Wiring Siemens LOGO! to ULCT 900/108 E Light Curtain Inputs

1. Overview

The ULCT 900/108 E is a Type 4 / SIL 3 safety light curtain used on packaging machinery, press lines, and assembly cells. Its outputs are OSSD1 and OSSD2 (Output Signal Switching Devices), each switching a 24 VDC sourcing signal in the clear state and dropping to 0 V in the obstructed or fault state. Because both outputs are short-circuit, cross-fault, and overload monitored by the light curtain's internal logic, they can be read as a single 24 V digital level on a Siemens LOGO! 8 base module (6ED1052-1xxxxx-xBAx) only when the application is a non-safety counting or presence-detection task. LOGO! is not certified to EN ISO 13849-1 PL d/e or IEC 61508 SIL 2/3 and must never be used in a safety path.

2. Prerequisites

Verify that your LOGO! base module is one of the DC variants. The 230 VAC unit's digital inputs are designed for mains-level signals and will be destroyed by direct connection of an OSSD output.

3. ULCT 900/108 E Pinout Reference

The light curtain receiver uses a standardized M12 8-pin A-coded connector. The transmitter is powered through the same cable harness via a separate M12 connector on the emitter head.

The transmitter end carries only +U_b, −U_b, and a periodic test pulse line; it is wired back to the same 24 V supply. There is no signal return from the emitter to the receiver.

4. Operation Mode Configuration

The ULCT 900/108 E ships in a default mode that requires EDM (External Device Monitoring) and restart interlock — both intended to drive a safety contactor. When feeding a single LOGO! input for non-safety counting, both functions should be disabled.

1. Open the receiver housing by removing the four end-cap screws.
2. Locate the 4-position DIP switch block labeled S1 on the receiver PCB (refer to page 7 of the operating instructions).
3. Set switch positions 1 and 2 to the -- configuration: no EDM, no restart interlock, automatic restart on clear.
4. Switch positions 3 and 4 select beam coding and range — leave at factory default unless long-range blanking is required.
5. Reinstall the end cap; torque the screws to 0.6 N·m to maintain IP65.

5. Three-Wire Wiring Procedure (Non-Safety Counting)

With the receiver configured to -- mode, only three conductors are required between the light curtain and the LOGO! base module.

1. Connect Pin 2 (brown, +U_b) to the +24 VDC terminal of the LOGO! power supply. The same rail can feed the LOGO! base module terminal L+.
2. Connect Pin 7 (blue, −U_b) to the M terminal of the LOGO! and to the negative rail of the power supply. Maintain a single-point ground to avoid ground-loop noise that could cause spurious OSSD dropouts.
3. Connect Pin 5 (grey, OSSD1) to any LOGO! digital input from I1 through I8. OSSD2 (Pin 6, pink) is left unconnected for this single-channel non-safety use; tie its shield pigtail to ground at the LOGO! end only.
4. Tie the cable shield to PE at one end only — typically at the cabinet entry gland.

5.1 SVG Wiring Diagram

6. Alternate Wiring — Default Mode (EDM + Restart Interlock Retained)

If the light curtain is left in factory default mode, Pins 1 and 4 must each be tied to +24 V to release the interlock and clear EDM. This mode is preferred when an inspector will later retrofit a safety relay, because the light curtain behaviour remains identical to a guarded machine installation.

1. Bridge Pin 1 (white) to the +24 V rail through a normally-open reset pushbutton (or jumper for non-safety).
2. Bridge Pin 4 (yellow) directly to +24 V to enable automatic restart.
3. Pin 3 (EDM, green) can be left open only if the light curtain's EDM is set to disabled in the same DIP block; otherwise loop it to +24 V through the N.C. auxiliary of the contactor being monitored.
4. Pin 6 (OSSD2, pink) is still not required for a single LOGO! input, but tying it to the same input terminal (in parallel with OSSD1) does not damage the device and is a useful diagnostic — both outputs will drop together on a beam break.

7. LOGO! 8 Digital Input Specifications

OSSD outputs are rated to source 24 VDC at up to 200 mA per channel with a 0.5 ms test pulse at 1 s intervals. The pulse is short enough that the LOGO! RC input filter does not see it as a logic-low transition provided the input debounce is left at the default 0 ms on the high-speed counter block. If you use a slow I input on I5–I8, leave the LOGO! software debounce at default 0 ms — increasing it to 50 ms may swallow legitimate beam-break events on a fast-moving part.

8. LOGO!Soft Comfort Program Example

The following program uses input I3 to increment an up/down counter that tallies parts falling past the light curtain on an assembly line. The counter value is shown on the LOGO! onboard display and, in a typical deployment, mirrored to a Modbus TCP register via the LOGO! CMR or the integrated Ethernet port.

// LOGO!Soft Comfort V8.4 - LAD/FBD style description
// Block list:
//   B001  --[ I3 ]--------( Up/Down Counter Cnt1, threshold 1000 )
//   B002  --[ I3 = 1 ]-----( Set Output Q1, count pulse indicator )
//   B003  --[ Cnt1 = CV ]--( Display message "PARTS: nn" on LOGO! display )
//   B004  --[ Reset pushbutton I4 ]--( Reset Cnt1 to 0 )

// Tag addresses for LOGO! 8 mapping (Ethernet):
//   I3  = 0x0003   (OSSD1 from ULCT 900/108 E)
//   I4  = 0x0004   (Operator reset pushbutton)
//   Q1  = 0x0001   (Count pulse indicator lamp, optional)
//   Cnt1.CV = VM 0 (current count, accessible via S7 communication)

For higher-speed counting (> 5 Hz on small parts), use the dedicated high-speed counter block HSC with I1 as the source input. The HSC block accepts a 24 VDC signal at up to 5 kHz, which exceeds the 1 kHz typical response of the ULCT receiver output stage.

9. Verification and Commissioning

1. Power up the light curtain and verify the status LED is solid green. An amber or red LED indicates a configuration mismatch — re-check the DIP switches.
2. Open LOGO!Soft Comfort, go online, and force-monitor input I3 using the online test function. The flag should be 0 in the obstructed state and 1 in the clear state.
3. Pass a hand, then a known 30 mm test rod, through the curtain. The input should toggle with a 10–30 ms propagation delay (ULCT receiver response time, typically 14 ms at 108 beams).
4. Run the program for 5 minutes while the line is producing parts. Confirm the counter increments on every beam break and that no double-counting occurs — if it does, increase the LOGO! input debounce to 5 ms (do not exceed 10 ms without consulting the light curtain datasheet).
5. Cycle power and verify the counter retains its value only if you have configured a retentive data block; by default, LOGO! counters reset on power cycle unless the Retentive flag is set in the Cnt1 properties.

10. Safety and Compliance Considerations

The ULCT 900/108 E is a Type 4 ESPE (Electro-Sensitive Protective Equipment) per EN 61496-1, rated for use in safety circuits up to PLe / SIL 3. Once the EDM and restart interlock are disabled at the DIP level, the device no longer functions as a complete safety function — the LOGO! is acting only as a non-safety signal sink.

• Per Siemens product documentation, LOGO! 8 base modules do not carry functional-safety certification and must not be relied on for personnel protection.
• If a future retrofit adds a safety relay (Pilz PNOZ s4, Sick FX3-XTIO, or Sirius 3SK1111), reconnect OSSD1 and OSSD2 to that device, restore the DIP defaults, and re-validate the safety function per EN ISO 13849-2.
• Equipment carrying the UL Certified mark for North America must be installed per the Canadian Electrical Code / NFPA 70. The LOGO! 8 family is UL Listed under file E204553; the ULCT 900/108 E is UL Recognized for industrial control panels.
• Use a 24 VDC SELV/PELV power supply that is itself UL Listed (Class 2 output) to keep the cabinet listing intact.

11. Alternative Sensor: E3FB-BP22 Photoelectric Switch

For higher-speed counting of small elements — for example, parts falling off an assembly conveyor — a diffuse-reflective photoelectric sensor such as the Omron E3FB-BP22 (PNP, 24 VDC, 200 mm range, M12 connector) is a common substitute. It offers:

• 1 kHz switching frequency — well above the 70 Hz typical of a light curtain
• Single PNP dark-on / light-on output (Pin 4 = brown +V, Pin 3 = blue 0 V, Pin 1 = black output, Pin 2 = white not used)
• Direct connection to the same LOGO! 24 V digital inputs with no DIP configuration step

Wire the E3FB-BP22 to a high-speed counter input on I1–I4. This avoids the OSSD test-pulse concern and gives a cleaner edge for short-duration events.

12. Troubleshooting Matrix

13. Frequently Asked Questions