LOGO! 8.3 Analog Inputs: Sizing AM2 Module Expansion Limits and Configuration Reference

The Siemens LOGO! 8.3 logic module (firmware designation 8.3, product family 0BA8) caps every CPU at eight (8) analog input channels and eight analog output channels. The 8 AI ceiling is a firmware constraint, not a physical slot limit. You can attach more AM2 expansion modules than the budget can absorb, but the surplus channels will be silently excluded by the program. This reference consolidates the base-module and AM2 specifications, the channel-arithmetic rules, the LOGO!Soft Comfort V8.3 configuration workflow, the TIA Selection Tool sizing procedure, and the field-commissioning checks an integrator needs to deploy an 8-AI LOGO! 8.3 stack without surprises.

1. LOGO! 8.3 System Architecture

The LOGO! 8.3 base module (BM) carries the CPU, integrated web server, Ethernet interface, supply terminals, the base I/O, and a bus interface on its right side. Up to 24 expansion modules can be snapped to the right of the base, daisy-chained via the integrated slide contacts. The system supports mixed digital and analog expansion modules; power and bus signals are passed module-to-module through the bus connector. Each module's bus address is set automatically by its position in the stack.

Two on-board interfaces matter for analog I/O engineering:

• Ethernet (RJ45): 10/100 Mbit/s, used for programming, the integrated web server, S7 communication, Modbus TCP, and MQTT (firmware 8.3 feature).
• microSD slot: Holds the program, the data log, and recipe storage. The data log is the primary mechanism for trending AI values over time.

The base module display (on -RCE and -CE variants) is a 6-line LCD that can show AI1 to AI8 values in engineering units when configured. The display is also the local interface for parameter changes when a PC is not present.

2. LOGO! 8.3 Base Module Variants

Three base-module variants dominate analog-input planning. The 12/24 RCE and 24 RCE both expose four native analog inputs (AI1 to AI4) on terminals I1 to I4. The 230 RCE exposes zero native AI; its eight inputs are digital-only. All three variants support the same 8-AI system ceiling.

3. AM2 and AM2 RTD Expansion Modules

Two analog expansion modules are relevant to AI capacity planning: the AM2 (general-purpose voltage / current input) and the AM2 RTD (PT100 / PT1000 / Ni1000 temperature input). Each contributes two channels to the 8-AI budget. The companion AM2 AQ is an analog output module and does not consume AI budget.

On the AM2, the input mode is set per channel: connect a voltage source to the U terminal, or a current source (with internal or external shunt) to the I terminal. Each channel can be independently configured for 0 to 10 V, 0 to 20 mA, or 4 to 20 mA in LOGO!Soft Comfort. The AM2 has no on-board 24 VDC sensor supply rail; two-wire 4 to 20 mA transmitters must be loop-powered from an external 24 VDC source.

The AM2 RTD accepts 2-wire, 3-wire, and 4-wire PT100 / PT1000 sensors. Three-wire connection is the field default; two-wire connection is permitted but requires the Enable Resistor parameter in LOGO!Soft Comfort to compensate for lead resistance. The maximum input voltage on the AM2 voltage-mode channels is 10 V; signals above 10 V will saturate the input and risk damage to the front end. For higher-voltage signals (the NI documentation for general-purpose multifunction DAQ hardware cites 42 V as a typical ±42 V maximum differential input ceiling), an external voltage divider or signal conditioner is required before the LOGO! AM2 terminal.

4. Channel Arithmetic and Maximum Configurations

The number of AI available to your program is governed by:

Total_AI = BM_native_AI + (2 × AM2_count) + (2 × AM2_RTD_count)

with the constraint Total_AI ≤ 8. The formula and the examples below assume the standard AM2 / AM2 RTD modules; the AM2 AQ analog-output module does not affect the AI total.

A common deployment is the 12/24 RCE base plus two AM2 modules: 4 + 2 + 2 = 8 AI. This is the maximum density stack without RTD inputs. A 230 RCE base plus four AM2 modules delivers the same 8 AI but in an AC-supplied installation where the on-base 0 to 10 V channels are not available.

5. Wiring, Shielding, and Sensor Power

LOGO! 8.3 base module terminals I1 to I4 are dual-use: they accept either a digital input (24 VDC) or an analog 0 to 10 V input. The selection is made per channel in LOGO!Soft Comfort, but the wiring must be consistent: once a channel is configured as analog, the digital threshold logic on that terminal is disabled.

Analog wiring best practice for the AM2:

• Use twisted, shielded pair (for example, LiYCY 2 × 0.25 mm²) for each analog channel.
• Terminate the shield at the AM2 end only. Do not bond the shield at the sensor end unless the sensor is also isolated.
• Route analog cables at least 200 mm away from VFD output cables, contactor coils, and welding leads.
• Install a 24 VDC supply near the AM2 to power 4 to 20 mA loops. Loop voltage must be at least 12 V at the transmitter terminals to guarantee linearity.
• For 0 to 10 V signals driving long cables (above 10 m), use a 0 to 20 mA current loop with a 500 Ω precision resistor at the AM2 input to convert to voltage.

The 0 to 10 V input on the AM2 and on the base module is single-ended with respect to the module's M terminal. Differential inputs are not supported. If a true differential source is required, use an external instrumentation amplifier or a Siemens SITRANS I or SITRANS P transmitter that converts to 4 to 20 mA at the source.

6. Configuring AI in LOGO!Soft Comfort V8.3

LOGO!Soft Comfort V8.3 (or later; V8.4 is the current shipping release) is required to program LOGO! 8.3 firmware. Earlier LOGO!Soft Comfort releases will read a V8.3 program but cannot access features added in 8.3 (custom web pages, MQTT, encrypted program download). Download LOGO!Soft Comfort from the Siemens Industry Online Support portal.

1. Create the project. Launch LOGO!Soft Comfort, choose File → New, and select 'LOGO! 8.3' in the device family. The right-hand catalog lists the available base modules; drag the desired variant to the project workspace.
2. Add the base module to the configuration. Double-click the LOGO! in the project tree. The Device Properties dialog shows the base module's native I/O. Confirm the AI count (0 or 4) before adding expansions.
3. Add AM2 / AM2 RTD modules. From the catalog, drag the AM2 or AM2 RTD block to the right of the base. The software will visually represent each module in the stack. The status bar shows running totals of AI used, AQ used, DI used, DQ used.
4. Open Block Properties for an AI. Click on the AI terminal of the base or AM2 module. The Block Properties dialog lists the available analog inputs (AI1 to AI8 maximum) and the input mode (0 to 10 V, 0 to 20 mA, 4 to 20 mA, PT100, PT1000). For 4 to 20 mA on the AM2, set the jumper in the channel's terminal area if required.
5. Wire AI to a function block. Drop an Analog Threshold, Analog Comparator, Analog Amplifier, Math, PI Controller, or similar block onto the diagram. Connect the AI terminal to the block's analog input. The block displays its assigned AI number.
6. Set Gain and Offset. The LOGO! 8.3 internal representation is 0 to 1000. For a 0 to 10 V input, Gain = 10.00 and Offset = 0.00 maps 0 to 1000 to 0.00 to 10.00 V. For a 4 to 20 mA input driving a 0 to 10 V input through a 500 Ω resistor (so 4 mA to 2 V, 20 mA to 10 V), set Gain = 1.25 and Offset = -2.5 to recover 4.00 to 20.00 mA at the output.
7. Compile and download. Click 'PC to LOGO!' to compile and push the program. The compiler will halt and report 'AI count exceeds maximum' if you have attempted to use more than 8 AI.

7. Sizing the Stack with the TIA Selection Tool

The TIA Selection Tool is Siemens' free browser- and desktop-based configurator for LOGO!, S7-1200, S7-1500, ET 200, and SIMOTION systems. For LOGO! 8.3, it is the authoritative source for MLFBs, slot count, supply compatibility, and module ordering.

1. Open the TIA Selection Tool and choose 'Start new configuration' → 'LOGO! Logic Module' → 'LOGO! 8'.
2. Filter the catalog by firmware designation 8.3 (search 'LOGO! 8.3' or sort by 'Newest').
3. Drag the desired base module into slot 0. The TIA Selection Tool records the native I/O automatically.
4. Drag AM2 and AM2 RTD modules into slots 1, 2, 3, and so on. The tool tracks the running AI and AQ totals in the right pane.
5. The tool highlights an over-budget configuration in red and refuses to export a valid bill of materials.
6. Click 'Export BOM' to generate the order list with MLFBs and quantities. Use this BOM when ordering from the Siemens Industry Mall or an authorized distributor.

The TIA Selection Tool also detects supply mismatches. For example, attaching a 24 VDC AM2 next to a 230 VAC base module is electrically valid (the AM2 takes its logic power from the bus), but the tool will surface the supply difference so that field wiring reflects the actual operating voltages. It will also reject combinations that violate the maximum 24 expansion module count.

8. Reading AI via Web Server, MQTT, and Modbus TCP

LOGO! 8.3 publishes AI values through three primary channels:

• Integrated Web Server: Browse to http://<logo-ip> and log in. The default view shows DI, DQ, AI, AQ status. Custom web pages can be defined in LOGO!Soft Comfort V8.3+ to display AI values with text labels and color-coded thresholds.
• MQTT (firmware 8.3 feature): Configure a broker IP, topic prefix, and payload format. AI values are published as JSON or numeric payloads on change events. The MQTT client on the LOGO! runs in the background and does not block scan time.
• Modbus TCP: The LOGO! 8.3 Modbus server exposes AI1 to AI8 at holding registers starting at address 0x0000. Each register holds a 16-bit signed integer (0 to 1000). The same register map is used for S7 PUT/GET communication.

The Modbus register map for AI is:

Use Modbus function code 03 (read holding registers) with a starting address of 0 and a count of 8 to read all AI values in a single transaction. The 0 to 1000 raw value requires client-side scaling to engineering units; LOGO! 8.3 does not perform scaling for Modbus output.

9. Data Logging AI Values

LOGO! 8.3 records AI1 to AI8 trends to the microSD card using the Data Log block. Each Data Log block stores a configurable number of samples at a fixed interval (minimum 100 ms). The log is a CSV file named by the block's identifier and can be pulled via the web server, FTP, or by removing the microSD card.

1. From the LOGO!Soft Comfort toolbox, drag a Data Log block onto the diagram.
2. Connect the desired AI to the block's input.
3. Configure the sample interval (for example, 1 s, 10 s, 60 s, 1 h) and the maximum number of samples (for example, 10,000 for a 24-hour trend at 10 s intervals).
4. Set the file name and the storage path on the microSD card.
5. Download the program and start logging. The LED on the LOGO! base will indicate active logging when configured.

The microSD card must be a Siemens-approved card or any FAT32-formatted SDHC card up to 32 GB. Industrial-grade cards are recommended for installations with high vibration or temperature extremes.

10. Practical Application Examples

10.1 HVAC zone controller (4 AI)

A 12/24 RCE base module covers a four-zone HVAC controller with no expansion: AI1 zone 1 supply air temperature (PT100 via on-board 0 to 10 V with scaling), AI2 zone 1 return air humidity (4 to 20 mA), AI3 zone 2 supply air temperature, AI4 zone 2 return air humidity. The 8-AI budget is left unused but available for future expansion (for example, outdoor air quality sensor on AI5 from an AM2 module).

10.2 Tank farm level monitoring (6 AI)

A 12/24 RCE base plus one AM2 module: AI1 to AI4 read hydrostatic level transmitters (4 to 20 mA) on tanks 1 to 4. AI5 reads a 0 to 10 V signal from a fifth tank's ultrasonic level sensor with 0 to 10 V output. AI6 reads a 4 to 20 mA signal from a common pressure transmitter on the manifold. Total 6 AI, 2 AI slots spare.

10.3 Boiler control with mixed AI and RTD (8 AI)

A 12/24 RCE base plus one AM2 plus one AM2 RTD: AI1 to AI4 read steam pressure, drum level, feedwater flow, and combustion air damper position (all 4 to 20 mA). AI5 reads the oxygen probe (0 to 10 V). AI6 reads the stack draft (4 to 20 mA). AI7 to AI8 read two flue gas temperatures (PT100 sensors on the AM2 RTD). This stack reaches the 8-AI ceiling with mixed voltage, current, and RTD inputs.

11. When 8 AI Is Not Enough: Migration to S7-1200 or ET 200SP

If the application requires more than 8 AI, the LOGO! 8.3 platform cannot be stretched. Common migration paths:

• S7-1200 CPU 1214C with SM 1231 AI modules: The SM 1231 AI4 (6ES7231-4HD32-0XB0) provides 4 AI per module. Up to 8 SMs can be added to a 1214C, yielding 32 AI per CPU. Resolution is 13-bit (default) or 16-bit (extended).
• S7-1200 CPU 1215C / 1217C with SM 1231 AI8: The higher-density SM variant supports 8 AI per module, allowing a 1215C + 1 SM to deliver 16 AI. Resolution is 15-bit plus sign.
• ET 200SP distributed I/O: AI Energy Meter, AI 2/4/8 modules, and the AI RTD module can be combined under a PROFINET or PROFIBUS head. ET 200SP supports per-channel diagnostics and hot-swap, which LOGO! does not.

The cost and complexity trade-off is significant: an S7-1200 + TIA Portal project has a higher per-channel cost and a steeper learning curve than LOGO!Soft Comfort, but the channel ceiling is far higher and the diagnostic capability is far richer. For a project that starts at 8 AI and is expected to grow, commissioning an S7-1200 from the outset is the conservative choice.

For applications that need more than 8 AI but cannot justify a full S7-1200, the Productivity2000 line from AutomationDirect is a comparable modular controller with 16-channel analog input modules; verify the application fit against the LOGO! instruction-set model before substituting controllers.

12. Troubleshooting Matrix and Field Verification

Field-Commissioning Verification Procedure

1. From the LOGO! base module display, navigate to AI1 through AI8. Each AI should show a plausible raw value (0 to 1000) consistent with the sensor input.
2. From LOGO!Soft Comfort, open Online → Test. For each wired AI, force a known input (for example, 5.00 V from a calibrator) and verify the function block outputs match the expected engineering units.
3. Cycle power to the LOGO! 8.3. The AI values should reappear within 2 seconds of boot, and the expansion module activity LEDs should be steady green.
4. If an AM2 was added at the end of an existing stack, re-check the slot count against the configured program. Mismatches between the physical and configured slot lists are a common cause of 'AI not found' errors at runtime.
5. Pull a Data Log from the microSD card and verify the timestamps and AI values match the engineering reference.
6. Verify the Modbus TCP register map (holding registers 0x0000 to 0x0007) against a SCADA poll.

13. Safety Considerations

LOGO! 8.3 is not a safety PLC. It is not certified to SIL 1 or higher per IEC 61508. If the application requires functional safety (for example, burner control, emergency stop, two-handed control), use a Siemens S7-1200F or S7-1500F CPU with F-I/O, or a dedicated safety relay. The 8-AI limit is not the only reason to migrate; safety integrity is a separate driver.

Analog inputs on the LOGO! 8.3 and the AM2 are not isolated channel-to-channel. If the application connects multiple sensors at different ground potentials, use isolated transmitters or external signal conditioners to prevent ground loops. If the analog input is at risk of over-voltage (for example, from a miswired transmitter or a lightning-induced surge), install a TVS diode or a Siemens SITRANS protector at the AM2 terminal.

Frequently Asked Questions