Siemens LOGO! 0BA8 Lift Control Timer, Interlock, and Display

1. System Overview: 4-Post Auto Lift Control

The application is a 4-post automotive lift (auto lift) controlled by a single Siemens LOGO! 0BA8 micro-PLC. Three momentary push-buttons share one pneumatic valve (PV1), one hydraulic valve (HV1), and one VFD-driven motor for vertical travel. The operator panel is hard-wired (non-latching contacts) and the LOGO! enforces the safe operating sequence: Up, Down-to-Safety, and Down with timed VFD assist. The control system must reject the simultaneous activation of any two motion inputs as a hard fault.

1.1 Functional Specification

The "1 second VFD pulse on DOWN" exists to drive the lift carriage off the mechanical safety pins, after which the operator lets gravity lower the platform with the valves still open. Holding I3 continuously must keep the valves open the whole time — only the VFD forward command is time-limited.

2. Hardware Selection: LOGO! 0BA8 Family

The 0BA8 hardware generation is required because earlier 0BA6/0BA7 firmware cannot drive the TDE text display with full three-color backlight control. The recommended build separates the basic module (no display) from a remote TDE so the operator panel can be flush-mounted at eye level while the BM is DIN-rail mounted inside the enclosure. This also gives a spare on-board display option by simply swapping the BM if the TDE is ever removed for service.

3. Power Supply Sizing and Protection

The 24 VDC rail feeds the LOGO! BM, the TDE, the valve solenoids, and the VFD control inputs. Sizing must cover the continuous load plus the worst-case solenoid inrush.

Continuous worst-case: 200 + 120 + 125 + 250 + 30 = 725 mA. The 4 A supply (6EP1332-1SH52) provides 5.5× headroom on continuous load and absorbs the ~1.9 A solenoid inrush event without sagging the LOGO! supply rail. If the lift will be expanded with additional valves or a 24 V buzzer, size the supply for the future load plus 30% reserve.

4. I/O Mapping and Terminal Assignment

For the transistor output variant (catalog suffix MD00), each DO sinks to M (24 V common) and is rated 0.3 A continuous. If your solenoids draw more than 0.3 A (typical 5 W pneumatic coils draw ~0.2 A, but DC hydraulic valves can hit 0.5–1.0 A), an interposing relay is mandatory. The relay-output variant 6ED1052-2MR00-0BA8 (5 A per output, 10 A common) is the safer choice when driving valve coils directly.

5. Interlock Logic: Mutual Exclusion of Motion Commands

The functional requirement is explicit: if two buttons are pressed at the same time, all outputs must go off. A two-button interlock also protects the VFD from a forward/reverse cross-connection. The cleanest implementation in LOGO! is one AND cluster per active command with NOT gates on the other two motion inputs, then a global disable from any pair-coincidence detector.

5.1 Per-Command Enable

UP_EN    = I1 AND NOT(I2) AND NOT(I3) AND I4
SAFE_EN  = NOT(I1) AND I2 AND NOT(I3) AND I4
DOWN_EN  = NOT(I1) AND NOT(I2) AND I3 AND I4

5.2 Pair-Coincidence (Fault) Detector

FAULT = (I1 AND I2) OR (I1 AND I3) OR (I2 AND I3)
Q1 = (UP_EN) AND NOT(FAULT)
Q2 = (UP_EN OR DOWN_EN) AND NOT(FAULT)
Q3 = (SAFE_EN OR DOWN_EN) AND NOT(FAULT)
Q4 = FAULT   ; amber backlight indicator

When FAULT is true, every output de-energizes within one LOGO! scan (typically < 10 ms on 0BA8), and the operator must release both buttons to clear the fault. This is a category-1 stop per ISO 13849-1, suitable for non-safety control of a single VFD; for safety-rated motion, migrate the E-stop chain to a dedicated safety relay or LOGO! 8 with the SIL variants.

5.3 Interlock Logic Schematic

6. Timer Logic: Generating a 1-Second VFD Pulse

The DOWN command requires the VFD to run for exactly 1 second, regardless of how long I3 is held. Two block patterns solve this in LOGO!Soft Comfort V8.

6.1 Solution A — Pulse Generator (Monostable, recommended)

The Pulse generator block (B004 in this design) outputs a fixed-width high pulse on the rising edge of its trigger. Configure the pulse time to T = 1.0 s. Wire the trigger to the rising edge of DOWN_EN. The output of the pulse generator feeds Q1 through the interlock gate.

; LOGO!Soft Comfort ladder excerpt
; B001: I1        (UP button, direct)
; B002: I2        (SAFE button, direct)
; B003: I3        (DOWN button, direct)
; B004: I3 — rising edge — PULSE GEN T=1.0s
; B005: AND(I1, NOT I2, NOT I3, I4) — UP_EN
; B006: AND(NOT I1, I2, NOT I3, I4) — SAFE_EN
; B007: AND(NOT I1, NOT I2, I3, I4) — DOWN_EN
; B008: OR(I1, I2) — FAULT (continue adding pairs)
; B009: AND(PULSE_OUT, UP_EN_OR_SAFE_EN, NOT FAULT) — Q1 VFD
; B010: AND((UP_EN OR DOWN_EN), NOT FAULT) — Q2 PV1
; B011: AND((SAFE_EN OR DOWN_EN), NOT FAULT) — Q3 HV1

6.2 Solution B — Off-Delay on a Derived Edge

If you prefer the TOF block (B005 = off-delay, T = 1.0 s), feed it a signal that is HIGH only on the rising edge of I3 and goes LOW immediately. This requires an extra XOR/edge-detection block combination. The cleanest version uses the built-in Edge-triggered pulse generator (different from the on-delay/off-delay pair) available in the Special Functions catalog of LSC V8.

6.3 Timing Diagram

The timing diagram confirms: while I3 is held from t = 100 to t = 400 (300 ms in this example), PV1 and HV1 follow I3 directly, while Q1 (VFD) produces a clean 1 s pulse starting on the rising edge of I3 and ending at t = 180, well before I3 is released. The valves close exactly when I3 is released. This is the behavior the operator expects.

7. Display Programming: TDE Message Texts and Backlight

The TDE 6ED1055-4MH00-0BA1 supports up to 50 message text blocks in LOGO!Soft Comfort V8. Each message has configurable trigger conditions, priority, and a backlight color (white, amber, or red). There is no native green backlight on either the BM with display or the TDE — the user’s earlier confusion was driven by community videos showing only the off-state (which appears greenish-white on a poor LCD panel).

7.1 Required Display States

7.2 Configuring the TDE in LOGO!Soft Comfort V8

1. Open Tools → Message Text Configuration.
2. Add a new message and assign a trigger (an M-flag, input, or function block output).
3. Set the Acknowledge (ACK) parameter to “No ACK” for status screens and “With ACK” for fault screens.
4. To change message text from German to English: the language follows the LOGO! system language setting (File → Properties → Language). Switch the project language to English (US) or English (UK), then re-type the message text strings. Existing German text is not auto-translated.
5. Compile and transfer. The TDE will not display any text in simulation mode; you must transfer the program to the actual 0BA8 hardware to verify the TDE output.

8. LOGO!Soft Comfort V8: Simulation vs Hardware Transfer

Three behavioral differences between LSC simulation and real-hardware execution cause most of the “works in SIM, fails in hardware” complaints.

8.1 Required Program Hygiene

• Every digital input must be assigned a unique input number; re-using I1 for two functions is not allowed.
• All function blocks must have their output connected to a Q, M, or another block input.
• The program must be transferred to the 0BA8 with the TDE powered (the TDE will refuse a download if its own firmware version does not match the BM).

9. Function Block Program Architecture

The complete FBD can be built from nine reusable blocks. Block numbering below matches the LSC convention where B001 is the first placed block.

Note: the B004 OR block can be built from three individual 2-input AND blocks followed by a 3-input OR, or directly as a single OR if the LSC version supports multi-input OR. In LSC V8 the OR block accepts up to four inputs.

10. Verification, Commissioning, and Field Checks

1. Pre-power check. With the SITOP supply de-energized, verify all wiring with a multimeter. Confirm there is no continuity between 24 V and PE anywhere on the LOGO! terminals.
2. BM-only power-up. Energize the supply and observe the BM status LED (green steady = RUN). If the LED is flashing red, the program is missing or invalid — re-transfer.
3. Input simulation in LSC online mode. Connect via Ethernet and force I1 HIGH for 5 seconds. Q1 and Q2 should follow. Force I3 HIGH and measure Q1 pulse width with a stopwatch; it should be 1.0 s ± 50 ms.
4. Two-button interlock test. Manually press I1 and I3 simultaneously. All three outputs (Q1, Q2, Q3) must drop within one scan. The TDE message should turn red and display the FAULT screen.
5. Air-pressure loss test. Vent the air supply so I4 goes LOW. All motion commands must be refused. The TDE should turn amber and show the “Air pressure LOW” message.
6. End-to-end motion test. With the lift unloaded and safety pins clear, run a full UP/DOWN-TO-SAFETY/DOWN cycle. Confirm VFD reverse coast time is ≥ 2 s before the next forward command to avoid DC-bus trip.
7. Power-loss ride-through. Cut 24 V while the lift is mid-travel. On power restore, no outputs should re-energize without a fresh button press (the LOGO! 0BA8 cold-starts with all outputs OFF, which is the correct safety behavior).

11. Troubleshooting Matrix

12. Field Notes and Lessons Learned

• Beginners instinctively reach for a TON timer for “run for X seconds” tasks. The TON is the wrong primitive; the pulse generator or the edge-triggered TOF is correct.
• LOGO! simulation is a logic simulator, not an HMI or backlight simulator. Plan a hardware dry-run for any change that affects the TDE.
• Every function block output must terminate. The simulator will let you ship broken logic; the hardware will not.
• The TDE has three backlight colors (white/amber/red) and a green-tinted off state, not a green on state. This is a common point of confusion in older YouTube tutorials where the unlit panel is photographed next to a green sticker.
• Use a SITOP 6EP1332-1SH52 (or its predecessor -1SH51) for the 24 V supply — it has the same footprint and an extra diagnostics contact, useful for fault logging.
• Add a 1000 µF / 35 V electrolytic capacitor across the LOGO! power terminals if you experience brown-outs during valve inrush; this is a well-known field fix for 0BA8 installations with long 24 V cable runs.