Configuring LOGO! 8 Text Display for Analog Amplifier Parameters

Overview

Siemens LOGO! 8 base modules integrate a PLC, an onboard text display (HMI) and digital/analog I/O into a single DIN-rail device. The on-board display (and the optional LOGO! TDE text display) is not a full graphical HMI, but it is fully programmable from LOGO! Soft Comfort and supports a powerful feature often missed by first-time users: runtime editing of message text parameters. This makes it possible to expose scaling parameters (gain, offset, minimum and maximum) of an analog amplifier block and let the operator change them from the front panel without re-flashing the PLC.

This article solves a common field problem: a 0–10 V analog input must accept multiple sensor ranges (for example 0–200 bar, 0–150 A, 0–50 A) and the operator needs to change the displayed engineering range from the LOGO! screen at runtime. The scaling math, the message-text configuration and the front-panel keystroke sequence are covered in full.

Prerequisites

• LOGO! 8 base module with at least 4 analog inputs (for example 6ED1052-1CC08-0BA1, the 12/24 V version with 8 DI / 4 AI / 4 DO that is referenced in the source project). Logic-only modules with 0 AI cannot be used.
• LOGO! Soft Comfort V8.x or V9.x installed on the engineering PC. The same procedure is valid for both major versions; V9 supports the LOGO! 8.3 (0BA9) devices.
• LOGO! TDE (optional but recommended) – catalog number 6ED1055-4MH08-0BA0 – for the 6-line × 20-character external display.
• One or more 0–10 V (or 0/4–20 mA) sensors with a known physical measuring range.
• A 12/24 V power supply and a programming cable (Ethernet for 0BA8/0BA9, or USB for older 0BA7).

LOGO! 8 Hardware – Analog Input Fundamentals

Every analog input of a LOGO! 8 base module contains a 10-bit A/D converter that always returns a raw value in the range 0–1000, regardless of the configured sensor type. The PLC does not return volts, milliamps or degrees – it returns 0 to 1000 "AI units". Converting those 0–1000 units into the engineering range of the sensor is the job of the Analog Amplifier (or the integrated scaling inside the analog input block when used inside a message text directly).

For the source project the four AI channels are used as 0–10 V voltage inputs. The PLC internally always sees 0–1000 raw units; the operator sees the engineering value after the analog amplifier.

Analog Amplifier Block – Parameters

The Analog Amplifier block (function block category Analog in LOGO! Soft Comfort) has the following editable parameters. The first four are stored in the program and are read/write from the display when the block is referenced inside a message text:

The transfer function applied by the block is:

Displayed value = ((Raw / 1000) × (Max − Min) + Min) × Gain + Offset

Calculating Gain and Offset for a 0–150 A Range

Use the following rules to convert the raw 0–1000 units into a sensor-specific range. The two most common cases from the source project are worked out below.

Case A – Integer display 0 to 150 (no decimals)

• Minimum: 0
• Maximum: 150
• Gain: 0.15 (150 / 1000)
• Offset: 0
• Decimal places in the message text: 0
• Result: at 0 V the display shows 0 A, at 10 V the display shows 150 A.

Case B – One decimal place, 0.0 to 150.0

• Minimum: 0
• Maximum: 1500
• Gain: 1.5 (1500 / 1000)
• Offset: 0
• Decimal places in the message text: 1
• Result: at 0 V the display shows 0.0 A, at 10 V the display shows 150.0 A.

General formula

For a desired display range [Dmin, Dmax] with the A/D raw range fixed at 0–1000:

Gain = (Dmax − Dmin) / 1000
Offset = Dmin

When Dmin = 0 (the usual case) the offset can be left at zero and the entire range is controlled by the gain. The number of decimal places is set in the message text and must be chosen at programming time.

Building the Message Text in LOGO! Soft Comfort

1. Open the project in LOGO! Soft Comfort and switch to Diagram Editor.
2. Drag an Analog Amplifier block onto the worksheet for each analog input that needs to be re-scaled. Wire the AI channel to the amplifier input.
3. Open the Message Text tool (it is the speech-bubble icon on the left toolbar) and drop a message text on the worksheet. Wire the block you want to display to the X input of the message text. You can also wire a Boolean trigger to the En input to enable/disable the message conditionally.
4. Double-click the message text to open the editor. Click in the body and add the block you want to make editable. Insert > Analog > the analog amplifier block you created.
5. The message text editor shows four lines by default (six on a TDE). For each line use the Insert menu to add text, tags, and parameters. To expose the gain of an analog amplifier, place the cursor on the gain parameter line, then from the menu select Insert > Parameter > Gain of the corresponding block.
6. Build a multi-screen menu:
   • Screen 1 (top of menu): Select AI to configure – use a cursor key to step through AI1 … AI4.
   • Screen 2: Selected AI = AIx
   • Screen 3: Gain = 0.150 with the gain parameter inserted and the focus on that line so it is editable.
   • Screen 4: Offset = 0 similarly.
   • Screen 5: Min = 0 and Max = 150 (only if you also need the operator to edit them; for sensor-type changes they are fixed).
7. Tick the option Parameter set for the gain and offset fields you want to make editable. Without this tick the value is shown read-only on the front panel.
8. Press F2 to compile the project, then PC → LOGO! to download the program to the base module.

Runtime Procedure – Editing Parameters from the LOGO! Display

The front-panel procedure is identical on the LOGO! base module and on the TDE, except that the TDE uses an Enter key where the base module uses OK and the TDE has dedicated cursor keys.

1. Use Cursor ↑ / Cursor ↓ to scroll to the message text that contains the gain/offset you want to change.
2. Press and hold ESC for longer than 2 seconds. A black bar appears across the first editable field of the active message text.
3. Use Cursor ↑ / Cursor ↓ to move the black bar to the field you want to edit (for example the Gain row).
4. Press OK (or Enter on the TDE) once. The black bar becomes a blinking box on the first digit of that field.
5. Use Cursor ← / Cursor → to position the blinking box on the digit you want to change.
6. Use Cursor ↑ / Cursor ↓ to change the digit (0–9). For a sign, hold the cursor to step to the + / − position.
7. Press OK to confirm the change. The blinking box reverts to the black bar.
8. Use the cursor keys to move to the next field, or press ESC to exit the edit session. All confirmed values are written to the analog amplifier immediately and retained across power cycles in the LOGO!'s non-volatile memory.

Practical Example – Multi-Range Sensor on AI1

The source project uses a single AI1 channel that is connected to a 0–200 bar pressure sensor one day and a 0–150 A current shunt on another day. The PLC program needs only one Analog Amplifier block for AI1, but two memory locations hold the Gain value to load, and the operator must be able to swap between them at runtime.

Program structure

1. Analog input AI1 → Analog Amplifier B001 (Sensor = 0–10 V, Min = 0, Max = 1500, Gain = 1.0, Offset = 0, decimals = 1).
2. Two Constant blocks for the two pre-computed gains: GAIN_BAR = 0.20 (0–200 bar with 0 decimals ⇒ Min = 0, Max = 200, Gain = 0.20) and GAIN_AMP = 0.15 (0–150 A with 0 decimals ⇒ Min = 0, Max = 150, Gain = 0.15).
3. A selector message text with two message texts – one for Bar sensor active and one for Amp sensor active – that uses a Digital Multiplexer in the diagram to push the correct gain into the amplifier when the operator presses OK.
4. A third message text that simply displays the current value at AI1 after the amplifier. Insert > Analog > B001 to make the live value visible without any parameter set ticked.

Operator procedure on the LOGO! screen

1. On the main menu, scroll to Analog input 1 and press OK.
2. Scroll to either Range 0–200 bar or Range 0–150 A and press OK to apply.
3. The display returns to the live value screen. The reading is now scaled with the new gain.
4. If the operator needs a range that is not pre-loaded, they can enter the Manual message text, press ESC > 2 s, and edit the gain field directly. The new gain is retained until the next manual edit or program download.

Limitations and Field Constraints

• Sensor type (0–10 V, 0/4–20 mA, PT100/1000) and the number of decimal places cannot be changed at runtime – they are part of the compiled program and require a download.
• The LOGO! 8 base display is a 4-line × 12-character LCD. The TDE is 6-line × 20 characters. Plan your message text to fit; long parameter names like Range of sensor must be abbreviated in 12-character mode.
• The display only shows the current value of the analog amplifier once you have wired it inside a message text. It does not by default render the range 0 – 150 A – it renders a single number. If you want to show the range as part of the screen you must use a static text line for the range string and a separate parameter line for the live value.
• Gain and offset are stored as 16-bit signed integers internally. Maximum editable value is ±32767 with three decimals. For 0–200 bar at 0 decimals the gain of 0.20 is well within range.
• Cursor key navigation on the base module only moves between message texts that are currently enabled (En = 1). A common bug is to wire the Enable input to a flag that is never set, which makes the message text invisible on the display.

Verification and Commissioning

1. Download the program, then power-cycle the LOGO! so any retained gain/offset values are cleared from the previous download.
2. Apply a known reference input – ideally a calibrator – to AI1: 0 V, 5 V, 10 V.
3. Read the value shown on the display and confirm the scaling. For 0–10 V, 0–200 bar, gain = 0.20: at 5 V the display must read 100 bar (±1 digit).
4. Enter the gain-edit message text, press ESC > 2 s, and deliberately set the gain to a wrong value, then to the correct value. Confirm the change is retained after a power cycle.
5. Simulate a sensor swap in the field: disconnect the 0–10 V sensor, connect the 0–150 A current sensor (with the appropriate shunt), scroll to the Range 0–150 A message, confirm the gain changes from 0.20 to 0.15, and verify the live value.
6. Document the procedure as a one-page operator card and tape it to the inside of the cabinet door.

Troubleshooting Matrix

Best Practices

• Keep the operator-facing gain in the same engineering units as the display – avoid mixing 0.20 (bar-per-volt) with 150 (ampere full scale) in different message texts.
• Pre-compute the most common ranges as named constants in the program so the operator only needs to pick a name, not type a number.
• Reserve a hidden Service message text guarded by a digital input for the raw engineering Gain edit – that way the operator can recover from a bad value without a PC.
• Print a one-page table of all selectable ranges and their gain value inside the cabinet. The LOGO! display is not a substitute for a paper reference.
• If you need more than 4–6 message texts, switch to a LOGO! TDE – the 6-line × 20-character screen is significantly easier to use than the 4-line × 12-character base display.
• Always keep the analog amplifier block in the program even if the message text is the only consumer. The Min, Max, Gain, Offset parameters of a bare AI inside a message text are read-only – they become editable only when the AI passes through an analog amplifier block.

Reference Documentation

• LOGO! 8 system manual – see the Siemens Industry Online Support portal (search for "LOGO! 8" or the catalog number 6ED1052-1xx08).
• LOGO! Soft Comfort V9 online help – bundled with the engineering software, includes the message-text and analog-amplifier block reference.
• LOGO! TDE (6ED1055-4MH08-0BA0) operating instructions – available on the Siemens support portal.