Connecting Two LOGO! TDE Panels to a LOGO! 8 PLC Limits

Overview

The LOGO! 8 logic module is widely used for small machine control, and the LOGO! TDE (Text Display Ethernet) is the matching operator interface. A recurring design question on machines with two operator stations (front and rear, or two sides of a long line) is whether a single LOGO! 8 can host two TDE panels. The short, documented answer is no: Siemens officially supports exactly one LOGO! TDE per LOGO! 8 base module. This reference explains the underlying hardware constraint, examines the unofficial dual-TDE workarounds that have been attempted in the field, and lays out the supported architectures when you genuinely need two operator stations: a second LOGO! 8, a migration to an S7-1200 with two SIMATIC Basic Panels, or a web-based operator interface served by the LOGO! 8 web server.

The constraint is rooted in the physical interface of the LOGO! 8 and the TDE discovery/addressing model, not in the LOGO! program itself. The same LOGO! program that drives a single TDE will be displayed correctly on any TDE that completes a valid Ethernet session, which is why field attempts at dual-TDE operation often appear to work during bench testing. The failure modes appear in production where the TDE state machine, alarm handling, and message buffering were not designed for two concurrent consumers.

LOGO! 8 System Architecture

LOGO! 8 is the 8th generation of the Siemens LOGO! logic module family. The base module integrates a CPU, digital inputs and outputs, and in most variants a built-in Ethernet port. A LOGO! 8 system can be expanded with digital and analog I/O modules, a CMK2000 communication module, a CMR2020 / CMR2040 wireless router, and up to one LOGO! TDE text display.

The configuration maximums are documented in the LOGO! 8 product overview and the LOGO! 8 system manual: a single base module can address up to 24 digital inputs, 20 digital outputs, 8 analog inputs, and 8 analog outputs when fully expanded. The I/O budget is independent of the HMI question, but it matters because it is the dominant reason people consider a second operator interface: the application grew and the second station is needed for visibility, not for additional I/O.

For the full product overview including catalog numbers and expansion rules, see the LOGO! 8 Product Overview (PDF) hosted on DigiKey and the Siemens Industry Online Support portal.

LOGO! TDE Hardware and Protocol

The LOGO! TDE is a 6-line text display available in two variants:

• 6ED1055-4MH00-0BA0 — TDE with Ethernet, monochrome, backlight selectable between white, amber, and red.
• 6ED1055-4MH08-0BA0 — TDE with Ethernet, color display.

Each TDE exposes two RJ45 Ethernet ports on the bottom of the housing. The dual ports are intended for daisy-chaining the TDE between the LOGO! 8 base module and an upstream switch or panel, not for connecting two TDEs in parallel. The TDE draws its power over the Ethernet connection (Power over Ethernet is not used; the TDE is powered by its own 12/24 V DC terminals), and the two Ethernet ports are an unmanaged internal switch.

Communication between the TDE and the LOGO! 8 base module is proprietary Siemens protocol carried over TCP/IP. Configuration of the TDE screen contents, message texts, and variable bindings is performed in LOGO! Soft Comfort (current version: LOGO! Soft Comfort V8.4 for LOGO! 8.3 / 8.4 base modules). The TDE does not use Modbus TCP for its display data; it uses a Siemens-internal protocol that maintains a session, synchronizes message state, and pushes alarm texts from the LOGO! 8 to the TDE.

The TDE supports a fixed maximum of 50 message texts (screen pages) and 50 alarm texts, with up to 6 lines per page and 16 characters per line in some character sets. The TDE does not support multi-user concurrent sessions. The protocol is designed for one TDE, one LOGO! 8.

Official Single-TDE Limitation

The LOGO! 8 system manual states that exactly one LOGO! TDE may be connected to a LOGO! 8 base module. This is an architectural limit, not a marketing preference. The reasons documented and implicit in the protocol design are:

1. Single TCP session model. The TDE protocol is a one-to-one request/response session. A second TDE would have to share the same state model (current message index, alarm queue, cursor position) and there is no defined arbitration.
2. Message text ownership. When a button on the TDE is pressed, it triggers actions on the LOGO! 8 (for example, acknowledging an alarm or switching to the next message). Two TDEs can issue conflicting commands; the LOGO! program cannot distinguish them.
3. Alarm acknowledgement. Alarm texts have an acknowledgement state. If operator A on TDE-1 acknowledges an alarm while operator B on TDE-2 is still reading it, the screen states diverge and the second operator sees a state change they did not trigger.
4. Configuration scope. LOGO! Soft Comfort's "Tools → TDE Settings" dialog binds one TDE target. There is no second binding slot.

Siemens' official guidance when a second operator station is required is to add a second LOGO! 8 and have the two base modules exchange data over Ethernet, or to migrate to a S7-1200 with two SIMATIC Basic Panels.

Physical Connection Constraints

The LOGO! 8 base module exposes one RJ45 Ethernet port. There is no second Ethernet port on the base. To attach two TDEs physically you need an external Ethernet switch. The TDE itself has two ports, but they form an internal switch intended for a single daisy-chain, not for two parallel upstream devices.

Topology options that have been attempted in the field:

Regardless of the switch topology, the underlying issue is the single TCP session model on the LOGO! 8 side, not the cabling.

Unofficial Dual-TDE Workarounds and Their Risks

It is technically possible to bring up two TDEs against one LOGO! 8 on a bench. Several users have reported intermittent success. The recurring failure modes in production are:

• Session preemption. After a power cycle or network glitch, only one TDE reconnects; the second sits in "Connect" until the first is removed.
• Acknowledgement desync. Alarm acknowledgement on one TDE does not propagate to the other; the second TDE shows the alarm as still active.
• Message scroll drift. The current message index is shared; pressing arrow keys on one TDE changes the page on the other.
• Variable update starvation. When both TDEs are refreshing simultaneously, the LOGO! 8's update rate halves and the display lag becomes visible, particularly with rapidly changing analog values.
• Non-deterministic restart. The first TDE to complete the TCP handshake after a restart "wins"; the assignment of which TDE is master is not predictable across firmware versions.

Recommended Architecture A: Two LOGO! 8 Modules with Ethernet Coupling

If the application must stay on LOGO! 8, the supported way to get two operator stations is to use two LOGO! 8 base modules, one TDE on each, and a small Ethernet coupling. The two base modules exchange the process image using the Network Inputs / Network Outputs mechanism supported by LOGO! Soft Comfort from V8.0 onward.

Steps:

1. Assign static IP addresses to both LOGO! 8 base modules on the same /24 subnet (for example, 192.168.1.10 and 192.168.1.11).
2. Connect a TDE to each base module using the TDE's own Ethernet port (single daisy-chain per base, which is the supported configuration).
3. Open the LOGO! program in LOGO! Soft Comfort. On the right-hand network editor, drag a Network Input (NI) block on the first base module and a Network Output (NO) block on the second, or vice versa. Up to 16 network inputs and 16 network outputs are supported per base module.
4. Bind the bits / bytes you need to share: machine state, start / stop commands, alarm summary, setpoints.
5. Build, download, and verify with the TDE on each station.

The two TDEs now display their own base module's local state plus the cross-coupled values from the partner base. Each TDE has its own single-session relationship with its own base, which is fully supported.

This pattern is useful when the two operator stations correspond to two physically distinct subsystems (for example, infeed and outfeed of a conveyor) but you still want a single coordinated control philosophy. Each base module is responsible for its own I/O and its own TDE; the network blocks act as a controlled interlock.

Recommended Architecture B: Migrate to S7-1200 with Two SIMATIC Basic Panels

If the application has outgrown a single LOGO! 8, the most natural upgrade is an S7-1200 CPU (for example, CPU 1214C DC/DC/DC, 6ES7214-1AG40-0XB0) and two SIMATIC Basic Panels (KTP400 Basic, KTP700 Basic, KTP700 Basic DP, KTP900 Basic, KTP1200 Basic). The S7-1200 supports up to 16 S7 connections; an HMI connection counts as one. Two Basic Panels therefore fit with headroom for a third connection to a TIA Portal engineering station or a SCADA gateway.

Advantages over the dual-TDE workaround:

• Officially supported by Siemens: the S7-1200 system manual explicitly lists S7-1200 ↔ Basic Panel as a supported HMI connection.
• Independent screen content per panel: each Basic Panel can have a fully custom WinCC Comfort / Basic project, not just the LOGO! TDE message model.
• Higher information density: 800 x 480 (KTP700) and 1280 x 800 (KTP1200) versus 6 lines of 16 characters on the TDE.
• Multi-user model: both panels can read and acknowledge alarms independently; alarm acknowledgement is a per-client state in WinCC.
• Smaller commissioning cost: TIA Portal handles both panels and the CPU in one project.

Migration effort scales with the size of the LOGO! program. Most LOGO! ladder blocks map directly to ladder (KOP) or FBD in TIA Portal. The Network Inputs / Network Outputs from the two-LOGO! architecture collapse into a single CPU with two HMI connections.

Recommended Architecture C: Use the LOGO! 8 Built-In Web Server

From firmware LOGO! 8.1 (6ED1052-xxx08-0BA1) and later, the base module exposes a configurable web server. From the LOGO! Soft Comfort project you can publish a small operator page with up to 8 user-defined web variables, current state, and a login-protected write page. The web page is accessible from any browser on the same network, including on a phone, tablet, or a low-cost panel PC mounted at the second station.

This is a supported configuration and is the lowest-cost way to add a second viewing station. It is not a replacement for a TDE if you need tactile operator buttons (start / stop / acknowledge), but it is sufficient for read-only monitoring, trend display, and password-protected setpoint entry.

Configuration is performed in LOGO! Soft Comfort under Tools → Webserver or via the base module's web administration page (default URL http://<LOGO_IP> after enabling the web server in the project).

Comparison of Options for a Second Operator Station

Commissioning and Verification

Whichever supported architecture you choose, run the following verification before handing the machine to production:

1. Session check. Power-cycle the LOGO! 8 and confirm exactly one TDE session (or one Basic Panel session, or one web session) is established, depending on the architecture. On the LOGO! 8 web administration page under Diagnostics, verify the number of open TCP connections.
2. Button independence. Press each F-key on each TDE / panel and confirm the action in the LOGO! or S7-1200 program. If using two LOGO! 8s, confirm that an F-key on TDE-1 drives the intended NI / NO block on its partner and not on itself.
3. Alarm acknowledgement. Trigger an alarm on TDE-1 (or Panel-1), acknowledge it there, and confirm the alarm clears on the partner station without further action. If using S7-1200, confirm the alarm state in the WinCC alarm log shows the correct user / station.
4. Network loss recovery. Disconnect the Ethernet cable to one station for 30 s and reconnect. Confirm the session re-establishes within the documented time and the display state is consistent with the LOGO! program.
5. Firmware consistency. If the architecture is two LOGO! 8 base modules, both must run the same firmware family (for example, both on 6ED1052-xxx08-0BA3 or both on -0BA4). Mixing 8.0 and 8.4 base modules in a network pair is not recommended because the network input / output feature set has evolved between generations.

Troubleshooting Matrix

Field-Proven Notes

• Do not assume the two TDEs share the same message index. They will appear to share it, then diverge on the first operator action. Treat any visible "two TDEs working" as a transient bench state.
• If you must keep LOGO! 8 hardware and need two TDEs, the second base module does not need a TDE assigned to it. The unused TDE port on the partner base is simply left empty. There is no penalty for running a LOGO! 8 with zero TDEs; the TDE slot is optional.
• For a clean migration to S7-1200, export the LOGO! program to LSC file, then use the LOGO! Soft Comfort "Convert" function to produce a TIA Portal-compatible ladder source. Manual review is still required because LOGO! blocks (for example, the analog threshold trigger, B001) do not have a 1:1 equivalent in TIA Portal.
• Reserve one of the S7-1200's 16 connection slots for engineering, two for the Basic Panels, and leave the rest for SCADA or a future third panel.
• The LOGO! 8 web server does not require the TDE to be installed. They are independent features and can coexist on the same base module.