Siemens LOGO! 8 TDE Large Font: Message Text Block Limits

The Siemens LOGO! 8 (0BA8) with the LOGO! TDE external text display is one of the most common operator-panel configurations for low-cost machine and building-automation retrofits. A recurring field request, after the project is deployed, is to enlarge the character size of a counter or process value that is shown on the TDE. The hard answer is documented in the LOGO! 8 System Manual and confirmed in the LOGO! Soft Comfort V8.x help: the TDE is a fixed-pitch text display. The font cannot be scaled. The only dynamic length-changing function inside the TDE is the Message Text ticker, and the ticker applies to static literal text only — variable tokens (counter value, analog value, I/O state) do not scroll and cannot be enlarged.

This reference walks through the LOGO! 8 FS4 system architecture, the TDE hardware limits, the Message Text function-block parameter set, the ticker behavior, the root cause of the variable-resize restriction, every practical workaround available inside LOGO! 8, and the migration path to a graphical HMI. Every step is anchored in the LOGO! 8 documentation and the LOGO! Soft Comfort V8.2 / V8.3 user interface.

1. LOGO! 8 FS4 System Overview

The LOGO! 8 (order code family 0BA8) is the sixth-generation universal logic module from Siemens. It combines a logic engine, digital and analog I/O, a real-time clock, retentive storage, and a 10/100 Mbit Ethernet interface in a single 72 mm DIN-rail housing. The base module is 8 DI / 4 DO with relay or transistor outputs. Up to 24 digital inputs, 20 digital outputs, 8 analog inputs, and 8 analog outputs can be added through digital and analog expansion modules daisy-chained on the internal bus. The base module is a class C Ethernet device and provides a 10/100 Mbit/s port for programming, peer-to-peer LOGO!–LOGO! communication, and S7-communication to a SIMATIC HMI or S7-1200/S7-1500 controller.

LOGO! 8 FS4 is the firmware build that introduced the extended special-character set used in modern Message Text blocks, expanded the analog value range, and added support for the LOGO! Access Tool and the LOGO! Web Editor. The build is supported by LOGO! Soft Comfort V8.2 (FS3 baseline) and V8.3 (FS4 specific). Confirm the firmware version on the LOGO! by navigating LOGO! → Diagnostics → FW Version on the TDE or by selecting the device in the LOGO! Soft Comfort online view. Typical FS4 build strings include LOGO! 8.FS4-1.01.00 and later patch levels.

Reference: LOGO! 8 (0BA8) System Manual — SIOS and Siemens LOGO! product page.

2. LOGO! TDE Hardware Specification

The LOGO! TDE catalog number 6ED1055-4MH08-0BA1 is the FS4 release of the external text display. The earlier FS3 release is 6ED1055-4MH00-0BA0 and should be replaced when the application requires the FS4 special-character set. The TDE is a 144 × 72 mm housing with a 6-line × 20-character FSTN monochrome LCD and a 16-key silicone keypad (cursor keys, ESC, OK, F1–F4). It connects to the second Ethernet port of the LOGO! base module with a standard Cat5 / Cat6 patch cable and is powered from a separate 24 V DC source.

Critical architectural fact: the TDE is a passive terminal. It has no processor that can render a custom font, no vector graphics engine, and no fieldbus slave of its own. The TDE communicates with the LOGO! base module over Ethernet using the LOGO! display protocol. The base module transmits pre-formatted character codes; the TDE places them in the 6 × 20 cell grid. There is no path through which a user program, parameter change, or TDE menu can change the font, the cell size, the dot matrix, or the character aspect ratio. The Message Text block is the only mechanism that determines what characters appear in what cell.

3. Character Set and Display Cell Geometry

The TDE cell is the atomic unit of display. The 6-row × 20-column grid is partitioned into 120 cells. Each cell is approximately 4.8 mm wide and 6.0 mm tall, leaving a 1-dot gap between columns and a 1-pixel gap between rows. The character rendered inside the cell is a 5 × 7 dot matrix, anchored to the upper-left corner of the cell. The remaining cell area is reserved for inter-character spacing and the row gap.

The LOGO! 8 character set is a fixed 256-codepoint table. The first 128 entries are the standard ASCII range (0x20–0x7F), the second 128 are LOGO! block-name characters, function names, and the extended special-character set that was expanded in FS4. The full set is documented in the LOGO! Soft Comfort help file under Message Text → Special characters. Notable entries include:

None of these special characters enlarge the cell. They are glyphs, not zoom factors. Selecting a special character in the editor inserts the codepoint at the cursor position and the TDE renders the glyph in the same 5 × 7 dot matrix as the standard ASCII characters.

4. Message Text Function Block (BM / Meldetext)

The Message Text block is one of the special function blocks in the LOGO! library. The block identifier in FBD is BM; in ladder it is labeled M. Up to 50 Message Text blocks can exist in a single LOGO! program, and any of them can target any of the four TDEs that may be attached to the same base module.

The variable token is inserted from the message-text editor. The dialog lists every numeric or boolean block currently in the program. When the message runs, the placeholder is replaced with the live value of the referenced block. The replacement respects the formatting defined by the block itself (decimal width, signed/unsigned, leading zeros) and is rendered in the standard 5 × 7 font at the same size as the literal text around it.

For numeric blocks, the variable renders as a decimal value with the number of decimal places configured in the source block. For boolean blocks, the value renders as 0 or 1 unless the message-text editor wraps the variable with a custom string (a typical pattern is State: {B005.QV} for a 0/1 status). A counter with on-threshold 999999 will display up to six digits; the rendered string grows from one to six characters as the count climbs. The variable width, in other words, is dynamic.

5. Ticker and Marquee Behavior

The ticker is the only TDE function that animates characters. When the line content exceeds 20 characters, the TDE either truncates (no ticker) or scrolls the line. The LOGO! Soft Comfort property page offers three modes:

• Ticker = No — only the first 20 characters are visible. Everything past position 20 is discarded. The static and the variable are both rendered in the same 5 × 7 font, in the same cell size, with no animation.
• Ticker = Character by character — the line scrolls horizontally, one column at a time, at a fixed cadence set by the LOGO! firmware (approximately one column per 500 ms in FS4, slightly faster in FS3). A space-padding separator is inserted so that the visible content cycles continuously through the message. The effect reads as a marquee.
• Ticker = Line by line — when the message has more than 6 lines, the entire 6-line block scrolls vertically. If the message fits in 6 lines, line-by-line ticker behaves identically to "Ticker = No".

The ticker engine is implemented in the LOGO! base module, not in the TDE. The base module pushes the visible 20-character window to the TDE at the ticker cadence. The TDE has no awareness of the underlying message; it simply displays what arrives. This is also why the ticker cannot react to the runtime width of a variable — the variable is updated in place, but the scrolling engine does not know the variable's position has shifted.

5.1 Why the ticker does not animate variable values

The ticker requires that the LOGO! runtime can pre-compute the rendered string. The runtime assembles a character buffer for each line. If the buffer contains a variable token, the runtime cannot know in advance how wide the token will be: a counter that is at 7 will be one character; at 12,000 it will be five. Pre-computing the buffer for a character-by-character scroll requires a fixed string length, and the LOGO! firmware does not do per-character interpolation across variable boundaries.

When the LOGO! runtime detects a variable token inside a ticker-enabled line, it disables the ticker for that line and renders the line with ticker = No behavior. The variable still appears, but it is truncated to 20 characters and is not animated. This is a hard, documented limitation, not a configuration error. Engineers upgrading from FS3 to FS4 sometimes expect the limitation to be lifted; it is not.

6. Why Variable Values Cannot Be Resized

Three independent reasons converge to make a large-font counter value impossible on the TDE:

1. Display cell is fixed. The TDE is a 6 × 20 character grid. The font is a 5 × 7 pixel matrix inside that cell. There is no command in the LOGO! display protocol to enlarge a cell, double a font, or run a sub-cell at 16-segment rendering. The TDE controller chip on the PCB samples the cell stream from the base module and latches the cells to the LCD; it has no instruction set to address a sub-cell.
2. Message Text has no font-size attribute. The Message Text block properties dialog exposes trigger, acknowledgement, line text, variable insertion, ticker, and target display. There is no font size, font family, character height, or weight parameter. The block format has not been changed since the TDE was introduced with LOGO! 6 (0BA6), and the LOGO! 8 firmware retains the same parameter set.
3. LOGO! TDE successor is a separate panel. Siemens does not sell a 6 × 20 text display with a larger font. The 0BA8 firmware carries forward the same 5 × 7 dot matrix that has been in the TDE since 2008. The migration target is the SIMATIC HMI Basic Panel family, not a refreshed TDE.

The combination of these three constraints means that the only way to make a counter value appear larger is to remove the TDE from the system and replace it with a panel that has a vector font engine. This is the migration path that field engineers adopt as soon as the application is reviewed and a graphical panel is justified.

7. Workarounds Inside the LOGO! 8

Although the variable cannot be enlarged, several layout tricks inside the existing TDE make the value visually dominate the screen and read at a distance.

7.1 Use a single line with no static prefix

Put the variable on its own line with no static characters around it. The counter value then fills the entire 20-character cell and reads at the maximum font size the TDE supports. Example layout:

Line 1: Operations today
Line 2: {C001}            <-- variable alone
Line 3:
Line 4: (blank)
Line 5: (blank)
Line 6: (blank)

This does not enlarge the digit itself, but it removes the visual competition of the surrounding text. The eye lands on the variable without parsing context first.

7.2 Use multiple message blocks to spread the value

If the value is a four-digit number such as 4271, you can dedicate one message block per digit. The variable placeholder is set to a counter of 1000s, 100s, 10s, or 1s. Each block places a single digit on its own line. From a distance the operator sees a four-line stack that reads as a tall numeral.

B010  Block: floor(C001 / 1000)  mod 10
       Line 1:  {B010}
B011  Block: floor(C001 /  100)  mod 10
       Line 1:  {B011}
B012  Block: floor(C001 /   10)  mod 10
       Line 1:  {B012}
B013  Block: C001                mod 10
       Line 1:  {B013}

Each block uses the LOGO! 8 FS4 arithmetic instruction (the analog math block and the threshold trigger can be cascaded for digit extraction). One message per line, no ticker, so the value updates instantly and shows each digit on its own line. From a 5 m viewing distance the four stacked digits read as a large counter.

7.3 Use the line-by-line ticker for a sweeping effect

With ticker = "Line by line" and a 7-line message, the line-by-line ticker scrolls vertically. The counter value appears to move from the bottom of the TDE to the top, drawing the eye toward it. This is a perception trick, not a true size increase.

7.4 Switch to the FS4 TDE and use the extended character set

The 6ED1055-4MH08-0BA1 (FS4) has a brighter white-LED backlight than the 6ED1055-4MH00-0BA0 (FS3). Combining the new TDE with high-contrast text (use the degree, ohm, and block-name characters from the extended set) on a single variable line improves legibility without changing the font size. The contrast ratio on a FS4 release at maximum backlight is typically 8:1 in office light and 12:1 in dim light, which is enough to make a 5 mm character readable at 4 m.

8. Migration to a Graphical HMI Panel

When the application truly needs a large counter, the correct path is to replace the TDE with a SIMATIC HMI Basic Panel. The Basic Panel range starts at 4" and supports TIA Portal / WinCC project engineering. The communication to LOGO! 8 is over Ethernet using the "LOGO! 8" driver that ships with WinCC Comfort (Comfort Panels) and WinCC Basic (Basic Panels).

Reference: SIMATIC HMI KTP400 / KTP700 Basic Panels operating instructions — SIOS and the Siemens SIMATIC HMI product page.

8.1 Communication wiring

Connect the Basic Panel Ethernet port to the LOGO! base module Ethernet port 1 (the IP-enabled port) with a Cat5 patch cable, max length 100 m. Configure the LOGO! with a static IP address (default 192.168.0.1 / 255.255.255.0). On the panel, add a connection of type "LOGO! 8" in the WinCC project, set the IP to 192.168.0.1, and confirm the port is 100 (default LOGO! port for the S7-communication layer). The driver polls the LOGO! variable database and exposes every LOGO! block as a tag of the matching data type. Counter blocks appear as INT (16-bit signed), analog blocks as WORD (16-bit unsigned), digital I/O as BOOL.

8.2 Display of a large counter on the KTP700 Basic

The WinCC Comfort project should contain a single screen with a numeric display object. Set the field to the LOGO! tag of the counter block, choose 16-bit signed representation, set the appearance font to a system vector font, and bind the font size to 144 pt or 192 pt. The counter value renders in a true large font that fills most of the screen. Adding a bar graph and a status indicator turns the screen into a 30-minute replacement for the original TDE-based counter page. The full engineering workflow is described in the WinCC Comfort V17 / V18 manual, in the section "Working with the LOGO! 8 driver".

8.3 Investment and risk

A KTP700 Basic mono is approximately three to four times the cost of a TDE in 2024–2025 catalogue pricing. The migration also requires a copy of WinCC Comfort (the engineering tool for Comfort Panels) or WinCC Basic (for Basic Panels). A floating or transferrable license can be acquired via the Siemens Industry Mall. Programming time is typically a single engineering day for a counter screen, and the LOGO! program itself does not change.

9. Configuration Example: Counter Display

This is a complete, deployable Message Text block definition for a counter on a LOGO! 8 FS4 with one TDE attached. The example uses DI 1 as the count trigger and DI 2 as the reset input.

9.1 Block diagram (textual)

DI 1  ──┐
         ├──> [ B001 Counter ] ──> B001.CV ──> [ B002 Message Text ]
         │                                       (display on TDE 1)
[ I1 ] ──┘

DI 2  ──> [ B003 Wiping pulse ] ──> B001.R

9.2 Block parameter tables

9.3 Expected TDE output

After download, the TDE shows:

Operations
            0
(blank)
(blank)
(blank)
(blank)

The counter is the only content on line 2. It is rendered in the standard 5 × 7 dot font at the same size as the title above. If the application requires a larger counter, this is the point at which a graphical panel migration is triggered.

10. Verification and Acceptance Test

The following checks confirm that the TDE configuration is correct.

1. Connect a PC running LOGO! Soft Comfort V8.2 or higher to the LOGO! base module Ethernet port 1 and start an online session. Confirm that the device shows up as LOGO! 8.FS4 in the project tree.
2. Verify the firmware version: LOGO! → Diagnostics → FW Version. The expected string contains FS4 (e.g., LOGO! 8.FS4-1.01.00). Earlier versions will not support the full special-character set and may have a different ticker cadence.
3. Force a count on the counter input. The TDE line 2 value must update on the next LOGO! scan (≤ 50 ms). If the value does not update, check that the message block trigger (En) is high and that the variable token references the correct block output (B001.CV, not B001.QV).
4. Apply the reset signal on DI 2. The TDE line 2 must drop to 0 within one LOGO! scan. The retentive flag in the counter settings determines whether the value is preserved across a power cycle.
5. Cycle power. Confirm the retentive behaviour matches the design intent. If the counter must survive power-down, set Remanence = On; the value is stored in the LOGO! non-volatile area and can be password-protected if required.
6. Trigger an overflow. With on-threshold = 999999, incrementing the counter past this value will latch it at 999999; verify that the application log records the overflow event and that no reset is required to clear it.

11. Troubleshooting Matrix

12. Frequently Asked Questions