Siemens LOGO! 0BA6 Output Wiring Polarity Rules and F1 Key Latch

Overview

When a Siemens LOGO! 0BA6 Pure 24V controller (order number 6ED1057-3BA10-0BA0) refuses to release output Q1 — and the program runs flawlessly in LOGO!Soft Comfort simulation — the cause is almost never the polarity of the wires on the Q1 output terminals. The classic “Q1 will not de-energise” symptom on a relay-output LOGO! traces back to one of three things:

1. The wiring order on the Q1 terminals (a real concern only for transistor-output LOGO! variants, not for the RC / relay Pure 24V in the case study).
2. The function-key behaviour of the LOGO! TD (Text Display), specifically F1, and the difference between the “switch” contact setting used in LOGO!Soft Comfort simulation and its actual hardware behaviour on the real controller.
3. The latch / flip-flop configuration that the F1 input drives — particularly the order in which Set and Reset pulses arrive on the same scan cycle.

This technical reference documents the diagnosis and resolution path for a real field case, the wiring rules that apply to the LOGO! Pure 24V (Type 0BA6) relay outputs, and the F1 key behaviour on the LOGO! TD that is the single most common cause of “Q1 will not de-energise” complaints on a freshly commissioned Pure system. The official LOGO! system documentation is available from Siemens at LOGO! System Manual (Siemens, PDF).

Identifying the Controller: LOGO! Pure 24V (6ED1057-3BA10-0BA0)

The order number decodes as follows:

The Pure designation means the base module ships without an integrated display or keypad. Operator interaction is delegated to the LOGO! TD (Text Display), a remote panel that connects via the dedicated bus cable and exposes the F1–F4 function keys, the OK and ESC keys, and the four cursor keys. The informal “RC” label — Relay with Clock — refers to a relay-output variant of the LOGO! 6 generation, which is the most common Pure 24V configuration.

Two hardware facts drive everything else in this article:

• The Q1, Q2, Q3, Q4 outputs on a relay-type LOGO! are dry contacts. They have no internal polarity reference and are not tied internally to 0 V or +24 V.
• The TD’s function keys (F1, F2, F3, F4, ESC, OK, ▲, ▼, ◄, ►) are read by the LOGO! firmware as ordinary digital inputs. They appear in the program editor as inputs I9 through I16 and obey the same logic-edge rules as I1–I8 on the base module.

Output Architecture: Relay vs Transistor LOGO! Variants

The Q outputs on a LOGO! come in one of two technologies, and the wiring rules differ fundamentally between them.

Relay outputs (the 6ED1057-3BA10-0BA0 variant)

• Mechanical changeover or normally-open contacts driven by an internal coil.
• Rated for both AC and DC loads, with the load circuit at any voltage up to the contact rating.
• Polarity-insensitive — the contact is electrically a switch, not a semiconductor.
• Switching speed limited to the relay pull-in and drop-out times (single-digit to low double-digit milliseconds).
• No internal short-circuit protection beyond the module’s overall fusing.

Transistor outputs

• Solid-state switches, typically MOSFET.
• Source (PNP, common on the 24 V Pure with transistor suffix) or sink (NPN) topology depending on the variant.
• Polarity-sensitive: the load must be wired between the switched terminal and the appropriate supply rail. Reversing the connection can hold the output in a partially-on state through the body diode or destroy the driver.
• Faster switching (sub-millisecond) and longer electrical life.
• Required for PWM, high-speed counters, and pulse-train output.

The 0BA6 Pure 24V with order code 6ED1057-3BA10-0BA0 is a relay-output variant. The two terminals of the Q1–Q4 pair are a switched contact; one terminal is the common and the other is the normally-open (or changeover) leg, depending on the printed marking on the housing. Either wire can be at +24 V and the other can return to 0 V; the contact closes and opens based purely on the internal coil, not on the polarity of the wires you connect to it.

Q1 Terminal Wiring: Polarity Rules in Practice

field reports connected +24 V to the left-hand Q1 terminal and routed the load to the right-hand Q1 terminal, then to 0 V. On a relay-type LOGO!, this is electrically equivalent to the manual’s suggested drawing (+24 V on the right, load on the left) and the output will function identically. The manual’s drawing is a documentation convention, not an electrical requirement.

Where polarity does matter:

• Transistor-output LOGO! modules (24 V supply): the switched terminal is the source; the load must return to 0 V. Reversing these wires on a sinking variant keeps the output effectively on because the MOSFET body diode is forward-biased through the load.
• Mixed-voltage installations where the relay contact is switching a different DC supply than the LOGO!: the polarity of the load-side wiring follows the load’s supply, not the LOGO!’s. AC loads are unaffected.
• Indicator LED wiring on the panel: if the LED indicator on the Q1 output terminal is monitored for the actual relay state, the indicator’s polarity matters; the relay contact does not.

For a deeper reference on connecting two-wire sensors (0–10 V / 4–20 mA) and other analogue devices into the LOGO! inputs, see the Siemens support article Connecting LOGO! inputs – ID 109741041.

The LOGO! Text Display: Function Keys as Digital Inputs

The LOGO! TD is not a passive display. The F1–F4 keys are wired into the LOGO! firmware as input bits and become available in the program editor alongside I1–I8. The default mapping on a 0BA6 system is:

These inputs are evaluated on the LOGO! base module’s scan cycle, which is single-digit to low double-digit milliseconds depending on program length. They behave exactly like a 24 V digital input wired to I1–I8, except that the logic level is generated by the TD’s keypad scanner and bus interface rather than by an external switch and a terminal voltage.

Push-button vs switch contact in the FBD

When you place an F1 contact into the FBD/LAD diagram in LOGO!Soft Comfort, the default contact is a normally-open push-button — it closes for the duration of the key press. To get toggle (“push on, push off”) behaviour, you must use one of the following constructs:

• An SR (Set/Reset) or RS (Reset/Set) flip-flop with the F1 contact driving the Set input and a second F1 contact driving the Reset input (the same key, the same contact, used twice).
• A pulse relay (current-leading edge or trailing-edge).
• A counter with a threshold trigger and retentive behaviour.

The “switch” contact option in the LOGO!Soft Comfort contact selector — the symbol that looks like a hand-actuated switch — is a simulation convenience only. It makes the F1 input appear latched in the simulator, but it does not change how the real LOGO! reads the F1 key. On real hardware, the F1 contact closes while the key is held and opens the moment it is released; the firmware does not stretch the pulse to a full scan, and it does not toggle. Double-pressing the key gives two scan-cycle pulses, not a toggle.

The “Switch” Contact Type: Simulation-Only Artefact

The single most common cause of a “Q1 will not de-energise” complaint on a freshly commissioned LOGO! Pure with a TD is the assumption that the “switch” contact type is real. It is not. The contact-type selector in LOGO!Soft Comfort includes:

• Normally-open (NO) push-button — real hardware: closes for the duration of the press.
• Normally-closed (NC) push-button — real hardware: opens for the duration of the press.
• “Switch” (toggle) — simulation only: toggles its state in the simulator on every click. On real hardware it behaves as a normally-open push-button.

This means that if you wire F1 (as a “switch” contact) into the Set input of an SR flip-flop, and the same F1 (again as a “switch” contact) into the Reset input of the same flip-flop, the program simulates perfectly: one click sets, the next click resets. On real hardware, a single press sends a Set pulse and a Reset pulse on the same scan, the Reset wins (RS flip-flop priority), and Q1 never goes high. A double press gives one Set pulse on the first press and one Reset pulse on the second press — or two of each, depending on the firmware’s edge handling — and the operator can end up in any of the four possible flip-flop states.

The correct pattern is:

1. Place two normally-open push-button contacts of F1 (not “switch”) on the ladder, one driving S, one driving R.
2. The first physical press sends a one-scan Set pulse; the output latches.
3. The second physical press sends a one-scan Reset pulse; the output releases.

If both contacts reference the same F1 input, the firmware still gives two distinct edge events because the key is released between presses. This is the same pattern as wiring two physical push-buttons to I1 and I2.

Root Cause: F1 Key Double-Press on a 0BA6

The original fault on the 6ED1057-3BA10-0BA0 was that Q1 would not de-energise after being commanded off via F1. The simulation worked, the wiring was correct (relay output, polarity-insensitive), and the program compiled. The root cause was a commissioning-time haste condition:

• F1 was configured as a “switch” contact in the simulator.
• During real-hardware testing, the operator pressed F1 once to turn the output on. The output came on.
• The operator pressed F1 a second time to turn the output off. The output stayed on.
• The operator pressed F1 a third, fourth, and fifth time. The output still stayed on.

Why? Because on real hardware, F1 as a “switch” contact is just a normally-open push-button. Each press fires a one-scan pulse on the F1 input bit. The SR flip-flop in the program received the pulses in a sequence that left Q1 latched. There was no “off” command being sent because the F1 input was never held down; it was only pulsed, and the flip-flop was wired such that a pulse with no second edge could not produce a Reset event.

Diagnostic sequence

Step-by-Step Resolution Procedure

The corrected program uses F1 as a digital input with the proper normally-open push-button contact type. The procedure to reproduce it is:

1. Open the project in LOGO!Soft Comfort. The 0BA6 generation is supported by V7.x and later versions; newer software can read older projects.
2. Delete the existing F1 “switch” contact references from the SR/RS flip-flop. Use the program editor, not the simulation monitor.
3. Insert a normally-open F1 contact into the Set (S) input of the flip-flop.
4. Insert a second normally-open F1 contact into the Reset (R) input of the same flip-flop.
5. Compile and simulate the program. With the corrected contact type, the simulation still behaves correctly.
6. Transfer the program to the LOGO! using the LOGO!Soft Comfort Transfer menu, or copy LOGO! → PC / PC → LOGO! via the TD menu tree. Verify the transfer reports success and no errors.
7. Test on real hardware. Press F1 once — Q1 should latch on. Press F1 a second time — Q1 should release. The TD message text should display the current Q state throughout.

Verification and Commissioning Checks

Common LOGO! Output Fault Matrix

Programming Notes and Best Practices

• Always use the normally-open push-button contact type for function keys. The “switch” symbol is a simulation-only artefact and a known trap for new LOGO! users.
• Use one TD key per Set/Reset input if you need deterministic, easily debuggable toggle behaviour. F1 = Set, F2 = Reset is far more robust than F1 = Set, F1 = Reset because it eliminates any chance of Set and Reset being interpreted on the same scan.
• Enable the message text on the TD to show Q1 (and any other output) state. The TD’s screen doubles as a one-line diagnostic that confirms the firmware is doing what you think it is doing.
• Add a watchdog or timeout in the program if the F1 key is safety-relevant. A button that requires the operator to keep it pressed for more than X seconds before latching Q1 is a simple de-bounce and intent-confirmation pattern.
• Avoid mixing “switch” and push-button contacts of the same F key in the same block. The simulator will treat the two as independent, but on real hardware the firmware reads the same input bit.
• Label the wires at the Q terminals with the function (e.g. “Q1 NO” / “Q1 COM”), not with the voltage. The relay contact has no polarity; the wire label should reflect the topology, not the polarity.
• Document the F-key mapping in a comment block at the top of the LOGO!Soft Comfort program so that anyone reading the project six months later knows which key does what.
• Pull the program from the LOGO! to LOGO!Soft Comfort on every service visit, even if you did not change the program. The on-line copy is the only ground truth of what is actually running on the controller.

Frequently Asked Questions