Cascading safety light curtains daisy-chain emitter/receiver pairs so a single OSSD pair and one EDM feedback loop supervise an L-shaped or perimeter-protected zone, cutting the safety relay count from one per pair to one per chain [S4][S5].
A chain of three or four Type 4 / SIL3 / PLe segments behaves as one virtual curtain to the safety relay or safety PLC, which only sees two OSSDs and one auxiliary EDM input regardless of segment count [S5][S6].
How a Cascade is Wired End-to-End
Banner's S4B cascading kit uses a black power cordset on every emitter and receiver and a yellow double-ended cable between adjacent units, with the yellow plug carrying OSSD pass-through between segments [S4]. Mouser's S4B datasheet confirms the same color convention: the black cordset is the local power/control feed and the yellow cordset is the inter-segment cascade link, with connector gender dictating whether a unit is a "first," "middle," or "last" segment [S6].
On the last segment of a chain the OSSD outputs and EDM return are wired into the evaluation device, while upstream segments only repeat the OSSD pair through the yellow cable; the evaluation device therefore cannot distinguish a two-segment chain from a four-segment one, which is precisely the engineering reason cascade segment count is a hard limit in the OEM manual [S4][S6].
Airindex's Model CA brochure states the same architecture verbatim: emitters and receivers are daisy-chained and "behave like a single light curtain" to the upstream protection relay, simplifying both the safety logic and the fault-diagnostic path [S5].
OSSD, EDM, and the Safety-Relay Interface
OSSD (Output Signal Switching Device) is the pair of self-monitored PNP outputs the chain presents to the safety relay; on a properly cascaded chain the two OSSDs must pulse-test within the window declared by the OEM, typically 100 µs to 1 ms depending on the segment [S5][S6].
EDM (External Device Monitoring) is the feedback loop from the contactors back into the receiver on the last segment; if the EDM input does not track the OSSD state within the declared cross-fault time, the curtain latches off and the chain must be power-cycled [S4][S5].
The forum case from PLCTalk illustrates the wiring failure mode: routing the light curtain OSSD into the same relay channel as the E-stop, instead of separate evaluation channels, defeats cross-fault detection and pulls the cell down to a lower PL than the individual components imply [S2]. Pilz's application note on cascading electrosensitive protective equipment repeats the rule: each chain must terminate in a single evaluation device, and mixing E-stops and curtain OSSDs on one input pair is explicitly out of scope [S8].
Selection Criteria and Cascade Limits
Four decision criteria separate a usable cascade from a misconfigured one: (1) all segments share a common Type/category (Type 4 / SIL3 / PLe is the typical industrial floor baseline); (2) the OEM maximum segment count per chain is respected (Banner S4B: up to 4 segments; many Type 4 families cap at 4-8); (3) protective height and resolution are matched across segments so the chain presents one homogeneous detection zone; (4) the safety relay is rated for the cumulative load and for the EDM wiring topology [S4][S5][S6].
Keyence's SL-V series datasheet page highlights an ultra-thin housing aimed at tight integration, but the cascade-capable SKU in that line is the SL-VHM or equivalent, not the entry-level body; datasheet verification of the cascade suffix is mandatory before ordering [S3]. DADISICK's DK-QT4 Series is the company's explicitly-named cascade light curtain SKU, segregated in the catalog from the standalone DK-QCE, DK-QT, DK-QO, DK-QRF, DK-KT, DK-QSA, DK-QBT, DK-QZ, and DK-QM lines so that the cascade wiring accessories are not cross-mated with non-cascade segments [S1].
A cascade is for: L- or U-shaped guarding on a robot cell, perimeter guarding around a press where a single straight run is geometrically impossible, and palletiser infeed/outfeed where two curtains cover a gap the operator must cross [S5][S8]. A cascade is NOT for: mixing brands on one chain (connector pinouts and OSSD pulse windows are OEM-specific), exceeding the OEM segment count, or installing segments whose microcomputer protection response time summed across the chain exceeds the safety distance calculation [S4][S6].
Standards, Distance, and Failure Modes
EN ISO 13855 (formerly EN 999) sets the safety-distance formula S = K*T + C, where K is the approach speed (typically 1.6 m/s or 2.0 m/s per hand/arm), T is the total stop time including the cascade's worst-case response time, and C is the intrusion allowance based on resolution; a 14 mm resolution curtain uses C = 0, while a 30 mm curtain adds 128 mm or 850 mm depending on reach direction [S7].
PL/SIL categorization for the chain is determined by the lowest-rated segment plus the safety relay, not the best segment; Type 4 segments chained through a Type 3 relay collapse the chain to Type 3 / PLd / SIL2 regardless of curtain capability [S5][S8]. The PLCTalk thread's practical resolution was to fit two redundant contactors in a Cat 4 / PLe configuration and keep the E-stop and the light curtain OSSD on separate relay channels, so that a cross-fault on one input pair is still detected by the other [S2].
Three failure modes dominate the field: (1) EDM jumper left open, which lets the contactors stick-weld undetected; (2) yellow cascade cable swapped for a black power cable, which puts 24 V onto the OSSD line; (3) cascaded segments of different categories, which silently downrates the chain [S4][S5][S6]. Automate.org's 2024 light-curtain guide adds a fourth: floating blanking, which is sometimes used to ignore material passing through but must be configured identically on every segment in the chain or the OSSD pulse window drifts out of spec [S7].
Cascade vs Standalone vs Multi-Sided Arrays
Standalone curtains use one OSSD pair per relay input and are appropriate for one straight detection zone [S3][S7]. Cascaded curtains use one OSSD pair per chain and are appropriate for two to four collinear or angled segments where a single safety relay can supervise the whole field [S4][S5][S6]. Multi-sided arrays (e.g., DADISICK DK-QSA) use mirrors or corner columns to fold a single curtain around a corner, which is mechanically different from cascading and does not multiply the OSSD count [S1].
On cost, a cascade saves one safety relay per extra segment (typical safety relay modules list around $200-400 each) but adds the price of the cascade cordsets and reduces the maximum segment count versus a true multi-channel safety-PLC approach [S3][S5]. On diagnostics, a cascade hides which segment tripped the OSSD unless the OEM offers per-segment status over IO-Link or a similar side-channel, while a multi-channel safety PLC resolves each segment to its own input bit [S3][S8].
Integration Notes for 2026 Robot and Press Cells
Banner's S4B and Pilz's PSEN op cascading family both publish OSSD response times under 15 ms per segment in 2026 datasheets, with chain-total response times staying inside 25-30 ms for a four-segment build, which keeps the safety-distance adder inside a workable envelope for cells with 250-400 ms hydraulic stop times [S4][S8].
For a 2026 robot cell with a 500 mm safety distance and a 250 ms measured stop time, a four-segment cascade at 30 ms chain response is borderline; spec the chain at the OEM's worst-case pulse-test number, not the typical, and re-validate with a stop-time meter after every curtain firmware update [S2][S5][S8]. The wiring duct routing the yellow cascade cable must be segregated from VFD and servo power cabling to keep OSSD pulse edges clean; cross-talk from a VFD can stretch the OSSD pulse and cause nuisance trips on the safety relay [S2].
Trackable signals for the next 90 days: confirm the cascade segment count for any new Banner S4B, Keyence SL-V, or Pilz PSEN op order against the 2026 datasheet revision, verify EDM wiring on the last segment is a closed loop back to the receiver (not the safety relay), and re-measure the chain-total response time after every safety PLC firmware update [S3][S4][S8]. A practical reference for the fixed-blanking option on a multi-segment chain is covered in this fixed-blanking guide, which describes the same per-segment configuration discipline that cascade wiring requires [S7].