Siemens LOGO! 8 TDE Patch Cable Length Limits and Network Topology

The Siemens LOGO! 8 family (often called LOGO! V8 in field shorthand) integrates an Ethernet interface directly on every base module. The optional LOGO! TDE (Text Display External) extends the HMI surface to a remote cabinet door or an office wall without disturbing the base module's location. Two recurring engineering questions arise during panel layout: how long can the patch cable be between the LOGO! base module and the TDE, and how long can it be between two LOGO! base modules that exchange data over the network? This reference consolidates the Siemens specification, the underlying Ethernet physical-layer rules, the catalog numbers involved, and the topology choices (direct, switched, daisy-chained) that solve the longer-run cases.

1. Overview and Application Context

The LOGO! 8 platform is Siemens' entry-level logic module with an integrated Web server, Ethernet interface, and micro-HMI. Three deployment patterns recur on small-machine projects:

1. Single LOGO! with remote TDE — the base module sits inside the control cabinet on the machine; the TDE is mounted on the cabinet door or in an office at the end of an Ethernet patch cord. Typical use: status and parameterization display for an operator who is not at the machine.
2. Two or more linked LOGO! base modules — two machines run cooperatively and exchange discrete I/O state via LOGO! network inputs/outputs. Typical use: a feed conveyor that hands off parts to a downstream station.
3. LOGO! base module with remote TDE plus peer network traffic — a hybrid where the LOGO! is networked with siblings and the TDE is also on the same Ethernet segment.

All three patterns use the same physical layer — 10/100 Mbit/s twisted-pair Ethernet over RJ45 — but Siemens publishes different maximum cable-length figures for the TDE because the device has a tighter power-budget and a single, dedicated host (the master LOGO!).

2. Catalog Numbers in the LOGO! 8 Family

Knowing the exact catalog (MLFB) number matters because firmware revisions and module revisions can shift the cable-length spec by a few meters or change the Auto-MDIX behavior. The following are the most commonly encountered MLFBs at the time of writing:

Every base module in the table ships with one RJ45 jack labeled ETHERNET on the bottom face. The TDE ships with two RJ45 jacks on its rear face, both labeled LOGO! TDE, wired 1:1 in parallel so that you can pass the network through the TDE to another device without a separate switch.

3. TDE Module Specifications

The TDE is a passive-membrane, 6-line by 20-character LCD with six function keys and a cursor pad. The electrical and mechanical data relevant to cabling is:

The TDE's two RJ45 jacks are passive — there is no internal Ethernet switch inside the TDE. They are bussed together. Plugging two patch cords in turns the TDE into a simple 2-port pass-through, which lets you place the TDE inline between a LOGO! and a peer LOGO! on the same 30 m ceiling. Beyond the 30 m point you must place a real switch.

4. LOGO! 8 Base Module Ethernet Interface

Because the LOGO! 8 Ethernet port supports Auto-MDIX, a crossover patch cord is not required for direct LOGO!-to-LOGO! connections. A standard T-568B straight-through patch cord works for every topology in this article.

5. Maximum Cable Length: The Three Reference Figures

Three distinct numbers govern the layout. Treat them as separate limits and apply the tightest one to each link.

5.1 Where the 10 m LOGO!-to-LOGO! figure comes from

The field report mentions a 10 m limit between two LOGO! base modules connected directly. That figure is not found in the current Siemens LOGO! 8 system manual; it appears in older LOGO! documentation or in third-party documentation that predates the Auto-MDIX-equipped LOGO! 8 hardware generation. The current Siemens guidance for LOGO! 8 base module to base module links follows standard 100BASE-TX rules: 100 m maximum per segment. Treat 10 m as an outdated or mis-remembered value and verify against the system manual shipped with your specific MLFB before committing to a layout that depends on it.

6. Cable Types and RJ45 Pinout

Every link in this article is wired as T-568B on both ends (straight-through). Siemens does not require crossover for any LOGO! 8 device because of Auto-MDIX.

Recommended cable grades:

• Cat 5e UTP — minimum acceptable for all LOGO! 8 links; supports 100 Mbit/s to 100 m.
• Cat 6 UTP — recommended for runs approaching 100 m or for noisy cabinets near VFDs.
• Cat 6A or Cat 7 S/FTP — only required if the run is bundled with 400 V cable trays without separation; otherwise overkill.

Industrial patch cords (Profinet-coded, green jacket, M12-to-RJ45 adapters) are supported but not required in cabinet-door or office applications.

7. Network Topologies for the Two Scenarios

7.1 Scenario A — Single LOGO! with Remote TDE

+----------+        Cat 5e ≤ 30 m        +-----------+
|  LOGO! 8 |------[ RJ45 patch cord ]------|  TDE 6ED1055-4MH08 |
| (master) |                              |   (cabinet door)    |
+----------+                              +-----------+

Use this when the TDE is within 30 m of the base module. The TDE's second RJ45 jack is left empty or capped.

7.2 Scenario A — Extended, TDE in a Separate Office

+----------+   Cat 5e ≤ 30 m   +-------+   Cat 5e ≤ 100 m   +-----------+
|  LOGO! 8 |---------------------| TDE   |---------------------| Office PC |
| (master) |   <-- Siemens 30 m   |       |   <-- Standard Eth  |   or      |
+----------+       ceiling        +-------+       100 m ceiling | another   |
                                                                  | LOGO!     |
                                                                  +-----------+

The 30 m Siemens ceiling applies to the LOGO!→TDE segment only. The TDE→switch segment can use the full 100 m because both endpoints support standard 100BASE-TX. This is the recommended topology when the office is 30 – 130 m from the cabinet.

7.3 Scenario B — Two LOGO! 8 Base Modules Linked by Network Functions

+----------+                       +----------+
|  LOGO! A |-----[ Cat 5e ]--------|  LOGO! B |
| (master) |   ≤ 100 m direct    | (slave)  |
+----------+                       +----------+

Standard 100BASE-TX: 100 m maximum. Auto-MDIX means a straight-through patch cord works either way. Configure LOGO! A as the master (192.168.0.10) and LOGO! B with a unique IP (for example 192.168.0.11), then bind network inputs/outputs in LOGO! Soft Comfort under Tools → Ethernet Connections.

7.4 Scenario B — Linked LOGO!s with a Central Office TDE

+----------+    100 m    +-------+    100 m    +-----------+
|  LOGO! A |-------------| SWITCH|-------------| Office PC |
+----------+             +-------+             +-----------+
                              |
                              | 30 m (Siemens)
                              |
                         +----------+
                         |  TDE     |
                         +----------+

Inserting a switch at the midpoint lets each segment respect its own ceiling. The TDE sits within 30 m of the switch because the TDE-to-network limit is the limiting factor for that leg. Total end-to-end reach is approximately 130 m (100 + 30) without further hops.

7.5 Scenario B — Long-Run (200 m or more) with Two Switches

+----------+ 100 m +-------+ 100 m +----------+ 30 m +-----------+
|  LOGO! A |-------| SW 1  |-------|  LOGO! B |------| Office PC  |
+----------+       +-------+       +----------+      +-----------+
                                                   (or TDE)

Each 100BASE-TX hop is 100 m maximum. With two unmanaged switches you can reach 200 m between LOGO!s and a TDE at the far end, provided the TDE is within 30 m of its master LOGO!. For runs over 200 m, convert one segment to fiber using an SC/ST media converter pair (Siemens MLFB 6GK1901-1BE00-0AA0 family or third-party equivalents); each fiber segment can extend to 2 km (multi-mode) or 40 km (single-mode) depending on optics.

8. Why 30 m and Not 100 m for the TDE?

Cat 5e UTP is specified for 100 m at 100 Mbit/s by IEEE 802.3u. The 30 m ceiling for the LOGO!-to-TDE link is a Siemens-imposed warranty and EMC limit, not a physical-layer limit. The reasons are:

1. EMC susceptibility of the TDE's front-panel membrane — the TDE is typically mounted outside the cabinet, where radiated noise from VFDs and switching power supplies can couple onto longer cable runs.
2. Single-master assumption — the TDE is a closed-loop device intended for one host LOGO!. Siemens does not test or warrant the TDE in multipoint or switched topologies even though the hardware is electrically capable.
3. Power-supply common-mode — the TDE draws power either from the master LOGO! or from a separate 24 V supply. Long runs can produce ground-potential difference that confuses the TDE's internal logic supply.

Operationally, the TDE almost always works at longer distances (60 – 90 m is reported as reliable in many field installations), but it is outside the Siemens warranty. If the application is commercial, follow the 30 m rule. If the application is internal R&D or non-critical monitoring, longer runs may be accepted at the integrator's risk after bench-testing.

9. Switch Selection for Extended Runs

An industrial unmanaged switch is sufficient unless the LOGO! is on a larger plant network. Recommended options:

For a 100 m hop between two LOGO!s, an unmanaged 5-port switch is more than adequate. The TDE does not consume a switch port (it is a slave), so a small 4- or 5-port switch keeps panel cost low.

10. Configuration Procedure (LOGO! Soft Comfort)

For a two-LOGO! link with an office-mounted TDE, the configuration steps are:

1. Open LOGO! Soft Comfort V8.x or later and connect to the master LOGO! via Ethernet.
2. Navigate to Tools → Ethernet and assign the master a static IP, e.g. 192.168.0.10 / 255.255.255.0.
3. Repeat on the slave LOGO! with 192.168.0.11 / 255.255.255.0.
4. Open Tools → Ethernet Connections and bind a network input on the slave to a network output on the master (and vice versa). Maximum 16 inputs and 16 outputs per peer.
5. Transfer the project to both modules.
6. Power the TDE. From the master LOGO!'s on-board display, navigate Card → Ethernet → TDE (firmware-dependent path) and confirm the TDE IP is on the same subnet.
7. From the TDE, hold ESC and OK for 3 seconds to enter the TDE's IP menu; assign 192.168.0.20.
8. Cycle power on the TDE and verify the six-line display shows the configured message text.

11. Verification and Commissioning Tests

12. Troubleshooting Matrix

13. Safety and EMC Considerations

• Run the Ethernet cable at least 200 mm away from any VFD output cables. Cross at 90° if a crossing is unavoidable.
• Bond the cabinet ground bus to the building ground at one point only (star topology) to avoid ground loops through the shield of long Cat 5e runs.
• Use shielded (SF/UTP) cable and bonded RJ45 plugs if the run passes near motors or switching power supplies; terminate the shield at the cabinet entry gland only.
• The TDE's membrane is IP65 only when the supplied gasket is correctly seated. A poorly mounted TDE allows moisture to creep into the RJ45 jack and corrupt the link.

14. Frequently Asked Questions