Siemens LOGO! Analog Scaling: Displaying Values Above 10,000

Siemens LOGO! 8 base modules and their AM2/AM2 RTD/AM2 PT1000 expansion modules resolve every analog input into 1,000 internal counts. That fixed mapping is the reason a 0–10 V sensor can never natively produce a reading of 11,299 or 12,235 in the LOGO! display or message text. This reference explains the root cause, defines the resolution boundary, and presents three engineering solutions that allow LOGO! to display 0–12,500 cleanly without losing diagnostic fidelity.

Problem Statement: Why 0–12,500 Cannot Display Natively

The original requirement is to display an analog engineering value in the range 0 to 12,500 on a LOGO! system. The default LOGO! analog input chain looks like this:

1. Voltage or current at the terminal block (0–10 V or 0/4–20 mA).
2. 10-bit successive-approximation ADC on the LOGO! 8 main unit, or 12-bit ADC on the AM2 expansion.
3. Internal value normalized to a fixed 0–1,000 range for 0–10 V or 0–20 mA.
4. Optional gain block (max ±10) producing 0–10,000 maximum after amplification.
5. Message text, LOGO! TDE, or web server display.

Because the last gain stage tops out at ±10 on the internal 0–1,000 stream, the largest possible engineering value is 1,000 × 10 = 10,000. To display 12,500, the engineer must move the gain stage to an arithmetic block with non-integer multiplier, or accept a coarser step size, or push the final display outside LOGO! to an external indicator.

LOGO! Analog Input Architecture and Internal Representation

The LOGO! 8 base module (order numbers 6ED1052-xxx08-0BA1/0BA2) exposes eight inputs I1–I8. The first four (I1–I4) are configurable as analog 0–10 V inputs with a 10-bit ADC; the remaining four are digital-only unless an AM2 module is added. For current loops, an AM2 expansion (6ED1055-1MA00-0BA2) is required, which has a true 12-bit ADC and produces 0–1,000 counts over 0–20 mA or 4–20 mA.

Reference: Siemens Industry Online Support – LOGO! 8 System Manual (entry ID 109741041), section 4.4 "Analog value processing". Although the AM2 silicon is 12-bit, LOGO! firmware intentionally normalizes every analog input to 0–1,000 counts so that all blocks share a common scaling model. The 12 leftover codes on the AM2 are filtered out and used for overrange diagnostics.

The 1,000-Step Resolution Boundary

Consider a real sensor that delivers 0–10 V proportional to 0–12,500 RPM, 0–12,500 Pa, or 0–12,500 mm. After the LOGO! ADC, the engineer holds 1,000 voltage buckets, not 12,500 engineering buckets. The display chain has only 1,001 distinct levels available, and the gain block cannot create more data than the ADC captured.

Mathematically, if N is the count after gain and Y is the engineering value, the relationship is always

Y = N × (E_max / 1,000)

where E_max is the desired full-scale engineering value. For E_max = 10,000 the increment is 10.0 per LSB. For E_max = 12,500 the increment becomes 12.5 per LSB. The engineer gets 1,000 displayed steps spread over 12,500 units, so the smallest visible change is 12 or 13 units. A request to display "11,299" or "12,235" has no physical meaning, because the closest possible readings are 11,287.5 and 12,300 (one LSB away from the requested value). This is the same mathematical reality the community has confirmed: scaling up reveals fewer intermediate values, not more.

Solution A: Cascade Two Analog Amplifier Blocks

The first engineering solution is to use the Analog Amplifier (block B042 in LOGO! Soft Comfort) twice in series. Each amplifier accepts a sensor input plus a gain factor and an offset. The maximum per-block gain is ±10, and the maximum resulting integer output per block is bounded by the firmware at ±10,000 in some firmwares (0BA7 and earlier) or 0–10,000 in newer ones.

For the 0–10 V → 0–10,000 internal stream, the two-stage cascade is implemented as:

1. First amplifier (B042) at gain = 10. Output 1 = 0–10,000.
2. Second amplifier (B042) at gain = 1.25. Output 2 = 0–12,500.

Because LOGO! amplifier blocks truncate to integer values, the final value is computed as floor(Output 1 × 1.25). With 0–10,000 on Output 1, the second stage yields 0, 12, 25, 37, 50, 62, 75, 87, 100, ... and ends at 12,500. There are still only 1,001 visible states — they are simply spread further apart.

The advantage of the cascaded amplifier is that the value remains an integer that LOGO! can display directly on the LOGO! TDE (6ED1055-4MH00-0BA0) or via LOGO! Soft Comfort message text without a floating-point formatter. The disadvantage is the loss of single-LSB precision: an analog input that moves 10 mV no longer produces a 1-digit change on the display.

Solution B: Linear Scaling with y = mx + b Arithmetic

For applications where 12.5 units per step is acceptable but the user still wants the math expressed cleanly, the Arithmetic block (B050) implements the slope-intercept equation in firmware:

Y = ((V_a − V_min) / (V_max − V_min)) × (E_max − E_min) + E_min

With V_min = 0, V_max = 10,000 (post first amplifier), E_min = 0, E_max = 12,500, the constants reduce to:

• Gain (m) = 1.25
• Offset (b) = 0

Configure the Arithmetic block in LOGO! Soft Comfort as follows:

1. Insert block B050 (Analog Arithmetic) into the program.
2. Set the value source V to the output of the first Analog Amplifier.
3. Set Gain1 = 1.25, Gain2 = 0 (unused), Offset = 0.
4. Enable output range 0–32,000 in the block properties.
5. Wire the output to a message text or to a network variable for the LOGO! TDE.

The Arithmetic block is the recommended solution when the engineering units are non-integer, the offset is non-zero (e.g., scaling a 4 mA = 0 kg to 20 mA = 12,500 kg load cell), or the scaling must compensate for sensor zero shift. The block also handles a 4–20 mA current-loop input where the live-zero must be subtracted; see the Schneider Electric 4–20 mA primer for a generic current-loop reference.

Solution C: External Digital Panel Meter / LOGO! TDE

If the engineer must show the user a value with single-unit resolution (e.g., 11,299 exactly), the LOGO! cannot do it alone because the underlying data has 12.5-unit granularity. Two paths exist:

C.1 Use the LOGO! TDE text display

The LOGO! TDE (6ED1055-4MH00-0BA0) is a 6-line text HMI that mirrors all LOGO! variables. It accepts message text entries with embedded bar graphs, status icons, and scaled values. By inserting a PI Controller block (block B060) with a manipulated variable setpoint range of 0–12,500, the operator sees the setpoint in the requested engineering units even though the controlled variable is still 0–1,000 internally. The TDE is not a true work-around for resolution; it is a re-labelling of the same 1,000 internal steps.

C.2 Use an external digital panel meter

For displays that require higher visual resolution than LOGO! provides, install a digital panel meter (DPM) with a 0–10 V or 4–20 mA input and a user-scalable readout. Typical DPMs in the 96 × 48 mm form factor accept 0–10 V and have a 4-digit LED display with configurable decimal point. Because the DPM is a separate instrument, the LOGO! is relieved of any responsibility for showing 0–12,500; it only transmits the 0–10 V (or 4–20 mA) signal. The trade-off is additional cost and panel cutout.

Engineering the Slope Equation: Worked Example 0–10,000 → 0–12,500

Worked numerical example for clarity. Suppose a 0–10 V pressure transducer is wired to LOGO! I3, and the engineering range required is 0 to 12,500 Pa. The expected sensor sensitivity is 1.25 Pa / mV.

To compensate a sensor with a non-zero live-zero (e.g., 4 mA = 0 Pa in a 4–20 mA loop), set Offset = 0 in the Arithmetic block and apply a pre-conditioning subtraction. With a 4 mA sensor at 0–12,500 Pa the live-zero takes up 20 % of the input range, so the arithmetic becomes:

Y = ((V_a − 200) / 800) × 12,500

Here V_a is the AM2 internal count of 0–1,000 representing 4–20 mA. The engineer subtracts 200 (= 4 mA in internal counts), divides by 800 (= 16 mA in counts), then multiplies by 12,500. The Arithmetic block performs this with Gain1 = 15.625, Gain2 = 0, Offset = 0, but only when the upstream amplifier is set to gain 1.

LOGO! Soft Comfort Step-by-Step Configuration

The following procedure produces a working program that displays 0–12,500 on the LOGO! onboard display and the LOGO! TDE. Open LOGO! Soft Comfort V8.4 (or later 8.x), create a new project, and follow the steps.

Prerequisites

• LOGO! 8 base module, firmware 8.3 or later (8.4 recommended for the 0–32,000 Arithmetic block range).
• LOGO! Soft Comfort V8.4 or later.
• One free analog input (I1–I4 on the base unit, or any AI on an AM2).
• Sensor wired 0–10 V (or 0/4–20 mA on AM2).

Procedure

1. Insert the analog input terminal. Drag I3 (Analog Input) onto the program. The default is 0–10 V, 0–1,000 internal.
2. Insert the first Analog Amplifier (B042). Set Sensor = I3, Gain = 10, Offset = 0. The output Ax is now 0–10,000.
3. Insert the second Analog Amplifier (B042). Set Sensor = output of block 2, Gain = 1.25, Offset = 0. Note that gain values in LOGO! are entered as floating-point numbers; firmware computes floor(Ax × 1.25) automatically.
4. Optional: replace the second amplifier with an Arithmetic block (B050). Set V = output of block 2, Gain1 = 1.25, Offset = 0. This is functionally identical but provides an explicit y = mx + b representation in the program.
5. Add a message text (B033). In the message text, drag "Analog Input" or "Function Block Output" onto the display line and select the output of the second amplifier (or the Arithmetic block). Enable "Bar graph" only if it is desired.
6. Set EN on each block to a high logic level (e.g., flag M8 or a constant 1). This avoids the default behavior of disabling the block on program download.
7. Compile and download via the LOGO! Soft Comfort PC → LOGO! Ethernet cable.
8. Verify on the LOGO! TDE or onboard display by sweeping the input from 0 V to 10 V and confirming 0, 12, 25, 37, 50, 62, 75, 87, 100, ... , 12,500 at the corresponding points.

Verification

• At 0 V input the display must show 0 ± 1 LSB (12 units).
• At 5 V input the display must show 6,250 ± 12 units.
• At 10 V input the display must show 12,500 ± 12 units.
• If the readout climbs only to 10,000, the second amplifier gain is still 1.0; check the gain field.

Solution Comparison Matrix

LOGO! 8.3 / 8.4 Firmware Considerations and Edge Cases

Siemens released the LOGO! 8.3 (0BA3) and 8.4 (0BA4) firmware generations with several analog-processing improvements documented in the release notes archived on the Siemens Industry Online Support portal:

• Arithmetic block range extension: Firmware 8.0 limited the Arithmetic block output to 0–9,999. Firmware 8.3 extended the working range to 0–32,767 in signed mode and 0–65,535 in unsigned mode. This is what enables a gain of 1.25 to produce 12,500 without an overflow flag.
• Higher analog input count for AM2 AQ: Firmware 8.4 added a non-linear curve option in the Analog Amplifier for sensor linearisation, but the integer step granularity remains 0.5 counts at the AM2 input.
• Web server variable scaling: Firmware 8.4 added scaling tags in the LOGO! web server, so an external HMI can read 0–12,500 over HTTP without requiring a separate display.

When upgrading from a pre-8.3 system, the engineer must recompile the program in Soft Comfort V8.3 or later to access the new analog range. Programs written for 0BA6/0BA7 firmwares may show an out-of-range warning if the second amplifier output exceeds 10,000; the upgrade clears the warning.

Field-Proven Pitfalls and Diagnostic Checklist

Engineers deploying the 0–12,500 solution in production should watch for the following known issues:

Resolution Recovery Strategies

If the application genuinely requires sub-12.5-unit resolution on a 0–12,500 full scale, the LOGO! ADC must be supplemented. Three strategies exist:

Strategy 1: External ADC

Mount a 16-bit ADC module (e.g., Siemens SIMATIC ET 200SP AI 4×RTD/TC) and transmit the 16-bit value to LOGO! via Modbus TCP. LOGO! 8.3 supports Modbus TCP server and client roles natively, so a 0–65,535 word from a higher-resolution ADC can be read into a network variable. The LOGO! then displays the value directly without the 12.5-unit granularity imposed by its own ADC. Resolution per LSB improves to 0.19 unit on a 0–12,500 scale.

Strategy 2: Two-point oversampling and averaging

The AM2 module supports software oversampling through the Math/Filter functions. By sampling the AM2 16 times and averaging, the effective resolution improves by log2(16) × 0.5 = 2 bits, giving a 14-bit equivalent. The resulting engineering value (after gain 12.5) reaches 0.78 units per LSB.

Strategy 3: Change sensor output type

If the sensor can output 0–20 mA instead of 0–10 V, the AM2's true 12-bit silicon and oversampling path will deliver 4,096 distinct codes (after firmware normalization to 1,000). Some industrial sensors (e.g., flowmeters, pressure transmitters) can be ordered with 0–20 mA output for this reason. Engineering units per LSB at 12,500 full scale drops to ~3 units, a 4× improvement over the 0–10 V case.

Frequently Asked Questions