Technical Specifications
| Parameter | Value |
|---|---|
| Product Model | 001818K (K revision; cross-referenced as 001818-AT) |
| Manufacturer | Opto 22 |
| Product Type | SNAP I/O Digital Input Module, 24 V AC |
| Input Channels | 4, optically isolated, sinking/sourcing field devices on 24 V AC common |
| Nominal Input Voltage | 24 V AC, 50/60 Hz (operating band 10–60 Hz) |
| Turn-On Threshold | 20 V AC typical, 14 V AC minimum |
| Turn-Off Threshold | 5 V AC maximum |
| Maximum Turn-Off Time | 30 ms (zero-crossing AC switching) |
| Input Impedance | 1.2 kΩ per channel |
| Input Current | ≈20 mA per channel at 24 V AC |
| Backplane Supply | 5 V DC from SNAP rack, ≈35 mA module draw |
| Isolation | 250 V AC continuous, field side to SNAP backplane/logic side |
| Module Power Dissipation | <0.2 W |
| Status Indication | Individual point-status LED per channel plus module-active indication |
| Compatibility | SNAP-B8M, SNAP-B16M, SNAP-16MC, SNAP-8MC carrier bases |
| Controller Compatibility | SNAP-B3000, SNAP2-BSeries brains, SNAP PAC R-series and EB-series |
| Software | PAC Control, OptoFlow, Legacy C/C++ SNAP library, Modbus/TCP mapped points |
| Operating Temp | –40 to 70 °C (–40 to 158 °F) |
| Storage Temp | –40 to 85 °C (–40 to 185 °F) |
| Humidity | 5–95% relative, non-condensing |
| Shock / Vibration | 50 G operational shock; 5 G, 10–150 Hz vibration |
| Environmental Rating | IP20 open-module; designed for sealed SNAP enclosure or panel mount |
| Certifications | UL 508, CSA, CE |
| Dimensions / Weight | ≈63 × 41 × 18 mm (2.5 × 1.6 × 0.7 in); ≈45 g (0.10 lb) |
| Lifecycle Status | Legacy mature part; sourced as new-surplus / tested equivalent spare |
Main Features and Advantages
AC sensing that was designed for contact quality, not just voltage:
A 24 V AC input is a harder real-world problem than the datasheet suggests. Contacts bounce, inductive loads ring, and a dry reed on a valve can present a less-than-clean sine wave after fifty metres of conduit. The 001818K addresses this with a 1.2 kΩ input impedance and a 20 V AC typical turn-on, so the channel ignores the low-level ringing that a sensitive input would mistake for a closure while still guaranteeing pickup on a weak but genuine 24 V AC signal. The 30 ms maximum turn-off is deliberately slow: it rides the AC zero crossing, which is exactly what you want when the input is a mechanical contact and you have no interest in seeing every half-cycle as a transition.
Isolation that protects the brain, not just the point:
Each field channel on the Opto 22 001818K is opto-coupled from the SNAP backplane, and the isolation rating is 250 V AC continuous. That lets a maintenance electrician land a 24 V AC pump-starter auxiliary or a float-switch leg without first verifying that the switch’s earth reference matches the panel’s. In a lift station or a wellsite skid, where instrument power and motor control power are routinely run from different transformers, that barrier prevents the classic “one bad ground and the whole rack goes noisy” failure. It also means a failed input stage generally takes out one point, not the SNAP brain behind it.
Drop-in replacement discipline:
The reason people search for the 001818-AT when they need a 001818K is that the two are form-fit and function-fit identical. Same 4-channel density, same 24 V AC threshold, same backplane keying, same point-to-terminal mapping. A rack that was commissioned with the AT revision therefore accepts the K revision with no I/O map change, no carrier swap and no point-rename in PAC Control. That is unusually valuable for a module whose host program may be fifteen years old and documented only on a paper printout in the operator’s cubicle.
Wide temperature and vibration margin:
Rated from –40 to 70 °C with a 50 G shock spec, the 001818K is built for the places Opto 22 traditionally sells: pump houses with no heat, desert solar skids, compressor stations and cold-storage loading docks. The open IP20 module is expected to live inside a SNAP enclosure, but the electronics do not flinch when the panel temperature cycles hard. That thermal headroom is why a site will stock one 001818K as a universal AC-input spare rather than keep four different SNAP input types on the shelf.
Software transparency:
Because the module presents standard SNAP digital points, it needs no special driver, no EDS and no add-on profile. PAC Control sees four Boolean inputs, the legacy OptoFlow project sees four points exactly where they were, and a Modbus/TCP or EtherNet/IP gateway maps them as four discrete inputs. There is no calibration, no range select, and no configuration register to corrupt — which is precisely the reliability model a remote pump station wants from a digital input.
