LOGO! TDE Large Number Display: 32767 Integer Limit Workarounds

LOGO! TDE Large Number Display: Workarounds for the 32767 Integer Limit

Siemens LOGO! 8 base modules and the LOGO! TDE external text display (6ED1055-4MH08-0BA1 family, also referenced as the LOGO! TD) use 16-bit signed integer math for nearly every analog operand, threshold, counter, and on-screen numeric field. This bounds every single displayed value to -32 768 ... +32 767. When a process variable legitimately exceeds that range - a 70 000 L tank, a 250 000 pulse count, a 1 000 000 m flow total - the TDE cannot show the real number. The variable clips, wraps, or is suppressed depending on the source block. This reference documents four field-proven methods to push past the limit without changing the controller: Analog Amplifier Gain/Offset scaling, decimal-point repositioning, multi-zone message composition, and unit-suffix scaling, with concrete configuration values, math, TDE commissioning checks, and a troubleshooting matrix.

1. Hardware and Software in Scope

The methods below apply to the LOGO! 0BA8 generation and its associated TDE. Earlier generations (0BA0 ... 0BA7) carry the same 16-bit limit; later firmware adds higher counter resolution but not a wider TDE numeric format.

Reference the official LOGO! 0BA8 system manual and the LOGO! TDE operating instructions for part-number details and pinout.

2. Why the Display Caps at 32 767

Inside the LOGO! 0BA8 runtime the CPU stores I/O flags, analog thresholds, message-text numeric inserts, on-delay / off-delay setpoints, counter values, and timer values as 16-bit signed words with the range -32 768 ... +32 767 (i.e. -2^15 ... 2^15 - 1). Any operand forwarded to the TDE that exceeds +32 767 is treated in one of three ways depending on the source block:

1. Clipping at +32 767 - typical of message-text numeric fields and the Analog Multiplexer.
2. Modulo wrapping - the value rolls over to a negative number. The 70 000 L case in the field report will appear as 70 000 - 2 x 32 768 = 4 464 if the underlying integer is treated as unsigned, or simply 0 if the value source is signed-clipped.
3. Suppression - the TDE message line is blanked or shows asterisks (*).

LOGO! 0BA8 does support a 32-bit high-speed counter (HSC) on the fast digital inputs I1 / I2 / I3 / I4 / I5 / I6 with counts up to 2 000 000 when using the high-speed counter block, and recent firmware exposes double-word (DWORD) for specific blocks. However, the TDE message numeric field format is still a 16-bit signed render, so a 32-bit HSC value displayed directly will still clip at 32 767. The fix is therefore always at the display boundary, not the counter boundary.

3. Worked Example - 70 000 L Tank Volume

The source scenario is a 70 000 L process tank whose actual level is multiplied by the tank cross-section to yield a volume word. If the analog input AI1 reports 0 ... 10 000 (representing 0 ... 100.00 % of probe span) and the geometry block multiplies by 7 to obtain 0 ... 70 000, the result exceeds 32 767 and is un-displayable.

From 46.6 % level upward the operator sees a value that no longer correlates to tank content. The four solutions below all bring the displayed value back into the 16-bit signed window.

4. Solution 1 - Analog Amplifier Gain / Offset Scaling

The Analog Amplifier block (block number B20 in LOGO!Soft Comfort) is the cleanest way to compress a 0 ... 70 000 signal into a 0 ... 7 000 word. Place the block on the FBD canvas, double-click, and configure:

With Gain = 0.1 and 0 offset the analog amplifier divides the input by 10. The 70 000 L maximum compresses to 7 000, which the TDE can render. Configure the on-screen message text to read Volume: [B20] x 10 L using a static text suffix, or place the unit "L" (or "kL") after the numeric field. Resolution becomes 10 L per step, which is acceptable for most tank-level applications.

An alternative is to keep Gain = 7 (as referenced in the field report) and set the display to two decimal places. The TDE message numeric format has limited decimal control; LOGO!Soft Comfort exposes the format via the message block's "Number of decimal places" attribute. With two decimal places a value of 0 ... 70 000 is rendered as 0.00 ... 700.00. If the application interprets the value as 100 L per unit (i.e. Gain 7 + divide by 100), the operator reads 70 000 L from a "700.00" display with a "x100 L" suffix.

5. Solution 2 - Decimal-Point Repositioning (kL Display)

If the process is happy in kilolitres, scale the underlying value by 0.001 before it reaches the TDE. With a 0 ... 70 000 L input and a divisor of 1 000, the displayed number is 0 ... 70. The TDE shows "70" with a "kL" suffix. This frees an entire order of magnitude and removes the need for decimal places.

Use the Math Instruction block (block number B25) or the Analog Amplifier for the scale. Both blocks accept a constant that LOGO!Soft Comfort can express as a float (e.g. 0.001), and the result is integerised by the runtime. This is the most readable solution for an operator: the TDE shows "70 kL" and the meaning is unambiguous.

6. Solution 3 - Multi-Field Display Zones

The TDE is 6 lines x 20 characters. For very large numbers you can split the value across two message lines and concatenate. The typical trick is to show the most-significant 2-3 digits on the top line and the least-significant 2-3 digits on the second line, separated by a header:

Line 1:  Tank 1 Volume
Line 2:  --------------------
Line 3:  70
Line 4:  000 L
Line 5:
Line 6:  -- High --

The top "70" is a B20 amplifier with Gain = 0.0001, the bottom "000" is the same source with Gain = 1 and Offset = 0, then masked to the lower 3 digits using the Analog Threshold Trigger combined with a Math Instruction. In practice, this is cumbersome; it is mostly useful when the existing two analog amplifiers are already present and only display formatting is needed. Most engineers will prefer Solution 1 or 2.

7. Solution 4 - Unit Conversion with the Math Instruction

The Math Instruction block (B25) accepts the formula A = (B + C) x D + E in free form, where B and C are the inputs and D, E are constants. The block is fully integer internally, so to scale 0 ... 70 000 down to 0 ... 7 000 use:

• Input Bx = raw volume word (0 ... 70 000)
• Operator = multiply, factor = 0.1 (gain)
• Operator = add, constant = 0 (offset)
• Output Ax = scaled value (0 ... 7 000)

LOGO!Soft Comfort allows the gain to be a non-integer constant; the runtime rounds the result to the nearest signed 16-bit word. Math Instruction is functionally equivalent to Analog Amplifier for simple scaling. Use Analog Amplifier when you also need to apply a sensor zero-shift (Offset <> 0) or when the result must drive an analog output AQ; use Math Instruction when the scaled value is only destined for the TDE message field.

8. TDE Text Alignment Constraints

The field report notes that the user attempted to append a literal text "0" to a numeric value on the TDE, expecting it to look like "70000". The TDE does not align free text against a numeric field pixel-for-pixel. The grid is fixed 6 x 20 characters with a single font and fixed character cell. You cannot right-align a custom text fragment against the rendered number, so the hack of appending a static "0" produces the typical "7000 0" result. There is no in-built leading-zero suppression, no decimal padding, and no sub-character positioning.

Consequences for the design:

• Choose a scaling factor (Solution 1, 2, 3 or 4) so the natural integer fits without manual padding.
• If the value must be shown as 5 digits and the source maxes at 4 digits, scale the source - do not pad with text.
• If the value is < 10 000, the TDE will left-pack the number with leading spaces; this is the natural behaviour and cannot be disabled.
• The negative sign occupies a full character cell, so the positive-only range 0 ... 32 767 effectively renders as up to 5 characters with the leading sign only on negatives.

9. Configuring the Analog Amplifier in LOGO!Soft Comfort V8.x

1. Open the project and switch to FBD view (the LOGO!Soft Comfort default).
2. Drag the Analog Amplifier block from the analog functions palette on to the canvas.
3. Double-click the block header to open the properties dialog.
4. Set the Sensor range to match the upstream source block (for example, 0 ... 10 000 if the source is the percentage output of an arithmetic block, or 0 ... 32 000 if the source is already a raw integer).
5. Set Gain to the desired scale (0.1 for the 70 000 L case).
6. Set Offset to 0 unless you need a zero shift.
7. Click OK; the block is now ready to drive a TDE message text numeric field.
8. Open the LOGO! Message Text configuration (drag the message block from the special functions palette), enable it, and in the message body place the analog amplifier tag (e.g. 001 if it is the first amplifier). In the right-hand attributes set "Number of decimal places" to 0 for an integer, or 1 / 2 if you prefer to keep precision in the visible digits.
9. Add a static text suffix such as " kL" to the message line. The suffix does not shift position when the numeric value changes digit count.
10. Save the project and transfer to the LOGO! via Ethernet or LOGO! USB cable. Trigger the message by enabling the message text enable flag from your program.

10. Verification and Commissioning Steps

1. Use the LOGO!Soft Comfort Simulation mode to drive the input block through its full range (0, 25 %, 50 %, 75 %, 100 %). Confirm the TDE message field shows the expected scaled value at each step.
2. Force the LOGO! into the Online Test view; right-click the Analog Amplifier block and choose "Watch". Verify the block output stays within -32 768 ... +32 767 at all reachable input levels. If the output ever exceeds the limit, the TDE will clip - reduce Gain or apply a divisor.
3. Disconnect simulation and connect to the live controller via the LOGO!Soft Comfort Ethernet connection. Transfer the project and observe the physical TDE.
4. Apply the real sensor input and walk the process from empty to full. Record the TDE readings at 0 %, 10 %, 25 %, 50 %, 75 %, 90 % and 100 % level. Compute the linearity error = (TDE reading - expected reading) / expected reading x 100. Acceptable for tank level indication is typically ± 2 %; tighter (± 0.5 %) requires re-calibration of the sensor, not the scaling.
5. Check the rollover boundary. If the TDE value ever wraps from 32 767 to -32 768, scaling is still wrong. Add a 10 % safety margin: keep peak TDE value ≤ 28 000.
6. Document the scaling in the project comments. Future engineers must be able to reconstruct the display logic from the project file alone.

11. Troubleshooting Matrix

12. Edge Cases and Field-Proven Caveats

• Sensor zero drift: If the analog input drifts (e.g. capacitive probe with temperature), a non-zero Offset on the Analog Amplifier can absorb up to 4 000 counts of zero shift without rescaling. Use Offset = -drift, Gain = 1.0 and then divide further downstream.
• Negative values: If the level sensor can read slightly negative (submersion, condensation), the TDE will reserve one character for the minus sign. Plan for 6 character positions when the value can swing -32 768 ... 0.
• Two-scale systems: Some applications need to show both litres and kilolitres on the same TDE - use the second line of the message for the secondary value with a different suffix. Keep them on separate blocks (B20 + B25) to avoid accidental cross-coupling.
• LOGO! 0BA7 compatibility: The same 16-bit limit applies; Analog Amplifier is block B13 on 0BA6 / 0BA7. The configuration dialog is similar but the decimal-place attribute is exposed differently. Check the firmware-specific manual.
• High-speed counters above 32 767: On 0BA8 the HSC can count to 2 000 000 but the TDE field is still signed 16-bit. If a pulse total is required, use a separate slow counter with a divisor of 100 or 1000 and a "pulse x 100" suffix on the TDE.
• Multiple TDEs on the same LOGO!: Up to two LOGO! TD/TDE modules can be chained; each shows the same message text by default. If you need different views per TDE, split the message into multiple Message Text blocks and select them in the TDE menu.

13. Frequently Asked Questions