LOGO! PLC Fiber Optic Communication: 3 km Ethernet Setup Guide

Overview

Siemens LOGO! logic modules do not ship with a native fiber-optic transceiver. They expose only a standard RJ45 Ethernet interface (LOGO! 8/8.3 BA: 10/100 Mbit/s Ethernet, RJ45, IEEE 802.3). To close a point-to-point link over a buried fiber run of 3–3.3 km, the copper RJ45 port of each LOGO! must be terminated into an industrial media converter that converts the 100BASE-TX electrical signal into an optical signal on 1300 nm fiber. This article documents the engineering selection, physical topology, IP addressing, and commissioning procedure required to make two LOGO! controllers exchange data reliably across several kilometers of single-mode or multi-mode fiber.

Prerequisites

• Two Siemens LOGO! 8 or 8.3 logic modules (6ED1052-xxx08-0BAx family or newer). Both CPUs must carry the same firmware train so the LOGO! communication feature set matches.
• LOGO! Soft Comfort V8.3 or later installed on the engineering PC for IP configuration and Ethernet connection setup.
• Two 24 VDC power supplies rated for industrial environment (LOGO! 8 PSU and the media converter PSU share the same SELV rail).
• One fiber-optic media converter per end of the link. Siemens SCALANCE or equivalent third-party unit with 100 Mbit/s electrical and 100BASE-FX optical port.
• Fiber-optic cable: single-mode 9/125 µm for distances greater than 5 km, or OM2/OM3 multi-mode 50/125 µm for runs up to 5 km. Outdoor-rated, gel-filled, armored (steel tape or corrugated steel) for direct burial.
• Fiber patch cords matched to the converter connector type (SC duplex for SCALANCE X101 series).
• Fusion splicer or pre-terminated pigtails plus a fiber cleaver for termination.
• OTDR or optical power meter (1310/1550 nm) for commissioning.

Technical Constraints and Signal Path

LOGO! 8 Ethernet is implemented with a Realtek RTL8211E PHY driving an internal RJ45 magnetics module. The MAC supports only copper 100BASE-TX/10BASE-T, full duplex. There is no SFP cage and no IEC 61754 fiber receptacle. Any fiber transport therefore requires an outboard media converter that maps the MDI/MDIX copper pair to an optical transmit/receive pair.

100BASE-FX links carry the physical layer requirements from IEEE 802.3u. The receiver sensitivity of a typical SCALANCE X101 optical port is around –31 dBm at 1310 nm; the transmit power is roughly –19 to –14 dBm. A 3 km single-mode link leaves 25+ dB of optical margin, which is more than sufficient for buried cable with two fusion splices and four SC connector pairs.

Component Selection: Media Converters

Siemens offers two industrial-grade converters from the SCALANCE X-100 family that are purpose-built for cabinet mounting on a 35 mm DIN rail, accepting 24 VDC and featuring diagnostic LEDs and fault relay contacts:

Both units present one RJ45 (10/100 Mbit/s) and one SC duplex fiber port. The –1LD variant is the correct choice for a 3.3 km buried link because single-mode cable is the universally deployed outside-plant medium and its dispersion characteristics at 1310 nm are essentially flat over the run.

Third-party alternatives such as the TP-Link MC210CS (Gigabit single-mode) are electrically compatible but do not carry the SCALANCE diagnostics. Industrial sites with harsh EMC or extended temperature requirements should default to the SCALANCE family.

Topology

The topology is a strict point-to-point link. There is no Ethernet switch between LOGO! #1 and LOGO! #2, only two media converters in the middle. No additional router or NAT is required; the converters are layer-1 transparent devices.

IP Address Plan

LOGO! communication uses standard TCP/IP over IPv4. The two stations must be on the same subnet and the subnet mask must be identical:

LOGO! Soft Comfort Configuration

Both LOGO! devices must be configured for Ethernet communication. Open the project in LOGO! Soft Comfort V8.3 or newer. On each LOGO! CPU configuration page:

1. Select Tools → Ethernet Connections (or right-click the LOGO! in the network view and choose Properties).
2. Enter the static IP, subnet mask, and disable DHCP client.
3. Enable Server mode on the master and Client mode on the slave.
4. In the project tree, drag a Network Input (NI) or Network Output (NQ) onto the workspace. Link the local VM bit to NI/NQ tags that point to the partner LOGO!.
5. Compile and download the project to each LOGO! via USB or the local Ethernet port.

The closed LOGO!-to-LOGO! data exchange uses the proprietary Siemens S7-compatible protocol on TCP port 102 (ISO-on-TCP). One LOGO! acts as the active partner (server), the other as passive (client). Up to 32 NI/NQ tags are supported per LOGO! 8 base module in a 1:1 point-to-point setup. With LOGO! 8.3 and a CMR/DM8 expansion, the maximum mapping rises to 64 NI plus 64 NQ.

Physical Installation

1. Mount both SCALANCE X101-1LD on a DIN rail in the same enclosure as the LOGO! 8 base module. Maintain ≥ 50 mm clearance above and below for fiber bend radius.
2. Wire the 24 VDC supply to the SCALANCE's 4-pin plug-in terminal. The same PSU that feeds the LOGO! may be used; total inrush on cold start of the SCALANCE is below 1 A.
3. Use a short (< 3 m) CAT5e or better patch cord between the LOGO! RJ45 jack and the SCALANCE RJ45 jack.
4. Route the single-mode cable through the enclosure wall via a proper cable gland. Do not exceed the SCALANCE SC port minimum bend radius of 30 mm.
5. Splice the outdoor cable onto SC/APC pigtails inside a fiber-termination box mounted to the enclosure. Use a fusion splicer; mechanical splices are acceptable only for repair, not commissioning.
6. Bond the metallic strength member of the armored cable to the cabinet PE bar at one end only to avoid ground loops.

Commissioning Procedure

1. With the fiber disconnected, verify each LOGO!-to-SCALANCE copper link: the SCALANCE LEDs for Port 1 Link and Activity should light green within 5 s of LOGO! boot.
2. Connect an OTDR to one end of the fiber run and shoot a trace at 1310 nm. Expect a single end-to-end event with insertion loss < 2 dB plus splice events ≤ 0.1 dB each.
3. Connect both fiber patch cords and power both SCALANCE units. The fiber-port Link LED on each converter must turn green; if it stays off, swap TX and RX on one patch cord (single-mode SC connectors are keyed but misalignment still produces zero link).
4. From the engineering PC, ping 192.168.0.10 and 192.168.0.11. Round-trip time on 3.3 km single-mode fiber should be ≈ 33 µs; expect ≈ 35 ms over ICMP due to LOGO! firmware handling.
5. Open LOGO! Soft Comfort, select Online → Ethernet Connections, and confirm both LOGO! modules appear in the access list.
6. Toggle a local output on LOGO! #1 wired to a Network Output tag. Verify the corresponding Network Input tag on LOGO! #2 changes state within two LOGO! scan cycles (≤ 200 ms typical).

Verification Checklist

Troubleshooting Matrix

Performance Limits

The LOGO! 8 firmware polls its partner connection at roughly 100 ms intervals when NI/NQ are configured, so the closed link is not suited to high-speed interlocking. Round-trip latency on a healthy 3 km fiber path measures 35–55 ms, dominated by LOGO! scan and stack handling rather than fiber propagation (≈ 16 µs per km). For deterministic sub-10 ms response, move to a SIMATIC S7-1200 with PN/PN coupler and an IRT-capable switch.

Alternative Architectures

For redundant paths or higher data volume, replace the direct SCALANCE pair with two SCALANCE XC-100 switches (managed) at each end and a redundant fiber ring. LOGO! 8.3 supports the SIMATIC LOGO! CMR2020 cellular router, allowing remote engineering access over LTE without disturbing the deterministic closed link.

FAQ