Special Function modules for the PACSystems RSTi‑EP line handle tasks beyond basic I/O, such as safe power feeds, PWM outputs, high‑speed counters, frequency measurement, serial communications and SSI encoder interfaces. They install with a ‘double‑click’ on DIN rails and offer IP20 protection and V‑0 flammability rating. These units enable safety circuits and specialty logic in a compact form. They’re useful for filling gaps in an RSTi system, but because they’re proprietary and limited to the RSTi ecosystem, they might lock you in; evaluate whether off‑the‑shelf safety relays or third‑party specialty modules could better meet your needs.