ms).
7. Map the read value to an Analog Flag or VM word by assigning the block's output Q connector.

For boolean reads, use the NDI block with function code 01 (Read Coils) or 02 (Read Discrete Inputs). Writes back to the remote server are supported by the NAQ (Network Analog Output) and NDQ (Network Digital Output) blocks using FC05, FC06, FC15, or FC16.

4. Register Mapping Table

Below is a worked example mapping three drive parameters (frequency, current, status word) from a hypothetical VFD with Modbus TCP support into LOGO! memory.

Sample LSC analog scaling in the diagram:

The Analog Gain block converts the Modbus INT16 scaled value (e.g. 500 for 50.0 Hz) into the engineering unit expected by the LOGO! program.

Using Modbus RTU-to-TCP Gateways

Because no LOGO! 0BA8 module can speak Modbus RTU natively, integrating RS-485 field devices requires an external gateway that converts TCP requests on its Ethernet side to RTU frames on its RS-485 side. The LOGO! simply sees another Modbus TCP server on the LAN.

Topology

LOGO! 8 (Modbus TCP client, 192.168.0.10)
        |
        | TCP/IP : 502
        v
+--------------------------+
|  Modbus RTU/TCP Gateway  |  IP: 192.168.0.50
|  e.g. Wago 750-362       |
+--------------------------+
        |
        | RS-485 (A+/B-/GND)
        v
+-------+-------+-------+
| VFD 1| VFD 2 | Meter |
| ID 1 | ID 2  | ID 3  |
+-------+-------+-------+

Gateway Configuration Checklist

1. Set the gateway's IP address, subnet mask, and default gateway so it is reachable from the LOGO! subnet.
2. Configure the RS-485 side: baud rate (e.g. 19200), parity (typically Even for Modbus), data bits (8), stop bits (1).
3. Map each downstream RTU slave (by Modbus unit ID 1-247) to a virtual TCP server or to a register offset accessible by the LOGO! client. Some gateways auto-forward with the same slave ID via the TCP request's Unit Identifier field (Modbus exception code 0x0A is used to mark gateway routing in MBAP); others require a static mapping table.
4. Set request timeout to ≥ 1000 ms for RTU; LOGO! NAI/NDI sample time must be greater than the gateway turnaround.

Verification and Diagnostics

1. Web server diagnostics: Open http://192.168.0.10 in a browser. The home page shows the active IP, firmware FS code, and the number of currently open Modbus TCP connections.
2. LOGO!Soft Comfort online test: From the menu, choose Tools → Ethernet Connections. The dialog lists active S7 and Modbus sessions with the partner IP and connection state. A green check indicates an established session.
3. External Modbus master: Use QModMaster or Modbus Poll to read VM0 (register 40001). The raw value should match the LOGO! program state. For example, if the LOGO! digital input I1 is HIGH and the program writes I1 -> VM0.0, then coil 0x0001 should read 1.
4. Wireshark capture: Capture on port 502 with filter modbus. Verify the transaction ID increments, the protocol ID is 0, the length field matches the payload, and the unit ID is correctly forwarded.
5. Read VM memory block statistics: In LSC online mode, place a Cursor on the VM connector. The status line shows the current value and the number of successful Modbus reads since last power-up.

Troubleshooting Matrix

Field-Proven Tips and Edge Cases

• Power budget: Modbus polling is single-threaded on the LOGO!. A long NAI poll (>100 registers) blocks other clients for the duration of the request. Keep poll lists short and use multiple smaller NAI blocks rather than one large one.
• Endianness: LOGO! stores VM words in big-endian on the wire (Modbus standard), but LSC's analog flag blocks interpret them as signed INT16 in the program's native byte order. No swap is required for single-register values. For 32-bit floats you must use two consecutive registers and apply a Math block to combine them.
• Write protection: LSC V8.2+ allows password-protected access to Modbus writes. If the client receives exception 04 (Slave Device Failure) on writes, the LOGO! program may have its password set with "Allow Modbus Write = No" in the project properties.
• Watchdog on client blocks: If the remote Modbus server disappears, the NAI block's output stays at the last good value. To detect loss, route a Network Status digital into the program and assert an alarm bit when the status flips to ERROR.
• Cybersecurity: LOGO! 8 has no built-in Modbus user authentication. Place the controller behind a firewall or VPN if exposed to an untrusted network. Disable the web server when not in use to reduce the attack surface.
• Firmware upgrade path: When upgrading from 8.1 to 8.2 in the field, plan a download of approximately 8 MB and a reboot of about 30 s. The project is preserved, but Modbus connection IDs may need to be re-saved if the project is round-tripped through LSC V8.2 from V8.1.

Sample Commissioning Workflow

1. Update LOGO! firmware to the latest FS via LSC Tools → Firmware Update. Confirm FS code via web page.
2. Update LOGO!Soft Comfort to the latest service pack and rebuild the project against the new libraries.
3. Set static IP on LOGO! and verify ping from engineering PC.
4. Insert NAI blocks for each remote parameter. Configure IP, port 502, FC03/04, sample time 250 ms.
5. Insert NAQ/NDQ blocks for writes. Mark all VM source addresses as non-overlapping with NAI destinations.
6. Compile and download the project via Ethernet.
7. Run QModMaster against the LOGO! IP. Read coils 0-15 and holding registers 0-15. Cross-check against the LSC online Cursor display.
8. If RTU slaves are involved, repeat the above against the gateway IP and verify Unit ID transparency.
9. Document the final register map (IP, port, function code, address, engineering unit) and back up the LSC project to the plant network share.

Standards and Reference Documentation

• Modbus Organization: Modbus Application Protocol V1.1b3 and Modbus TCP/IP V1.0b — refer to the Modbus Organization for the wire-format specification.
• Siemens LOGO! 8 System Manual — chapter on Ethernet communication and Modbus device integration, available as Siemens Support entry 109751606.
• LOGO! Product Page: Siemens LOGO! Logic Module overview with order numbers and firmware history.
• LOGO!Soft Comfort Download: Available from the Siemens Industry Online Support portal under the LOGO! product tree. A free of charge license unlocks the full feature set including Modbus client configuration.