Siemens LOGO! 8 I/O Configuration: 8DI 2AI 4DO Setup Guide

Overview

Siemens LOGO! 8 (the 6ED1052 family of logic modules) covers a very specific niche: low-cost, small-machine control where a full S7-1200 would be overkill. One of the most common sizing questions engineers ask is whether a single LOGO! base unit can cover a required count of 8 digital inputs, 2 analog inputs, and 4 digital outputs. The answer depends on two facts that are easy to miss in the catalog: (1) the digital inputs I1 through I8 of the LOGO! 12/24 RCE base unit are dual-function, accepting either 24 V DC discrete signals or 0–10 V analog signals on the same terminal, and (2) those two functions cannot be used simultaneously on the same channel. Once two of the eight channels are committed to analog, only six digital inputs remain. That single constraint determines whether you stay on the base module or add an AM2 analog extension, a DM8/DM16 digital extension, or both.

This article walks through the engineering decisions, part numbers, wiring rules, firmware behavior, relay-contact life, and motor-switching caveats needed to build a reliable LOGO! 8 solution around an 8DI / 2AI / 4DO requirement. All part numbers and ranges below are referenced against the Siemens LOGO! manual set and the Siemens Industry Online Support (SIOS) product pages, which should always be cross-checked against the current edition before placing an order.

LOGO! 8 Base Module Identification

LOGO! 8 has shipped in three hardware generations. The base unit is identified by the MLFB (machine-readable product number) suffix:

For an 8DI / 4DO requirement with no onboard relay concern other than standard 24 V DC loads, the most common choice is the LOGO! 12/24 RCE in its current variant. Confirm the exact MLFB against the SIOS product page for your region, as Siemens updates these part numbers periodically.

The 12/24 RCE designation breaks down as follows:

• 12/24 – supply voltage 12 V DC or 24 V DC (the same module accepts either supply, useful in mobile or battery-backed applications).
• R – relay outputs. The four outputs are potential-free changeover contacts, not transistors.
• C – integral clock/real-time calendar.
• E – Ethernet interface (RJ45) for programming and S7/Modbus TCP communication.

Base Module I/O Count and Channel Reuse

The LOGO! 12/24 RCE base module exposes the following physical terminals on its front connector:

This is the single most important rule for an 8DI / 2AI / 4DO application: each input can only be used for one purpose at a time. When you select I1 and I2 as analog inputs in LOGO!Soft Comfort, those two channels are no longer available as discrete inputs. You are left with six digital inputs (I3–I8) and two analog inputs (I1–I2 in AI mode).

LOGO!Soft Comfort shows this clearly in the I/O configuration dialog. A small colored overlay marks the terminals that have been re-purposed. Always confirm the count before generating the switch program, because a missing digital input will not produce a hard fault — it will simply appear as "always 0" to your logic, which is one of the most common LOGO! commissioning bugs.

Choosing the Right Extension Architecture

Once you know the base module covers 6DI + 2AI + 4DO, the question is how to add the missing two digital inputs while preserving the two analog inputs. The Siemens extension catalog offers three practical paths:

For the canonical 8DI / 2AI / 4DO target where the two analog signals really are 0–10 V, the cleanest engineering choice is:

1. Use LOGO! 12/24 RCE base.
2. Configure I1 and I2 as the two analog inputs (0–10 V).
3. Add a DM8 12/24R extension to recover the two missing digital inputs and gain four spare outputs.

This gives 8 + 4 = 12 digital inputs, 2 analog inputs, and 4 + 4 = 8 digital outputs. Leave spare I/O for future expansion as discussed in the source thread — the cost of one extra module is much lower than the cost of a panel rework six months later.

Wiring and Power Budget

Each extension module must receive its own 12/24 V DC supply on its power terminals (L+, M). The base module does not back-feed power through the side connector. Common wiring errors include:

• Sharing a single 24 V source across base and extension without fusing each tap individually. Use a 1 A fast-blow fuse or PTC per module.
• Mixing the LOGO! PE terminal with the 24 V return. The PE terminal is for shielding and is internally bonded; do not use it as a current-carrying conductor.
• Routing analog signal cables in the same conduit as the relay-contacts of Q1–Q4. The base AI has only modest filtering; a contactor coil wired next to it will inject transients.

Total worst-case power consumption for a 12/24 RCE plus a DM8 12/24R and an AM2 sits in the 200–400 mA range at 24 V DC. Always size the power supply with 30% headroom and verify the inrush of any solenoids or contactors on the same rail.

Relay Output Ratings and Motor Switching Caveats

The base unit Q1–Q4 and the DM8 12/24R outputs are rated by Siemens for the following typical loads (verify against the current manual for your MLFB):

These ratings look generous, but the practical electrical life collapses fast when switching DC motor windings or solenoid valves. The DC arc at contact break is much harder to extinguish than the AC arc, and 24 V DC inductive loads routinely kill LOGO! relays in weeks rather than years. The most common field failure pattern is a relay whose NO contact welds shut because a motor stalled.

Two practical remedies:

1. Always interpose an external contactor (Schneider LC1D, Siemens Sirius 3RT, etc.) between the LOGO! output and any motor larger than a small instrument fan. The contactor is sized for the motor's inrush and is field-replaceable in minutes. The LOGO! relay only sees the contactor's coil current, which is well within its rating.
2. Add a flyback diode (1N4007 across the coil) or an RC snubber across every DC inductive load that the LOGO! switches directly. This is essential for 24 V DC solenoid valves.

LOGO! Contactors for Direct Motor Loads

Where you want to keep the LOGO!-style form factor but still need to switch a small motor, Siemens offers the LOGO! Contact module. It physically clips next to the LOGO! base unit and accepts the same 24 V control voltage. Two MLFBs cover the common cases:

The LOGO! Contact module typically carries an AC-3 rating of around 9 A at 400 V 3-phase for a 4 kW motor, but always confirm the exact value on the device label and in the current product data sheet — Siemens updates these ratings between hardware revisions. The big advantage over a generic DIN-rail contactor is the visual continuity on the panel: the LOGO! Contact looks like an extension module and does not require a separate DIN section or labeling convention.

Step-by-Step Configuration in LOGO!Soft Comfort

Assuming LOGO!Soft Comfort V8.3 (or current equivalent) and a 12/24 RCE base, the configuration flow is:

1. Open the project and select the hardware. Right-click the LOGO! in the project tree and choose the correct MLFB for the base unit. If you add a DM8 extension, right-click and add it in the correct slot order. The order matters: LOGO! reads modules left-to-right, starting with the base.
2. Configure the analog inputs. Open the I/O list, click I1, and set "Used as" to Analog Input. Repeat for I2. The display in the I/O list should show two AI tags and six DI tags on the base.
3. Set the analog input scaling. In the Properties panel of each AI, set the sensor type to 0–10 V and define the engineering range (e.g., 0 V = 0 °C, 10 V = 100 °C). The block "Analog Input" in the program will return a value normalized to 0–1000.
4. Add the extension modules. Add the DM8 12/24R. The four new digital inputs appear as I9–I12 in the program; the four outputs appear as Q5–Q8.
5. Wire the program. Use the FBD editor (or ladder view) to map physical I/O to logic blocks. Drag physical tags onto inputs of blocks rather than typing the address — this catches re-mapping errors when the hardware list changes.
6. Assign message text and clock functions. If you need a real-time schedule, the C in RCE stands for the integrated real-time clock. Use the "Weekly Timer" and "Astronomical Timer" blocks for sunrise/sunset-driven loads.
7. Simulate before downloading. Press F11 (or click the simulation icon) and exercise every input combination. Watch the analog value trace.
8. Download to the LOGO!. Connect via Ethernet (PC → LOGO! direct cable, or through a switch). The transfer includes the circuit program and the hardware configuration.

Verification and Commissioning Checks

A reliable commissioning procedure for the 8DI / 2AI / 4DO architecture is:

1. Verify the I/O list physically. With the program running in run mode, force each input by shorting the terminal to 24 V and confirm the matching tag turns on in the online monitor. A wrong tag wiring is the number-one reason a LOGO! program "almost works."
2. Verify the analog scaling. Apply 0 V, 5 V, and 10 V from a calibrator to AI1 and AI2. Confirm the program sees the expected 0, 500, and 1000 values. If you see 9999 or a clipping value, the scaling or input wiring is wrong.
3. Verify relay contact operation. With the output off, measure continuity across the NO contact — it should be open. Energize the output and confirm the contact closes. Listen for the click; if you don't hear it, the relay driver or contact is faulty.
4. Verify extension module addressing. Cycle power to the base unit only. The extension should retain its addressing and re-appear within 5 seconds. If it doesn't, check the side connector for bent pins.
5. Verify Ethernet communication. Ping the LOGO! from your engineering station. If the LOGO! is unreachable, check that you are on the same subnet (LOGO! defaults to 192.168.0.1, with newer 0BA2/0BA3 units in the 169.254.x.x range on first boot).

Troubleshooting Matrix

Spare I/O and Future Growth

A practical recommendation that echoes the source thread: always leave spare capacity. The DM8 12/24R extension adds four digital inputs and four relay outputs in the same physical slot height. The cost difference between a populated 4-I/O and a populated 8-I/O panel during commissioning is small; the cost difference during a one-year retrofit is large. Siemens supports up to 24 digital inputs, 20 digital outputs, and 8 analog inputs on a single LOGO! base (counting all extension slots). Realistically, that is the upper bound for which a single LOGO! 8 makes sense — beyond that, an S7-1200 is more cost-effective.

Safety Notes

LOGO! 8 is not a safety PLC. It does not carry TÜV certification for use in safety functions up to SIL 3 or PL e. For e-stop circuits, guard doors, or two-hand control, use a Sirius 3SK safety relay or a fail-safe S7-1200F CPU. Wiring the e-stop into a LOGO! digital input and treating the LOGO! output as the safety-rated switch is a common but life-threatening error. Even for non-safety applications, always follow the local electrical code (NEC, IEC 60204-1) for grounding, fusing, and disconnect devices.

References for Further Verification

Always cross-check the specific MLFBs and ratings above against the live Siemens Industry Online Support (SIOS) portal and the current LOGO! manual set. The main entry points are:

• LOGO! product family page on SIOS (search for "6ED1052" and "LOGO! 8").
• LOGO! manual set, which includes the device manual, programming manual, and the LOGO!Soft Comfort online help.
• The LOGO! Web Configurator for sizing extensions against your I/O list.