Wiring Pushbuttons and Limit Switches to LOGO! 8 Digital Inputs

System Overview

Siemens LOGO! 8 miniature logic modules (programmable relays) accept 24 V DC digital signals directly from mechanical switches, pushbuttons, limit switches, and proximity sensors without any signal conditioning. A recurring commissioning question concerns the ability of a LOGO! input to handle the current delivered by a 24 V DC power supply sized for high-current loads such as solenoid valves. The premise itself is the result of a category error: a power supply rated 10 A does not force 10 A into the LOGO! input. The current drawn is determined by the input impedance of the receiver, not the maximum capability of the source. A LOGO! digital input draws approximately 0.3 mA at 24 V DC, so a 10 A supply has more than six orders of magnitude of headroom for the input circuit.

This article covers the wiring of a momentary normally open (NO) pushbutton and two mechanical limit switches to a LOGO! 8 base module (order code 6ED1052-1MD08-0BA1 as reported by the operator) for the purpose of controlling a reciprocating hydraulic cylinder. It addresses:

• Hardware identification of the LOGO! 8.2 base module
• Input electrical characteristics
• Pushbutton and limit switch wiring topology
• Analog/digital multiplexing on terminals I1, I2, I7, I8
• Recommended ladder/FBD architecture for start/stop, direction change, and timeout protection
• Verification procedure and troubleshooting matrix

Hardware Identification

Siemens LOGO! 8 base modules are ordered under a structured catalog code. The reported order number 6ED1052-1MD08-0BA1 is decoded in the following table. Note that standard catalog listings for the LOGO! 8.2 family most commonly carry the suffix BA0; the trailing BA1 should be verified against the physical rating plate on the right side of the module before commissioning. The electrical and logical characteristics described below apply to the entire 6ED1052-1MD08 family.

Confirm the following on the physical nameplate before proceeding:

1. Supply range: 24 V DC (allowed 20.4–28.8 V DC)
2. Eight digital inputs (I1–I8) sourced from the internal 24 V rail
3. Four relay outputs rated 10 A resistive / 2 A inductive at 24 V DC / 230 V AC
4. Analog function available on I1, I2, I7, I8 (0–10 V, 10-bit resolution)

Power Supply Architecture and 24 V Distribution

A 24 V DC power supply with a 10 A rating is typically selected because the load includes solenoid coils, each of which can draw 1–3 A inrush and 200–500 mA holding. The LOGO! base module itself draws approximately 45 mA at 24 V (with all relays de-energized and the display off) and roughly 95 mA with the display at full brightness. The eight digital inputs collectively draw on the order of 2.4 mA. Total LOGO! consumption is therefore below 150 mA — a small fraction of the supply capability.

Recommended distribution topology:

1. Single 24 V DC / 10 A power supply with electronic current limiting.
2. Branch 1 (high current, fused at 6–8 A) feeds the solenoid valves via a common 24 V rail.
3. Branch 2 (logic, fused at 1 A slow-blow) feeds the LOGO! L+ / M terminals.
4. Mechanical switches (pushbutton, limit switches) are wired between the LOGO! input terminal (I1–I8) and the LOGO! M (24 V common/ground reference) terminal. Do not wire the switch between an input terminal and 24 V positive — the LOGO! inputs are type 1 (IEC 61131-2) and sink current to M.

Digital Input Electrical Characteristics

Specifications below apply to the 24 V DC input section of the LOGO! 8 base module (per the LOGO! 8.2 system manual, Section A.2).

These figures make clear why the 10 A power supply is irrelevant to the input: the input current is set by the internal pull-up network (or, in this case, the input's reference to M), not by the supply. A mechanical switch contact simply closes a path between an input terminal and M, and the input then sources (or sinks, depending on convention) microamps through that contact.

Wiring a Normally Open Pushbutton to I1–I8

A momentary NO pushbutton is the simplest possible input device. Wire it as a "dry contact" between the LOGO! input terminal and the M terminal on the same module.

Connection List (Start Pushbutton)

No external resistor, capacitor, or power supply is required. When the pushbutton is open, the input reads 0 (LOW). When the pushbutton is closed, the input reads 1 (HIGH). The input threshold is internally defined; no user calibration is necessary.

Wiring Mechanical Limit Switches

Mechanical limit switches for hydraulic cylinder end-of-stroke position feedback are typically SPDT (single pole, double throw) devices with NO, NC, and common terminals. The recommended practice is to use the NO contact in series with the LOGO! input to M.

Connection List (Two Limit Switches)

Mechanical limit switches bounce on contact closure. The LOGO! input has no built-in debounce filter; the firmware will register the bounce as multiple transitions. The recommended practice is to enable the input debounce filter in LOGO! Soft Comfort (Tool → Input Debounce) for each mechanical input, or to handle debounce in program logic with a TON (on-delay timer) of 20–50 ms before the limit switch signal is allowed to influence the cycle.

Analog and Digital Multiplexing on I1, I2, I7, I8

On 24 V DC base modules, terminals I1, I2, I7, and I8 are dual-function: each can be used as a 24 V DC digital input or as a 0–10 V analog input. The selection is made in LOGO! Soft Comfort under File → Properties → Inputs/Outputs, by editing the input block's parameter and selecting either Digital or Analog usage. The setting is per-terminal; the unused function is then disabled.

If a 0–10 V analog sensor is connected to I1, I2, I7, or I8, the corresponding digital input function for that terminal becomes unavailable. For the hydraulic cylinder application described, where the start pushbutton and two limit switches are the only field devices, all three signals should be assigned to non-multiplexed terminals (e.g. I3, I4, I5) to avoid accidentally losing a digital function to an unused analog configuration.

Two-Wire versus Three-Wire Sensors

Mechanical switches and 3-wire PNP sensors are wired as described above. 2-wire sensors (e.g. mechanical proximity switches, reed switches, or inductive sensors that share the input pair as both power and signal) present a special case:

• 2-wire mechanical switch: Connect the two switch leads to the LOGO! input and M. The input sees a low-impedance path when closed; no residual current is required. The LOGO! 24 V digital input detects the closure reliably because its internal threshold (12 V ON, 5 V OFF) is well above the contact drop of a closed mechanical pair.
• 2-wire electronic sensor (e.g. NAMUR-style): A small leakage current flows even in the OFF state. Verify that the LOGO! OFF threshold of 5 V is not exceeded by the residual voltage of the sensor in its open state. If the sensor's leakage is high enough to register as ON, a pull-down resistor or a relay interface is required.
• 3-wire PNP sensor (sourcing): Connect the brown wire to LOGO! 24 V (L+), the blue wire to LOGO! M, and the black (output) wire to the LOGO! input. The output switches 24 V to the input, which is the opposite of the dry-contact convention. To convert, use the input as active-LOW and invert the logic in the program, or use a relay interface.

Input Debouncing in LOGO! Soft Comfort

Mechanical contacts typically bounce for 1–20 ms. To prevent the LOGO! from registering multiple transitions on a single button press:

1. Open the program in LOGO! Soft Comfort.
2. Select Tools → Input Debounce (or, on older versions, Edit → Properties on the input block).
3. Set debounce time to 20 ms for pushbuttons and 50 ms for mechanical limit switches.
4. Transfer the program to the LOGO! base module.

Alternatively, implement debounce in logic: a 30 ms on-delay timer (B001 in the program) blocks transient contacts from propagating to the latching circuit.

Program Architecture: Start/Stop with Hydraulic Cylinder

The functional requirement is a reciprocating hydraulic cylinder driven by two solenoid outputs:

• Q1 — Extend solenoid (energises directional valve to extend cylinder)
• Q2 — Retract solenoid (energises directional valve to retract cylinder)
• I3 — Start pushbutton (NO, momentary)
• I4 — LS1 (limit switch, cylinder fully extended)
• I5 — LS2 (limit switch, cylinder fully retracted)

Three program requirements must be implemented explicitly; their omission is a common commissioning defect that produces the "looping without limit switch feedback" symptom reported in the field report.

1. Latching (RS flip-flop): Use an SR (set/reset) block, B001. The Start pushbutton (I3) sets the latch. A Stop pushbutton (I6, NC contact for fail-safe) or a fault condition resets the latch. The latch output drives the cycle enable.
2. Direction selection: A second RS flip-flop, B002, toggles between Extend and Retract on each cycle. Set on Start and on LS1; reset on LS2.
3. Timeout protection: A watchdog timer (B003) is enabled whenever a direction output is active. If the corresponding limit switch does not activate within a configurable window (e.g. 10 seconds), the timer elapses and forces a fault, de-energising both solenoids and requiring operator intervention.

Ladder/FBD Implementation

The following FBD excerpt shows the core logic. It is written for LOGO! Soft Comfort V8.x and later.

Inputs:
  I3  -- Start pushbutton (NO, momentary)
  I4  -- LS1, cylinder fully extended (NO contact)
  I5  -- LS2, cylinder fully retracted (NO contact)
  I6  -- Stop pushbutton (NC, fail-safe)

Blocks:
  B001  SR flip-flop (cycle latch)
  B002  RS flip-flop (direction toggle)
  B003  TON, preset 10.0 s, base 1 s (extend timeout)
  B004  TON, preset 10.0 s, base 1 s (retract timeout)
  B005  AND (extend enable)
  B006  AND (retract enable)
  B007  OR (fault OR stop OR timeout reset)

Wiring:
  I3 --> B001.S
  I6 --> B007.in1
  B003.Q --> B007.in2
  B004.Q --> B007.in3
  B007    --> B001.R

  B001.Q --> B005.in1
  B002.Q --> B005.in2
  B005    --> Q1 (extend solenoid)

  B001.Q --> B006.in1
  NOT B002.Q --> B006.in2
  B006    --> Q2 (retract solenoid)

  Q1 --> B003.IN
  I4 --> B002.S  (set on reaching extended position)
  B003.Q --> B002.R (fault clears direction)

  Q2 --> B004.IN
  I5 --> B002.R  (reset on reaching retracted position)
  B004.Q --> B002.S (fault sets opposite direction? review)

Outputs:
  Q1  -- Extend solenoid driver
  Q2  -- Retract solenoid driver
  Q3  -- Fault indicator (e.g. beacon or HMI tag)

The exact reset/set assignment must be reviewed for the specific cycle desired. The key engineering point is that limit switch state must always be evaluated: the program must not assume a limit switch is reached on time without explicit verification. A program that simply toggles a direction flag on a timer — without reading I4 and I5 — is unsafe and will continue cycling even if the cylinder stalls or the limit switch fails.

Wiring Topology

Timing Diagram

Verification Procedure

After wiring and program transfer, perform the following verification in order.

1. Power-on test. Apply 24 V DC to L+/M. The LOGO! should power up, display the date/time, and remain in RUN. The display should read "LOGO!" and the current program name.
2. Input forcing (offline). In LOGO! Soft Comfort, open Tools → Simulation. Force I3 high. The cycle latch (B001) should set. Force I4 high; the direction flip-flop (B002) should toggle and Q1 should de-energise while Q2 energises.
3. Input forcing (online). With the program transferred to the module, press the Start pushbutton. The cycle should begin. Observe the LOGO! display or use the LOGO! Soft Comfort online monitor to verify I3, I4, I5, Q1, Q2 state.
4. Voltage measurement at each input. With the field device open, the input terminal should read 24 V relative to M. With the field device closed, the input terminal should read < 5 V relative to M. Any reading between 5 V and 12 V indicates a partial fault (high-resistance contact, poor crimp, or a damaged sensor).
5. Limit switch test. Manually actuate the cylinder to each end position. The corresponding limit switch LED on the LOGO! (visible on the front of the base module) should illuminate. The corresponding input variable in the online monitor should read 1.
6. Timeout test. Block the cylinder mechanically so it cannot reach the limit switch. After the configured timeout (e.g. 10 s), the fault output (Q3 or an HMI tag) should activate and the directional outputs should de-energise.
7. Stop pushbutton test. Press Stop (NC contact). The cycle latch should reset. Both Q1 and Q2 should de-energise immediately.

Troubleshooting Matrix

Notes on Catalog Number and Documentation

For the most accurate and up-to-date information, always refer to the official Siemens documentation:

• Siemens Industry Online Support — search for "LOGO! 8.2 system manual" (entry ID 109751616) for the full hardware and programming reference.
• The LOGO! product page on siemens.com lists current catalog numbers, firmware versions, and accessories.
• LOGO! Soft Comfort (V8.x and later) is the official programming environment; download from the Siemens support portal after registration.